Abstract:
An improved ski binding heel unit comprising a lateral heel release mechanism to release the heel of a ski boot from a ski is disclosed. The ski binding heel unit typically includes an independent vertical heel release mechanism, an independent lateral heel release mechanism, and a longitudinal pressure compensator. The improved ski binding heel unit allows a reduction in the height of the heel pad. This reduction in heel pad height is generally achieved by eccentrically positioning two or more of the springs within the heel unit while superpositioning the resultant centroid of the forces of friction between the lower housing and mating heel track structures that house at least one of the eccentrically positioned springs to foster smooth longitudinal displacement of the lower heel housing when the ski flexes and counterflexes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application claims the benefits of priority of commonly assigned U.S. patent application Ser. No. 61/803,922, entitled “SKI BINDING HEEL UNIT” and filed at the United States Patent and Trademark Office on Mar. 21, 2013. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to alpine ski bindings, all-terrain (AT) ski bindings, tech-bindings and, in particular, to multi-directional release alpine, AT and tech ski binding heel units that release in the vertical and lateral directions. 
     BACKGROUND OF THE INVENTION 
     Toe-heel type bindings that provide lateral heel release and which mitigate inadvertent pre-release have been proposed. However, during aggressive edging on hard-pack snow or ice, or especially in the presence of Phantom Foot or Slip-Catch events (for more information about these injury mechanisms, please refer to: Effect of Ski Binding Parameters on Knee Biomechanics; A Three-Dimensional Computational Study,  Med. Sci. Sports Exerc.,  2004; July; 36(7):1218-25; as well as, Kinematics of Anterior Cruciate Ligament Ruptures in World Cup Alpine Skiing: 2 Case Reports of the Slip-Catch Mechanism,  Am J Sports Med,  2013 May; 41(5):1067-73; and Critical Load Cases for Knee Ligaments at Skiing—An Engineering Approach, Skiing Trauma and Safety: Thirteenth Volume ASTM STP 1397, 2000:160-74; incorporated herein by reference), those bindings with their correspondingly tall heel pads increase the effective lever arm over which the abduction force component of a Phantom Foot or Slip-Catch event acts, thereby increasing valgus torque within the knee. Excessive valgus torque within the knee causes excessive strain across the anterior cruciate ligament (ACL), excessive strain across the medial collateral ligament (MCL), excessive compressive loads on the lateral surfaces of the proximal tibial plateau, excessive compressive loads on the lateral surfaces of the menisci, and excessive compressive loads on the lateral condyles of the distal end of the femur. As such, the presence of excessive strain and compressive loading has required proposed bindings to release laterally at the heel in order to limit the strain and compressive loads. Those prior art lateral heel release bindings with their tall heel pads also have the capacity to generate large tensile loading of the ACL during Boot Induced Anterior Drawer (BIAD) events, which pure or nearly-pure BIAD-events, lateral heel release bindings cannot address. The inherently tall heel pads of those lateral heel release bindings are a consequence of the stacking-height of the lateral heel release mechanism and the longitudinal pressure spring(s). The heel pads of those lateral heel release bindings have heights of 30 mm and more when measured from the ski top surface to the upper surface of the heel pad, where the boot typically sits. 
     Tall heel pads extend the distance from the snow surface to the knee, thereby increasing the effective lever arm from the point at which abduction forces enter the medial edge of the ski (under, or near, the projected axis of the tibia) at the ski-snow boundary—to the center of the knee. Increasing the distance over which an abduction force generates valgus torque to the knee—together with the addition of tibia torque—increases strain across the ACL (Valgus Plus Internal Rotation Moments Increase Anterior Cruciate Ligament Strain More Than Either Alone.  Med. Sci. Sports Exerc.,  2011 August; 43(8):1484-91. incorporated herein by reference). Increased strain across the ACL then increases the possibility that the ACL will reach its elastic limit, causing a skier to sustain a Grade-I ACL sprain (semi-mild sprain), Grade-II ACL sprain (significant sprain); or causing a skier to sustain a Grade-III ACL rupture. ACL injuries are the most frequent injury in alpine skiing (see the peer reviewed journal paper on ski-injury epidemiology, Snowboarding Injuries: Trends Over Time and Comparisons with Alpine Skiing Injuries,  Am J Sports Med , Jan. 12, 2012; published online as a preview, incorporated herein by reference) and are severe in terms of the level of debility imparted because they require at least 8-months of aggressive rehabilitation after reconstructive surgery. Recently, it was estimated that approximately 50,000 skiing-ACL injuries occurred during the 2011-12 alpine ski season at an estimated US$20,000 cost per injury, or a total accumulated cost of US$1-billion per year. The estimated US$20,000 cost per injury includes diagnosis, treatment and rehabilitation, but does not include the costs associated with the typical onset of early PTOA (post-traumatic osteoarthritis) or the social cost of worker-compensation during the course of rehabilitation. Alpine skiing ACL injuries are therefore both frequent and severe. As such, an alpine ski binding, an AT ski binding and/or a tech ski binding that is able to mitigate such injuries would be highly desirable. 
