Patent Publication Number: US-11642578-B2

Title: Iron-type golf clubs and golf club heads

Description:
This is a continuation of U.S. patent application Ser. No. 16/882,523, filed on May 24, 2020, now U.S. Pat. No. 11,148,019, which is a continuation of U.S. patent application Ser. No. 16/125,108, filed Sep. 7, 2018, now U.S. Pat. No. 10,661,130, which is a continuation of U.S. patent application Ser. No. 15/485,687, filed Apr. 12, 2017, now U.S. Pat. No. 10,092,800, which is a continuation of U.S. patent application Ser. No. 15/077,418, filed Mar. 22, 2016, now U.S. Pat. No. 9,623,300, which is a continuation of U.S. patent application Ser. No. 14/534,915, filed Nov. 6, 2014, now U.S. Pat. No. 9,427,633, which is a continuation of U.S. patent application Ser. No. 14/284,968, filed May 22, 2014, now U.S. Pat. No. 9,295,887, which claims the benefit to U.S. Provisional Application No. 61/922,756, filed Dec. 31, 2013, which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to golf clubs and golf club heads, and more particularly muscle-back or blade iron golf clubs and golf club heads. 
     BACKGROUND 
     Golf clubs are well known in the art for use in the game of golf. Iron type golf clubs generally either have a cavity back configuration or a muscle-back or blade-type configuration. Amateur golfers generally prefer cavity back perimeter-weighted clubs because they produce better shots when not struck near the center of the face. Blade type irons are generally preferred by professional golfers and golfers of higher skill levels because they provide better feel when a golf ball is struck in the center of the face and more feedback when not struck on the center of the face. Blade irons also permit golfers to more readily shape shots by adding different types of spin to the ball, whereas cavity-backs minimize the ability to shape shots. 
     Cavity-back iron type club heads, also known as perimeter weighted irons, are known to have a concentration of mass about the periphery of a rear surface of the club head. This concentration of mass is in a raised, rib-like, perimeter weighting element that substantially surrounds a rear cavity, which comprises a major portion of the rear surface of the club head. In addition to locating a substantial amount of mass away from the center of the club head behind the club face, the rib-like perimeter weighting element acts as a structural stiffener, which compensates for reduction in face thickness in the cavity region. 
     Muscle-back or blade irons are characterized by a thick lower portion known as the “muscle”, which extends along the entire length of the head. A thin upper portion extends upwardly from the muscle and behind the face of the club, and is commonly referred to as the blade portion. The blade portion may not have reinforcement ribs or perimeter weighting, and may have substantial mass concentrated in the muscle of the club extending along the sole and the entire length of the club head. Typically, a muscle-back club head is smaller than a cavity-back head. 
     Generally, muscle-back or blade irons have a center of gravity located away from the face center location, typically on the heel and sole side of the face center location. It is generally understood that the closer the center of gravity of the club head is to the face center, the better the club will feel and perform at impact when hitting the golf ball on the face center location. 
     The present invention seeks to overcome these limitations and other drawbacks of known muscle-back or blade iron golf clubs and golf club heads. 
     SUMMARY 
     The following presents a general summary of aspects of the invention in order to provide a basic understanding of the invention and various features of it. This summary is not intended to limit the scope of the invention in any way, but it simply provides a general overview and context for the more detailed description that follows. 
     According to aspects of this invention, an iron-type golf club head may comprise a top surface, a sole, a heel, and a toe. The iron-type golf club head may be a blade-type iron golf club head further defined with the top surface having a width of no greater than 8 mm and the sole having a width of no greater than 16 mm. The iron-type golf club head may further comprise a ball striking surface configured for striking a ball. The iron-type golf club head may include a leading edge defined as a forward most surface connecting the sole and the ball striking face. The iron-type golf club head may include a leading edge defined as a forward most surface connecting the sole and the ball striking face. The ball striking surface may have a ball striking area that defines a heel-side boundary line, a toe-side boundary line, and a ball striking centerline located equidistant between the heel-side boundary line and the toe-side boundary line. The iron-type golf club head may further comprise a rear surface opposite the ball striking surface. The rear surface may have a separate upper blade portion and a lower muscle portion. The upper blade portion may be separated from the lower muscle portion by a blade interface. The upper blade portion and the lower muscle portion may extend across the rear surface from the heel to the toe. The upper blade portion may extend from the top surface to the blade interface. The lower muscle portion may extend from the blade interface to the sole. Additionally, the upper blade portion may have a generally uniform thickness from the heel to the toe which is between approximately 6 mm and 8 mm. The lower muscle portion may have a thickness greater than the upper blade portion thickness. The lower muscle portion thickness may be between approximately 8 mm and 16 mm. The lower muscle portion may have a height measured from the blade interface to the leading edge. Additionally, an average height of the lower muscle portion from the ball striking centerline to the toe-side boundary line may be at least 50% greater than an average height of the lower muscle portion from the heel-side boundary line to the ball striking centerline. 
     Additionally, another aspect of this invention may relate to the blade interface. The blade interface may be a smooth arcuate surface that forms a transition area between the upper blade portion and the lower muscle portion. Additionally, the blade interface may include a first blade interface and a second blade interface intersecting the first blade interface at a convergence point. The first blade interface may be generally parallel to the leading edge extending from the heel to the convergence point. The second blade interface may extend upward from the convergence point to the toe. The convergence point may be located approximately along the ball striking centerline. The second blade interface may extend upward at an interface angle between approximately 105 degrees and 155 degrees. 
     Additionally, another aspect of this invention may relate to a blade-type iron golf club in accordance with examples of this invention. The blade-type iron golf club may comprise a blade-type golf club head and a shaft attached to the blade-type golf club head. The blade-type golf club head may comprise a body forged of a metal material. The body may include a hosel, a top surface, a sole, a heel, and a toe. The blade-type golf club head may further comprise a ball striking surface configured for striking a ball. The iron-type golf club head may include a leading edge defined as a forward most surface connecting the sole and the ball striking face. The ball striking surface may define a heel-side boundary line, a toe-side boundary line, and a ball striking centerline located equidistant between the heel-side boundary line and the toe-side boundary line. The club head may further comprise a rear surface opposite the ball striking surface. The rear surface may have an upper blade portion and a lower muscle portion with the upper blade portion separated from the lower muscle portion by a blade interface. The upper blade portion and the lower muscle portion may extend across the rear surface from the heel to the toe. The upper blade portion may extend from the top surface to the blade interface. The lower muscle portion may extend from the blade interface to the sole. The upper blade portion may have a generally uniform thickness from the heel to the toe. The lower muscle portion may have a thickness greater than the upper blade portion thickness. The lower muscle portion may have a height measured from the blade interface to the leading edge. The height at the toe-side boundary line may be greater than approximately 26 mm and the height at the heel-side boundary line may be less than approximately 14 mm. In another club head embodiment, the height at the toe-side boundary line may be greater than approximately 30 mm and the height at the heel-side boundary line may be less than approximately 16 mm. In yet another club head embodiment, the height at the toe-side boundary line may be greater than approximately 36 mm and the height at the heel-side boundary line may be less than approximately 18 mm. 
     Additional aspects of this invention relate to a blade-type iron golf club head. The blade-type golf club head may comprise a ball striking surface and a rear surface opposite the ball striking surface. The ball striking surface may be configured for striking a ball. The ball striking surface may have a ball striking area that defines a heel-side boundary line, a toe-side boundary line, and a ball striking centerline located equidistant between the heel-side boundary line and the toe-side boundary line. The rear surface may have a separate upper blade portion and a lower muscle portion. The upper blade portion may be separated from the lower muscle portion by a blade interface. The upper blade portion and the lower muscle portion may laterally extend across the rear surface. The upper blade portion may extend upward from the blade interface. The lower muscle portion may extend downward from the blade interface. The upper blade portion may have a generally uniform thickness of between approximately 6 mm and 8 mm. The lower muscle portion may have a thickness greater than the upper blade portion thickness. The lower muscle portion thickness may be between approximately 8 mm and 16 mm. The lower muscle portion may have an average height from the ball striking centerline to the toe-side boundary line that is at least 50% greater than an average height of the lower muscle portion from the heel-side boundary line to the ball striking centerline. Additionally, the blade-type golf club head may further include a toe surface area of the lower muscle portion located between the toe-side boundary line to the ball striking centerline that is 2 time greater than a heel surface area of the lower muscle portion located between the heel-side boundary line to the ball striking centerline. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention and certain advantages thereof may be acquired by referring to the following detailed description in consideration with the accompanying drawings, in which: 
         FIG.  1    generally illustrates a perspective view of an example golf club according to this invention; 
         FIG.  2 A  generally illustrates a perspective rear view of a prior art golf club head; 
         FIG.  2 B  generally illustrates a rear view of the prior art golf club head of  FIG.  2 A ; 
         FIGS.  3  through  7 B  generally illustrate various views of an example golf club head and various performance parameters and characteristics according to this invention; 
         FIG.  8    illustrates a perspective front view of a golf club head according to this invention; 
         FIG.  9    illustrates a perspective rear view of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  10    illustrates a front view of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  11    illustrates a rear view of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  12    illustrates a top view of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  13    illustrates a bottom view of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  14    illustrates a side view (on the toe side) of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  15    illustrates another side view (on the heel side) of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIGS.  16 A through  16 C  illustrate cross-sectional views along lines L 1 , L 3  and L 2  respectively of the golf club head illustrated in  FIG.  8    according to this invention; 
         FIG.  17    illustrates a perspective rear view of another golf club head according to this invention; and 
         FIG.  18    illustrates a rear view of the golf club head illustrated in  FIG.  17    according to this invention. 
     
    
    
     The reader is advised that the attached drawings are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     In the following description of various example structures in accordance with the invention, reference is made to the accompanying drawings, which form a part hereof, and in which are shown by way of illustration various example adjustment members, golf club heads, and golf club structures in accordance with the invention. Additionally, it is to be understood that other specific arrangements of parts and structures may be utilized, and structural and functional modifications may be made without departing from the scope of the present invention. Also, while the terms “top,” “bottom,” “front,” “back,” “rear,” “side,” “underside,” “overhead,” and the like may be used in this specification to describe various example features and elements of the invention, these terms are used herein as a matter of convenience, e.g., based on the example orientations shown in the figures and/or the orientations in typical use. Nothing in this specification should be construed as requiring a specific three dimensional or spatial orientation of structures in order to fall within the scope of this invention. 
     A. General Description of Muscle-Back or Blade-Type Iron Clubs and Club Heads 
     In general, aspects of this invention relate to a set of golf clubs, golf clubs, or golf club heads with a blade-type golf club head.  FIG.  1    illustrates an example blade-type golf club head in accordance with aspects of this invention. A blade-type golf club head does not contain any cavities or depressions in the rear surface as distinguished from perimeter-weighted clubs which contain one or more rear cavities.  FIGS.  2 A and  2 B  illustrate a prior art golf club head, shown having a traditional muscle-back or blade-type iron configuration. The iron club head  1102  in  FIGS.  2 A and  2 B  includes a face or striking face  1110 , a top surface  1112 , a sole  1114 , a heel  1116 , a toe  1118 , and a rear surface  1120  as was described above. The rear surface  1120  comprises a substantially flat area, which defines a blade portion  1210  of the club head, and a contoured area which defines a muscle portion  1220  of the club head  1102 . The blade portion  1210  generally occupies the entire upper portion of the club head  1102 , and has a substantially constant thickness that may be less than, for example, approximately 6 mm. The muscle portion  1220  generally constitutes a lower portion of the club head  1102 , and has a varying thickness that is everywhere greater than that of blade portion  1210 . 
     The muscle portion  1220  may be generally separated from the upper blade portion  1210  by a blade interface  1202 , represented by a phantom line. The blade interface  1202  may be a smooth, arcuate surface forming the transition area between the upper blade portion  1210  and the muscle portion  1220 . If there is no distinct boundary separating the muscle portion  1220  and the upper blade portion  1210 , the transition between the muscle portion  1220  and the upper blade portion  1210  may occur via a gradual surface curvature. As illustrated in  FIGS.  2 A and  2 B , the blade interface  1202  is a straight line extending across the rear surface  1120  of the club head  1102  from the heel  1116  to the toe  1118 . 
     Additionally, other features and characteristics may be identified with a blade-type or muscle-back iron club head. The blade-type or muscle-back iron club head may be formed of forged metal such as carbon steel in order to increase the feel provided to the golfer. Additionally, the sole width of a blade-type or muscle-back iron club head may be generally thin and constant along the length of the sole. For example, the sole width for blade-type or muscle-back iron club heads may be approximately 10 mm to 19 mm in width. Additionally, the top surface width or blade width (also known as top-line width) may be generally constant along the length of the top surface. For example, the top surface width for blade-type or muscle-back iron club heads may be approximately 3.2 mm to 6.4 mm in width. Additionally, the hosel length of blade-type or muscle-back iron club heads may be approximately 60 mm to 90 mm in length measured from the origin point  132 A where the axis at the center of the hosel intersects the ground plane  124  to the center at the top of the hosel  126 . Generally, the top lines on a blade-type or muscle-back iron club head are thin and set. For example, the top view from a reference position, a golfer looking down on the club head can see only the thin top surface  112  and the striking face  110 , with none of the rear surface  120  being visible. 
     B. Description of Muscle-Back or Blade-Type Iron Clubs and Club Heads in Accordance with Examples of this Invention 
       FIG.  1    generally illustrates an example muscle-back or blade iron golf club  100  in accordance with at least some examples of this invention. This club  100  includes a club head  102 , a shaft  106  (which will be described in more detail below), and a grip member  103  engaged with the shaft  106 . While a low loft iron golf club head  102  is illustrated in these figures, aspects of this invention may be applied to any type of iron club head, including, for example: low, middle, and high loft club heads (of any desired loft, e.g., 1-iron, 2-iron, 3-iron, etc. to 9-iron and wedges with loft angles ranging from 20-64 degrees). The iron club heads may be made from any desired materials, in any desired construction and/or in any desired manner, including from conventional materials, in conventional constructions, in conventional manners, as are known and/or used in the art, optionally modified (if necessary, e.g., in size, shape, inclusion of structures, etc.) as required for aspects of this invention as described in more detail below. 
     Any desired materials also may be used for the shaft  106 , including conventional materials that are known and/or used in the art, such as steel, graphite based materials, polymers, composite materials, combinations of these materials, etc. Optionally, if necessary or desired, the shaft  106  may be modified (e.g., in size, shape, etc.) to accommodate releasable club head/shaft connection parts. The grip member  103  may be engaged with the shaft  106  in any desired manner, including in conventional manners that are known and/or used in the art (e.g., via cements or adhesives, via mechanical connections, etc.). Any desired materials may be used for the grip member  103 , including conventional materials that are known and/or used in the art, such as rubber, polymeric materials, cork, rubber or polymeric materials with cord or other fabric elements embedded therein, cloth or fabric, tape, etc. 
     Generally, all iron club heads  102  include various parts.  FIG.  3    illustrates various parts of the golf club head  102  as will be referenced throughout the remainder of this application (as referenced from USGA Rules of Golf). An iron club head  102  has a face or striking face  110 , a top surface  112 , a sole  114 , a heel  116 , a toe  118 , and a rear surface  120 . The top surface  112  may be defined as the upper portion of the head  102 . The sole  114  may be defined as the bottom or underside portion of the head  102 , and is generally opposite the top surface  112 . The sole  114  may include an area on the club head  102  that rests on the ground when a golfer soles the golf club  100 . The sole  114  may generally rest on a ground plane  124 , wherein the ground plane  124  is a horizontal plane tangent with the bottom of the club head  102 . The heel  116  may the part of the club head  102  nearer to and including a hosel  126 . The toe  118  may be the area of the golf club  100  that is the farthest from the shaft  106 . The rear surface  120  of the club head  102  is generally opposite the face  110 . The shaft  106  attaches to the head  102  at the heel  116  via a hosel  126 . The shaft  106  has a center axis. The hosel  126  may have a bore for receiving the shaft  106 , or a shaft adapter (not shown). The hosel bore has a center axis or a hosel axis  128 . If the shaft  106  is inserted and attached directly to hosel bore, the hosel axis  128  may be substantially coincident with shaft axis. For club configurations including a shaft adapter, the shaft  106  may be received in a shaft adapter bore. The shaft adapter bore may have a center axis or shaft adapter axis, which may be substantially coincident with shaft axis. The shaft adapter axis may be offset angularly and/or linearly from the hosel axis  128  to permit adjustment of club parameters via rotation of the shaft adapter with respect to club head  102 , as is known by persons skilled in the art. 
     According to aspects of this invention, a golf club  100  may be oriented in a reference position. In the reference position, the golf club  100  may include a number of parameters or characteristics that may include, but are not limited to: a face center location, a loft angle, a face angle, a lie angle, and a center of gravity location. Parameters or characteristics as well as methods and procedures for measuring them will be described and detailed below. 
     As illustrated in  FIG.  3   , a lie angle  130  is defined as the angle formed between the shaft axis or hosel axis  128  and a horizontal plane contacting the sole  114 , which may be the ground plane  124  when the iron is positioned such that the scorelines on the face are parallel to the ground plane  124 . 
       FIG.  4    illustrates the face center location  140  on a fixtured club head  102 . The face center  140  is determined using Unites States Golf Association (USGA) standard measuring procedures and methods. For example, the current USGA procedure requires finding the center point along a horizontal line  142  along the club face  110  until the heel  116  and the toe  118  measurements from the edges of the roughened area of the face  110  of the club head  102  are equal. After finding the face center point, the face center plane  144  is defined as a vertical plane through the center point. Then, the center point on a face center plane  144  is found along the club face  110  when the upper portion  112  and the sole  114  measurements at the edges of the face  110  of the club head  102  are also equal. When the heel  116  and the toe  118  measurements are equal and the upper portion  112  and the sole  114  measurements are equal, the intersecting point of these lines is defined as the face center location  140 . For irons, the heel and toe measurement is made at the edges of the roughened area of the face. 
       FIG.  5    illustrates an example of a loft angle  150  and a leading edge  122  of the golf club head  102 . As illustrated in  FIG.  5   , the loft angle  150  is defined as a measurement between an axis normal  152  or perpendicular to a face center axis  154  and an axis normal  156  or perpendicular to the ground plane  124 . The face center axis  154  is defined as the axis from the face center  140  and normal to the face. Additionally, the loft angle  150  may be defined as a measurement between the face center axis  154  and the ground plane  124 . It is recognized that each of these loft angle  150  definitions may yield a similar or exactly the same loft angle measurement. The leading edge  122  is the forward most surface connecting the sole  114  and the striking face  110 . The leading edge  122  may be a constant radius or may have a curvature that changes along the heel to the toe of the golf club head. 
     An origin point  132  may be defined on the golf club  100  or golf club head  102 , or a point defined in relation to certain elements of the club or head. Various other points, such as the center of gravity, sole contact, and face center, may be described and/or measured in relation to the origin point  132 .  FIG.  3    illustrates two different examples of where the origin point  132  may be located. A first location  132 A, defined as a ground origin point  132 A, is generally located at the ground plane  124 . The ground origin point  132 A is defined as the point at which the ground plane  124  and the hosel axis  128  intersect. The second location  132 B, defined as the hosel origin point  132 B, is generally located on the hosel  126 . The hosel origin point  132 B is located on the hosel axis  128  and coincident with the uppermost edge  126 B of the hosel  126 . Either location for the origin point  132  may be utilized without departing from this invention. Additionally, other locations for the origin point  132  may be utilized without departing from this invention. Throughout the remainder of this application, the ground origin point  132 A will be utilized for all reference locations, tolerances, and calculations. 
     As illustrated in  FIG.  3   , a primary coordinate system may be defined on the origin point  132 A, e.g., the origin point  132 A has an X axis  135  parallel to the ground plane  124  and generally parallel to the leading edge  122  of the golf club head  102 , a Y axis  137  perpendicular to the X axis  135  and oriented away from the rear surface of the golf club  120 , and a Z axis  133  perpendicular to the ground plane  124 . 
     Additionally, as illustrated in  FIG.  4   , a secondary coordinate system may be defined on the origin point  132 B, e.g., with a Z′ axis  134  extending along the direction of the shaft axis  107  (and/or the hosel axis  128 ), an X′ axis  136  parallel with the vertical plane and normal to the Z′ axis, and a Y′ axis  138  normal to the X′ and Z′ axes. 
       FIG.  6    illustrates an example of a face angle  160  of a golf club head  102 . As illustrated in  FIG.  6   , the face angle  160  is measured by utilizing the face center axis  154  and a right plane  162  (a plane perpendicular to the X axis  135 ). 
       FIGS.  7 A and  7 B  illustrate an example of a center of gravity location  170  as a specified parameter of the golf club head  102 . The center of gravity of the golf club head  102  may be determined using various methods and procedures known and used in the art. The golf club head  102  center of gravity location  170  is provided with reference to its position from the origin point  132 A. As illustrated in  FIGS.  7 A and  7 B , the center of gravity location  170  is defined by a distance from the origin point  132 A along the X axis  135  named CGX  171 , Y axis  137  named CGY  173 , and Z axis  133  named CGZ  175 . The center of gravity  170  of the golf club head  102  may be provided with reference to its position from the hosel origin point  132 B. As illustrated in  FIGS.  7 A and  7 B , the center of gravity  170  location may also be defined by a distance from the hosel origin point  132 B along the X′ axis  136 , named ΔX  172 , the Y′ axis  138 , named ΔY  174 , and the Z′ axis  134 , named ΔZ  176 . 
     The moment of inertia is a clubhead property whose importance is well known to one skilled in the art. There are three moment of inertia properties that this application may reference. As  FIG.  7 A  illustrates the MOI x-x, which is the moment of inertia of an axis through the center of gravity of the clubhead around an axis parallel to the X-axis  135  of the origin coordinate system. Similarly, the MOI z-z is the moment of inertia of an axis through the center of gravity around an axis parallel to the Z-axis  133  as illustrated in  FIG.  4   . Lastly, the MOI h-h, is the moment of inertia around the shaft axis or Z′-axis  134  as illustrated in  FIG.  7 A . The MOI h-h is important in looking at how the clubhead may resist the golfer&#39;s ability to close the clubface during the swing. 
     Additionally,  FIGS.  4  and  5    show the face center location  140  may be defined from the ground origin point  132 A and the ground plane coordinate system, where CFX  141  is the distance along the X axis  135  from the origin point  132 A, CFY  143  is the distance along the Y-axis  137 , and CFZ  145  is distance along the Z-axis  133 . Also, the head length  147  of the golf club head can be defined from the origin point  132 A as measured along X-axis  135  to the furthest extent of the toe  118  of the golf club head  102 . 
       FIGS.  3 - 7 B  illustrate a golf club head  102  oriented in a reference position. In the reference position, the hosel axis  128  or shaft axis lies in a vertical plane as shown in  FIG.  5   . As illustrated in  FIG.  3   , the hosel axis  128  may be oriented at a lie angle  130 . The lie angle selected for the reference position may be the golf club  100  manufacturer&#39;s specified lie angle. If a specified lie angle is not available from the manufacturer, a lie angle can be determined using the parallel scoreline method described herein. Furthermore for the reference position, as illustrated in  FIG.  5   , the striking face  110  may be oriented at a loft angle  150 . The loft angle selected for the reference position may be the golf club manufacturer&#39;s specified loft angle. Table 1, below, provides typical loft and lie angles for various blade-type iron golf club heads in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Example Loft and Lie Angle for 
               
               
                 a Blade-Type Iron Club Heads 
               
            
           
           
               
               
               
            
               
                 Blade-Type  
                 Typical 
                 Typical 
               
               
                 Iron Golf 
                 Loft 
                 Lie 
               
               
                 Club Head 
                 Angle 
                 Angle 
               
               
                   
               
               
                 #2 
                 18.0° 
                 59.0° 
               
               
                 #3 
                 21.0° 
                 59.0° 
               
               
                 #4 
                 24.0° 
                 60.0° 
               
               
                 #5 
                 27.0° 
                 61.0° 
               
               
                 #6 
                 31.0° 
                 62.0° 
               
               
                 #7 
                 35.0° 
                 62.5° 
               
               
                 #8 
                 39.0° 
                 63.0° 
               
               
                 #9 
                 43.0° 
                 63.5° 
               
               
                 PW 
                 47.0° 
                 64.0° 
               
               
                   
               
            
           
         
       
     
     Club head parameters or characteristics may be measured physically, or in a computer-aided-design (CAD) environment. Generally, if a 3 dimensional (3D) model of club head  102  is not readily available, one may be created by performing a 3D scan of the club head  102  and creating a model file from the scan data and/or physical measurements, such that the model is substantially representative of the physical club head. In the CAD environment, the model of club head  102  may be set in the reference position with the face  110  oriented at the manufacturer&#39;s loft, lie, and face angles within the CAD environment such that the model is fully constrained. 
     Additionally, the golf club  100  may be physically oriented in the reference position using a fixturing system known and used in the art. As was described above, the shaft axis may be aligned at a lie angle according to the golf club manufacturer&#39;s specification, or at an appropriate lie angle as determined means described above. The golf club head  102  may rest with its sole  114  contacting a horizontal surface  124  with the club face  110  positioned at the manufacturer&#39;s face angle and/or loft angle using conventional loft and face angle measurement gauges known to one of skill in the art. 
     The present invention provides a blade-type or muscle-back iron club head.  FIGS.  8  through  16 C  illustrate a blade-type or muscle-back iron club head in accordance with aspects of this invention.  FIG.  8    illustrates a perspective front view of the golf club head  102 .  FIG.  9    illustrates a perspective rear view of the golf club head  102 .  FIG.  10    illustrates a front view of the golf club head  102 .  FIG.  11    illustrates a rear view of the golf club head  102 .  FIG.  12    illustrates a top view of the golf club head  102 .  FIG.  13    illustrates a bottom view of the golf club head  102 .  FIG.  14    illustrates a side view (on the toe side) of the golf club head  102 .  FIG.  15    illustrates another side view (on the heel side) of the golf club head  102 .  FIGS.  16 A  through  16 C illustrate cross-sectional views along lines L 1 , L 3  and L 2  respectively of the golf club head  102 . 
     As described above, the club head  102  includes a face or striking face  110 , a top surface  112 , a sole  114 , a heel  116 , a toe  118 , and a rear surface  120 . The rear surface  120  comprises a substantially flat area, which defines a blade portion  210  of the club head  102 , and a thickened area which defines a muscle portion  220  of the club head  102 . The blade portion  210  generally occupies the entire upper portion of the club head  102 , and has a substantially constant thickness that may be less than, for example, approximately 6 mm. The muscle portion  220  generally constitutes a lower portion of the club head  102 , and has a varying thickness that is everywhere greater than that of blade portion  210 . 
     The striking face  110  may be provided with score lines, grooves, or other surface features or textures enhancing the ability of the club head to grip the golf ball during impact. A grip-enhanced area or a ball striking area  110 A may be generally defined by a heel-side score line (or other grip-enhancing surface feature) boundary L 1  (shown dashed) that is perpendicular to the ground plane  124  and a toe-side score line (or other grip-enhancing surface feature) boundary L 2  (shown dashed) that is perpendicular to the ground plane  124  and by segments of the top surface  112  and sole  114  perimeter edges of the striking face  110  extending between by those heel-side and toe-side boundary lines L 1 , L 2 . Thus, the ball striking area  110 A has a length Ls (extending between boundary L 1  and boundary L 2 ) and a height Hs (extending between top surface  112  edge and the leading edge  122 ). Generally, this height Hs increases in the toe direction. The height Hs may be a minimum at the heel most extent of the ball striking area  110 A, and a maximum at some point in the toe direction. A centerline L 3  of the ball striking area  110 A may be located halfway along the length Ls of the ball striking area  110 A along center face plane  144 . Lines L 1 , L 2  and L 3  may be associated with through-the-thickness cross-sections as illustrated in  FIG.  10    and  FIGS.  16 A- 16 C , respectively. Thus, points on the rear surface  120  may be defined by reference to lines L 1 , L 2  and/or L 3 . 
     As illustrated in  FIGS.  9  and  11   , the muscle portion  220  adds mass and material to the top surface  112  and toe  118  and removes mass and material from sole  114  and heel  116 . This mass distribution of the muscle portion  220  moves the center of gravity  170  of the club head  102  closer to the face center  140  location. For most conventional blade-type irons (as illustrated in  FIGS.  2 A and  2 B ), the center of gravity  170  is located on the heel and sole side of the face center location  140  and approximately 2 to 3 mm away from the face center plane  144  location in the X-axis direction. For the club heads  102  in accordance with aspects of this invention with the sloped muscle portion  220 , the center of gravity  170  moves towards the toe side of the club head by approximately 0.5 to 1.0 mm closer to the face center plane  144 . This is an approximate 17% to 50% change in the location of the center of gravity  170  with respect to the face center plane  144 , moving the center of gravity  170  17 to 50% closer to the face center  140  as compared to the conventional blade-type irons. 
     As illustrated in  FIGS.  9  and  11   , the muscle portion  220  of the club head  102  is larger or has a greater height at the toe as compared to the heel. The area and the mass of the muscle portion  220  increases as the muscle portion  220  extends from the heel to the toe. The height of the muscle portion  220  increases extending across the rear surface from the heel to the toe. Additionally, the height of the muscle portion  220  generally increases at a greater rate from the middle of the club head  102  at approximately L 3  to the toe of the club head  102  at approximately L 1  as compared to the heel of the club head  102  at approximately L 2  to the middle of the club head  102  at approximately L 3 . 
     The muscle portion  220  may be separated from the upper blade portion  210  by a blade interface  202 . The blade interface  202  may be a smooth, arcuate surface forming the transition area between the upper blade portion  210  and the muscle portion  220 . The blade interface  202  may extend across the rear surface  120  of the club head  102  from the heel  116  to the toe  118 . In an aspect of the invention, there may be no distinct boundary separating the muscle portion  220  and the upper blade portion  210 , wherein the transition between the muscle portion  220  and the upper blade portion  210  may occur via a gradual surface curvature. 
     As illustrated in  FIGS.  9  and  11   , the blade interface  202  includes a first blade interface  202 A and a second blade interface  202 B. The first blade interface  202 A and the second blade interface  202 B are not along the same line. The first blade interface  202 A and the second blade interface  202 B may be intersecting lines, intersecting at a convergence point  204 . The convergence point  204  may be located along or near to the face center plane  144  of the ball striking area  110 A. The first blade interface  202 A may extend from the heel  116  to the convergence point  204 . The second blade interface  202 B may extend from the convergence point  204  to the toe  118 . The first blade interface  202 A may extend generally parallel to the sole  114  or the leading edge  122  or to the ground plane  124 . The second blade interface  202 B may then extend from the convergence point at an angle  206  upward toward the toe  118  and the top surface  112 . 
     The angle  206  may be an obtuse angle in accordance with aspects of this invention. For example, the angle  206  may be approximately 125 degrees. In another exemplary embodiment, the angle  206  may be between approximately 105 and 155 degrees. The convergence point  204  is located at a position in the X-axis direction from the ground plane origin  132 A to the convergence point  204  between 28 to 34 mm. Table 2 shows an exemplary embodiment of the location of the muscle corner and the angle  206  of the muscle throughout the set. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Example Muscle Convergence Point 
               
               
                 Location and Angle Dimensions for 
               
               
                 3-, 6-, and 9-Iron Club Heads 
               
            
           
           
               
               
               
            
               
                   
                 Location of 
                   
               
               
                   
                 Convergence Point 
                 Angle of 
               
               
                   
                 (ref #204) 
                 Muscle 
               
               
                 Iron 
                 [mm] 
                 (ref #206) 
               
               
                   
               
               
                 3 
                 32.2 
                 120 
               
               
                 6 
                 32.2 
                 124 
               
               
                 9 
                 32.3 
                 127 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIGS.  16 A,  16 B, and  16 C , the club head  102  may include three different heights, a muscle height H M , a blade height H B , and a face height H F . Each of these heights as illustrated in  FIGS.  16 A,  16 B, and  16 C  may be defined or measured across the club head at various cross-sections, such as at the toe L 1 , heel L 2  or the face center L 3 . 
     As illustrated in  FIGS.  11 ,  16 A,  16 B, and  16 C , the lower muscle portion  220  may include a muscle height H M . The muscle height H M  may be measured parallel to the striking face from the leading edge  122  to the corner of intersection of the lower muscle surface  220  and the blade interface surface  202 . Since most clubs will have a small fillet radius at the corner and not a sharp edge, the measurement will be taken to the midpoint of the radius. The muscle height H M  of the muscle portion  220  may vary across the rear surface  112  of the club head  102 . The muscle height H MX  may be defined or measured across the club head, where X corresponds to the appropriate cross-section L 1 , L 2 , or L 3 . 
     For example, the muscle height H M1  of a 3-iron or 21-degree loft club head at L 1  or near the toe may be approximately 28.2 mm. The muscle height H M2  of a 3-iron or 21-degree loft club head at L 2  or near the heel may be approximately 12.6 mm. The muscle height H M3    220  of a 3-iron or 21-degree loft club head at L 3  or near the face center  140  may be approximately 15.2 mm. Additionally, for example, the muscle height H M1  of a 6-iron or 31-degree loft club head at L 1  or near the toe may be approximately 31.3 mm. The muscle height H M2  of a 6-iron or 31-degree loft club head at L 2  or near the heel may be approximately 14.2 mm. The muscle height H M3  of a 6-iron or 31-degree loft club head at L 3  or near the face center  140  may be approximately 16.4 mm. Additionally, for example, the muscle height H M1  of a 9-iron or 43-degree loft club head at L 1  or near the toe may be approximately 37.8 mm. The muscle height H M2  of a 9-iron or 43-degree loft club head at L 2  or near the heel may be approximately 16.5 mm. The muscle height H M3  of a 9-iron or 43-degree loft club head at L 3  or near the face center  140  may be approximately 20.1 mm. These dimensions are summarized below in Table 3A for an exemplary 3-, 6-, and 9-iron or 21, 31, 43-degree loft blade type club head respectively in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 3A 
               
             
            
               
                   
               
               
                 Summary of Dimensions for Muscle Height H M  for an Exemplary 
               
               
                 3-, 6-, and 9-Iron Club Head in Accordance with this Invention 
               
            
           
           
               
               
               
               
            
               
                   
                 Muscle Height at 
                 Muscle Height at 
                 Muscle Height at the 
               
               
                   
                 the Toe, H M1   
                 the Heel, H M2   
                 Face Center, H M3   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 28.2 
                 12.6 
                 15.2 
               
               
                 6 
                 31.3 
                 14.2 
                 16.4 
               
               
                 9 
                 37.8 
                 16.5 
                 20.1 
               
               
                   
               
            
           
         
       
     
     The muscle height at the toe H M1  may be generally greater than the muscle height at the heel H M2 . For example, a muscle height toe-heel percentage may be the percentage increase of the muscle height of the toe H M1  over the muscle height of the heel H M2 , or (H M2 −H M1 )/H M1 . For an exemplary 3-iron golf club head, the muscle height toe-heel percentage may be approximately 123%. For an exemplary 6-iron golf club head, the muscle height toe-heel percentage may be approximately 120%. For an exemplary 9-iron golf club head, the muscle height toe-heel percentage may be approximately 128%. The muscle height toe-heel percentage may range from 75% to 150%. In another embodiment, the muscle height toe-heel percentage may be greater than 150%. 
     In another example, a muscle height toe-heel ratio may be the ratio of the muscle height of the toe H M1  over the muscle height of the heel H M2 , or H M2 /H M1 . For an exemplary 3-iron golf club head, the muscle height toe-heel ratio may be approximately 2.2. For an exemplary 6-iron golf club head, the muscle height toe-heel ratio may be approximately 2.2. For an exemplary 9-iron golf club head, the muscle height toe-heel ratio may be approximately 2.3. The muscle height toe-heel ratio may range from 1.5 to 3. In another embodiment, the muscle height toe-heel ratio may be greater than 3. 
     As illustrated in  FIGS.  16 A,  16 B, and  16 C , the upper blade portion  210  may include a blade height H B . The blade height H B  may be measured parallel to the striking face from the corner of intersection of the lower muscle surface  220  and the blade interface surface  202  to the top surface  112 . Since most clubs will have a small fillet radius at the corner and not a sharp edge, the measurement will be taken to the midpoint of the radius. The blade height H B  of the blade portion  210  may vary across the rear surface  112  of the club head  102 . The blade height H BX  may be defined or measured across the club head, where X corresponds to the appropriate cross-section L 1 , L 2 , or L 3 . 
     For example, the blade height H B1  of a 3-iron or 21-degree loft club head at L 1  or near the toe may be approximately 21.5 mm. The blade height H B2  of a 3-iron or 21-degree loft club head at L 2  or near the heel may be approximately 16.6 mm. The blade height H B3  of a 3-iron or 21-degree loft club head at L 3  or near the face center  140  may be approximately 25.8 mm. Additionally, for example, the blade height H B1  of a 6-iron or 31-degree loft club head at L 1  or near the toe may be approximately 20.4 mm. The blade height H B2  of a 6-iron or 31-degree loft club head at L 2  or near the heel may be approximately 16.6 mm. The blade height H B3  of a 6-iron or 31-degree loft club head at L 3  or near the face center  140  may be approximately 26.4 mm. Additionally, for example, the blade height H B1  of a 9-iron or 43-degree loft club head at L 1  or near the toe may be approximately 17.7 mm. The blade height H B2  of a 9-iron or 43-degree loft club head at L 2  or near the heel may be approximately 17.3 mm. The blade height H B3  of a 9-iron or 43-degree loft club head at L 3  or near the face center  140  may be approximately 28.8 mm. These dimensions are summarized below in Table 3B for an exemplary 3-, 6-, and 9-iron or 21, 31, 43-degree loft blade type club head respectively in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 3B 
               
             
            
               
                   
               
               
                 Summary of Dimensions for Blade Height H B  for an Exemplary 
               
               
                 3-, 6-, and 9-Iron Club Head in Accordance with this Invention 
               
            
           
           
               
               
               
               
            
               
                   
                 Blade Height at 
                 Blade Height at 
                 Blade Height at the 
               
               
                   
                 the Toe, H B1   
                 the Heel, H B2   
                 Face Center, H B3   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 21.5 
                 16.6 
                 25.8 
               
               
                 6 
                 20.4 
                 16.6 
                 26.4 
               
               
                 9 
                 17.7 
                 17.3 
                 28.8 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIGS.  16 A,  16 B, and  16 C , the club head  102  may include a face height H F . The face height H F  may be measured from the leading edge  122  to the top surface  112  parallel to the loft plane of the golf club head. Since most clubs will have a small fillet radius at the corner and not a sharp edge, the measurement will be taken to the midpoint of the radius. The face height H F  may vary across the rear surface  112  of the club head  102 . The face height H FX  may be defined or measured across the club head, where X corresponds to the appropriate cross-section L 1 , L 2 , or L 3 . 
     For example, the face height H F1  of a 3-iron or 21-degree loft club head at L 1  or near the toe may be approximately 49.7 mm. The face height H F2  of a 3-iron or 21-degree loft club head at L 2  or near the heel may be approximately 29.2 mm. The face height H F3  of a 3-iron or 21-degree loft club head at L 3  or near the face center  140  may be approximately 41.0 mm. Additionally, for example, the face height H F1  of a 6-iron or 31-degree loft club head at L 1  or near the toe may be approximately 51.7 mm. The face height H F2  of a 6-iron or 31-degree loft club head at L 2  or near the heel may be approximately 30.8 mm. The face height H F3  of a 6-iron or 31-degree loft club head at L 3  or near the face center  140  may be approximately 42.8 mm. Additionally, for example, the face height H F1  of a 9-iron or 43-degree loft club head at L 1  or near the toe may be approximately 55.5 mm. The face height H F2  of a 9-iron or 43-degree loft club head at L 2  or near the heel may be approximately 33.8 mm. The face height H F3  of a 9-iron or 43-degree loft club head at L 3  or near the face center  140  may be approximately 48.9 mm. These dimensions are summarized below in Table 3C for an exemplary 3-, 6-, and 9-iron or 21, 31, 43-degree loft blade type club head respectively in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 3C 
               
             
            
               
                   
               
               
                 Summary of Dimensions for Face Height H F  for an Exemplary 
               
               
                 3-, 6-, and 9-Iron Club Head in Accordance with this Invention 
               
            
           
           
               
               
               
               
            
               
                   
                 Face Height 
                 Face Height 
                 Face Height at the 
               
               
                   
                 at the Toe, H F1   
                 at the Heel, H F2   
                 Face Center, H F3   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 49.7 
                 29.2 
                 41.0 
               
               
                 6 
                 51.7 
                 30.8 
                 42.8 
               
               
                 9 
                 55.5 
                 33.8 
                 48.9 
               
               
                   
               
            
           
         
       
     
     Furthermore, the lower muscle portion  220  may be defined having an average muscle height H MAVG  (measured from the blade interface  202  to the leading edge  122 ) extending from the heel to the toe. Additionally, the muscle portion  220  may be defined having an average heel muscle height H M(L2−L3)  extending between the heel-side score line L 2  and the face center score line L 3 . Additionally, the muscle portion  220  may be defined having an average toe muscle height H M(L3−L1)  extending between the face center score line L 3  and the toe-side score line L 1 . Exemplary dimensions of average muscle height are summarized below in Table 4A for an exemplary 3-, 6-, and 9-iron or 21, 31, 43-degree loft blade type club head respectively in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 4A 
               
             
            
               
                   
               
               
                 Summary of Average Muscle Height for an Exemplary 
               
               
                 3-, 6-, and 9-Iron Club Head in Accordance with this Invention 
               
            
           
           
               
               
               
               
            
               
                   
                 Average 
                 Average Heel 
                 Average Toe 
               
               
                   
                 Muscle Height, 
                 Muscle Height, 
                 Muscle Height, 
               
               
                   
                 H MAVG   
                 H M(L2-L3)   
                 H M(L3-L1)   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 20.4 
                 13.9 
                 21.7 
               
               
                 6 
                 22.8 
                 15.3 
                 23.9 
               
               
                 9 
                 27.2 
                 18.3 
                 28.9 
               
               
                   
               
            
           
         
       
     
     Additionally, the upper blade portion  210  may be defined having an average blade height H BAVG . Generally, the average blade height H BAVG  may be defined as the average distance measured from the blade interface  202  to the top surface  112  and extending from the heel to the toe. Additionally, the upper blade portion  210  may be defined having an average heel blade height H B(L2−L3) . Generally, the average heel blade height H B(L2−L3)  may be defined as average distance measured from the blade interface  202  to the top surface  112  and extending between the heel-side score line L 2  and the face center score line L 3 . Additionally, the upper blade portion  210  may be defined having an average toe blade height H B(L3−L1) . Generally, the average toe blade height H B(L3−L1)  may be defined as average distance measured from the blade interface  202  to the top surface  112  and extending between the face center score line L 3  and the toe-side score line L 1 . Exemplary dimensions of average blade height are summarized below in Table 4B for an exemplary 3-, 6-, and 9-iron or 21, 31, 43-degree loft blade type club head respectively in accordance with an embodiment of this invention. 
     
       
         
           
               
             
               
                 TABLE 4B 
               
             
            
               
                   
               
               
                 Summary of Average Blade Height for an Exemplary 
               
               
                 3-, 6-, and 9-Iron Club Head in Accordance with this Invention 
               
            
           
           
               
               
               
               
            
               
                   
                 Average 
                 Average Heel 
                 Average Toe 
               
               
                   
                 Blade Height,  
                 Blade Height, 
                 Blade Height,  
               
               
                   
                 H BAVG    
                 H B(L2-L3)   
                 H B(L3-L1)   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 19.0 
                 21.2 
                 23.6 
               
               
                 6 
                 18.5 
                 21.5 
                 23.4 
               
               
                 9 
                 17.5 
                 23.0 
                 23.2 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIGS.  11 ,  16 A,  16 B, and  16 C , the lower muscle portion  220  may have a muscle thickness or muscle width W M  at each of the locations of the heel, center and toe. The width at each location is identified as W MX , where X corresponds to the appropriate cross-section L 1 , L 2 , or L 3 . W MX  is measured perpendicular from the striking face to the corner of intersection of the lower muscle surface  220  and the blade interface surface  120  at the designated cross-section. Since most clubs will a small fillet radius at the corner and not a sharp edge, the measurement will be taken to the midpoint of the radius. 
     Additionally, the lower muscle portion  220  may have a sole thickness or sole width W S  at each of the locations of the heel, center and toe. The width at each location is identified as W SX , where X corresponds to the appropriate number depending upon the measurement at cross-section L 1 , L 2 , or L 3 . W SX  is measured perpendicular from the striking face to the leading edge  122 . The sole width W SX  may range from approximately 8 mm to approximately 16 mm across the rear surface extending from the heel to the toe.  FIGS.  16 A through  16 C  illustrate the muscle portion  220  thickness W M  at each of the cross-sections L 1 , L 2 , and L 3 . Tables 5A and 5B below summarize exemplary embodiments of the 3-iron, 6-iron, and 9-iron and their muscle width, W MX , and sole width, W SX . 
     
       
         
           
               
             
               
                 TABLE 5A 
               
             
            
               
                   
               
               
                 Example Dimensions for Muscle Width 
               
               
                 Dimensions for a 3-, 6- and 9-Iron Club Heads 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Muscle Width at 
                 Muscle Width at 
                 Muscle Width at 
               
               
                   
                   
                 the Toe, W M1   
                 the Heel, W M2   
                 the Mid, W M3   
               
               
                   
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
                   
               
               
                   
                 3 
                 8.8 
                 7.6 
                 10.1 
               
               
                   
                 6 
                 9.1 
                 8.0 
                 11.4 
               
               
                   
                 9 
                 8.9 
                 8.0 
                 11.1 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5B 
               
             
            
               
                   
               
               
                 Example Dimensions for Sole Width 
               
               
                 Dimensions for a 3-, 6- and 9-Iron Club Heads 
               
            
           
           
               
               
               
               
            
               
                   
                 Sole Width at the 
                 Sole Width at the 
                 Sole Width at the 
               
               
                   
                 Toe, W S1   
                 Heel, W S2   
                 Mid, W S3   
               
               
                 Iron 
                 [mm] 
                 [mm] 
                 [mm] 
               
               
                   
               
               
                 3 
                 16.1 
                 11.6 
                 14.6 
               
               
                 6 
                 16.1 
                 11.5 
                 14.9 
               
               
                 9 
                 16.0 
                 11.5 
                 14.9 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIGS.  12  and  14   , the upper blade portion  210  may have an average thickness or blade width W B  (measured from the rear surface  120  to the striking face  110 ) extending from the heel  116  to the toe  118 . For example, an average thickness or blade width W B  for the upper blade portion  210  may be approximately 6.5 mm. 
     Additionally, the lower muscle portion  220  may be defined as having a muscle surface area of the muscle portion. The muscle surface area may be divided between a heel muscle surface area and a toe muscle surface area. The heel muscle surface area may be defined as the surface area of the muscle portion between the heel-side boundary line L 1  and the face center plane  144  or L 3 . The toe muscle surface area may be defined as the surface area of the muscle portion between the toe-side boundary line L 2  and the face center plane  144  or L 3 . For example, the heel surface area of the muscle portion  220  of a 3-iron or 21-degree loft club head may be approximately 328 mm 2 . While, the toe surface area of the muscle portion  220  of a 3-iron or 21-degree loft club head may be approximately 725 mm 2 . Further, the total surface area of the back of the muscle  220  of a 3-iron or 21-degree loft club head may be between 900 and 1200 mm 2 . Approximately 30% of the total surface area of the muscle  220  may be defined on the heel side of the golf club head and correspondingly 70% of the total surface area of the muscle  220  may be defined on the toe side of the golf club head. In another embodiment, approximately 25% to 35% of the total surface area of the muscle  220  may be defined on the heel side of the golf club head and correspondingly 75% to 65% of the total surface area of the muscle may be defined on the toe side of the golf club head. Table 6 below summarizes the surface area of exemplary 3-, 6-, and 9-iron golf club heads in accordance with aspects of this invention. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Example Dimensions for Surface Area for 3-, 6- and 9-Iron Club Heads 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Total 
                 Heel 
                 Toe 
               
               
                   
                   
                   
                 Surface 
                 Surface 
                 Surface 
               
               
                   
                 Heel Side 
                 Toe Side 
                 Area of 
                 Area-% 
                 Area-% 
               
               
                   
                 Surface 
                 Surface 
                 Muscle 
                 of Total 
                 of Total 
               
               
                   
                 Area 
                 Area 
                 Portion 
                 Surface 
                 Surface 
               
               
                 Iron 
                 [mm 2 ] 
                 [mm 2 ] 
                 [mm 2 ] 
                 Area 
                 Area 
               
               
                   
               
               
                 3 
                 328 
                 725 
                 1053 
                 31% 
                 69% 
               
               
                 6 
                 294 
                 698 
                  992 
                 30% 
                 70% 
               
               
                 9 
                 325 
                 761 
                 1086 
                 30% 
                 70% 
               
               
                   
               
            
           
         
       
     
       FIGS.  17  and  18    illustrate another blade-type iron club head in accordance with this invention. As illustrated in  FIGS.  17  and  18   , a rear surface  120  of the club head  102  may include an indentation positioned within the upper blade portion  210  of the club head  102 . 
     BENEFITS 
     Embodiments of this invention present many benefits to the golf industry and the different participants in the golf industry. 
     First, the mass distribution of the muscle portion moves the center of gravity of the club head closer to the face center location. As was described above, for most conventional blade-type irons, the center of gravity is located to the heel side and sole side of the face center location and approximately 3 mm away from the face center location. For the club heads in accordance with aspects of this invention with the sloped muscle portion, the center of gravity moves towards the toe and top surface of the club head and approximately 0.5 to 1.0 mm closer to the face center location. This is an approximate 17% to 50% change in the location of the center of gravity with respect to the face center, moving the center of gravity 17% to 50% closer to the face center as compared to the conventional blade-type irons. By placing the center of gravity closer to the face center location, the impact efficiency is improved which can improve the ball speed, the vertical and horizontal launch angles, and the corresponding back spin and side spin rates of the golf ball. If this efficiency is improved, the distance and accuracy of the golf shot will be improved. Table 7 below illustrates example dimensions from the ground origin point  132 A in the ground coordinate system for the center of gravity locations and the moment of inertia for exemplary 3-, 6-, and 9-iron club heads in accordance with this invention. Table 8 below illustrates example dimensions from the hosel origin point  132 B in the hosel coordinate system for the center of gravity locations and the moment of inertia for exemplary 3-, 6-, and 9-iron club heads in accordance with this invention. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Example Dimensions from Ground Origin Point 132A in Ground 
               
               
                 Coordinate System for Center of Gravity Locations 
               
               
                 and Moment of Inertia for a 3-, 6-, and 9-Iron Club 
               
               
                 Head (Absolute Values of CG Dimensions Shown) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 CG X 
                 CGY 
                 CGZ 
                 MOI x-x 
                 MOI z-z 
               
               
                   
                 [mm] 
                 [mm] 
                 [mm] 
                 [g*cm 2 ] 
                 [g*cm 2 ] 
               
               
                 Iron 
                 [+/− 2 mm] 
                 [+/− 2 mm] 
                 [+/− 2 mm] 
                 [+/− 400] 
                 [+/− 400] 
               
               
                   
               
               
                 3 
                 28.3 
                 5.2 
                 19.4 
                 2580 
                 2230 
               
               
                 6 
                 29.1 
                 7.4 
                 19.3 
                 2630 
                 2310 
               
               
                 9 
                 29.3 
                 10.3  
                 19.0 
                 2708 
                 2500 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Example Dimensions from Hosel Origin Point 132B in Hosel 
               
               
                 Coordinate System for Center of Gravity Locations 
               
               
                 and Moment of Inertia for a 3-, 6-, and 9-Iron Club 
               
               
                 Head (Absolute Values of CG Dimensions Shown) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Delta X 
                 Delta Y 
                 Delta Z 
                 MOI h-h 
               
               
                   
                 [mm] 
                 [mm] 
                 [mm] 
                 [g*cm 2 ] 
               
               
                 Iron 
                 [+/− 2 mm] 
                 [+/− 2 mm] 
                 [+/− 2 mm] 
                 [+/− 2 mm] 
               
               
                   
               
               
                 3 
                 34.3 
                 5.2 
                 66.5 
                 4420 
               
               
                 6 
                 34.7 
                 7.4 
                 65.6 
                 4950 
               
               
                 9 
                 34.7 
                 10.3  
                 65.1 
                 5600 
               
               
                   
               
            
           
         
       
     
     In addition to improved mass distribution, this muscle configuration provides unexpectedly a change in modal frequencies, resulting in improved feel and sound when impacting a golf ball. It has been found that increasing the amount of mass in the high-toe region of a blade type iron as taught herein increases the overall structural stiffness of the head, resulting in a higher first non-rigid body mode natural frequency when compared to a traditional muscle blade design of the same sized face, hosel, and sole profile. Correspondingly, the reduced muscle height proximate the center of the club head creates a lower second mode natural frequency when compared to a traditional muscle blade design of the same sized face, hosel, and sole profile. Table 9 displays the predicted natural frequencies computed using finite element analysis of the first three non-rigid body modes of two configurations of a 7-iron of the golf club as taught herein compared to its corresponding traditional muscle blade design 7-iron. Both of the first two modes have frequencies within the more sensitive human hearing range of 2000 Hz to 5000 Hz. The improved blade design leads to a unique overall audible response that is perceived by the golfer as an improved feel during impact. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Example Natural Frequencies as calculated by Finite Element 
               
               
                 Analysis of 3-Dimensional CAD files for a 3, 6 and 9 
               
               
                 Blade Iron Club Heads of Present Invention. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Mode 1 
                 Mode 2 
                 Mode 3 
               
               
                   
                 7-Iron 
                 [Hz] 
                 [Hz] 
                 [Hz] 
               
               
                   
                   
               
               
                   
                 Current Design 1 
                 3421 
                 4682 
                 7403 
               
               
                   
                 Prior Art-Traditional 
                 3307 
                 4786 
                 7401 
               
               
                   
                 Design 1 
                   
                   
                   
               
               
                   
                 Current Design 2 
                 3463 
                 4678 
                 7250 
               
               
                   
                 Prior Art-Traditional 
                 3449 
                 4871 
                 7445 
               
               
                   
                 Design 2 
               
               
                   
                   
               
            
           
         
       
     
     CONCLUSION 
     While the invention has been described in detail in terms of specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and methods. Thus, the spirit and scope of the invention should be construed broadly as set forth in the appended claims.