     There is thus a need for a ski binding that provides the benefits of lateral heel release and retention while minimizing the forces, torques and bending moments that are transferred to the musculoskeletal structures of the leg (e.g. the knee, the ACL, the MCL, the tibial plateau, the menisci and the condyles of the femur) by the lever arm from the snow surface to the knee. Similarly, the minimization of forces, torques and bending moments acting upon these musculoskeletal structures will result in a reduced number of instances whereupon the lateral heel release of a binding should be actuated. 
     SUMMARY OF THE INVENTION 
     The shortcomings of prior art ski bindings with respect to lateral heel release are at least mitigated by an alpine, AT, or tech ski binding that provides additional protection against knee injuries, including injuries to the ACL, MCL, tibial plateau, menisci, and femoral condyles by reducing the length of the lever arm from the snow-surface to the knee. In accordance with the principles of the present invention, additional mitigation of knee injuries is obtained by providing a laterally-releasing heel unit of a ski binding wherein the heel unit has a lowered heel pad. 
     The lowered heel pad lateral heel release binding will generally reduce peak strain across the ACL and MCL and generally reduce compressive loading on the tibial plateau, the menisci and femoral condyles by reducing valgus torque and valgus bending moments to the knee. Such lowered configuration generally also allows a reduction in cumulative stress to these musculoskeletal structures, and also serves to reduce the frequency in which a lateral heel release binding needs to release the ski boot (or in an equal-magnitude and opposite-direction perspective—release the ski) because both the frequency and the severity of loading is reduced. Consequently, reducing the frequency of lateral heel releases also serves to mitigate second-order-injury effects during release-events. Second-order-injuries include events such as having the released ski hitting the skier (or hitting other skiers) during the time between when the ski releases and when the ski comes to a full stop; or reducing the need for a skier with only one remaining ski to ski to a controlled stop without first striking a tree, lift-tower, other skier, or from impacting the snow surface. Each such release-event and possible subsequent impact could cause an upper-body or head injury that could result in a more severe injury than an injury to the ACL, MCL, tibial plateau, menisci or femoral condyles. 
     In a lateral heel release ski binding, the height of the heel pad is typically guided by the height of the heel cup of the upper heel housing. Accordingly, in order to reduce the height of the heel pad, the upper heel housing also requires lowering. One way in practice to lower the upper heel housing is to lower the lower heel housing to which the upper heel housing is typically indirectly connected. 
     In accordance with the principles of the present invention, lowering the heel pad is generally achieved by reducing the “stacking height” of the mechanical elements inside the lower heel housing. Such reduction in stacking height must take into account significant restrictions such as the width limits of narrow skis, whereon a binding heel unit, including the binding heel unit&#39;s related ski-brake elements, should not significantly overhang the width of a narrow ski. Furthermore, the lowered lower heel housing should translate, longitudinally, within its mating heel track, as smoothly and as rapidly as possible during rapid flex and counter-flex of the ski, even in the presence of the eccentric lateral loads that cause the lower heel housing to partially-rotate within its corresponding heel track. Partial-rotation should not cause longitudinal jamming in ways similar to the jamming of dresser-drawers. For example, during the induction of an ACL-straining abduction force generated under or near the projected axis of the tibia, the abduction force can sometimes enter the ski-binding-boot-leg system aftward of the effective lateral-centroid of the lower heel housing and heel track assembly thereby potentially causing a jamming-effect unless a novel heel track system is deployed to avert such jamming effect. 
     In accordance with the principles of the present invention, the lowering of the heel pad is generally achieved by reducing the “stacking height” of the mechanical elements inside the lower heel housing. Such reduction in stacking height is typically achieved through a reorganisation of the mechanical elements, such as the springs. The reorganisation of the springs will generally result in having the longitudinal pressure spring positioned eccentrically. For instance, one way to reduce the stacking height referred to herein would be to have the lateral heel release spring(s) and the longitudinal pressure spring(s) in a side by side configuration. However, such side by side configuration could potentially lead to a lower heel housing having springs disposed too asymmetrically to one another. Typically, the reorganisation of the mechanical elements that lower the height of the top surface of the heel pad will result in the lower heel housing being secured, slidingly-longitudinally within the heel track through surfaces of the heel track that are not adjacent to the counter-resistive forces of the longitudinal pressure mechanism, thereby inducing too much friction between the lower heel housing and mating heel track, potentially impeding proper movement of the lower heel housing within the mating heel track during rapid flex and counter-flex of the ski. 
     In accordance with the principles of the present invention, the configuration of the interface between the lower heel housing and heel track assembly maximizes the balance of the opposing forces of the eccentrically positioned longitudinal pressure spring against the resultant-refracted force of friction formed at the interfaces between the lower heel housing and heel track—in order to promote smooth longitudinal displacement between the lower heel housing and heel track. 
     Other and further aspects and advantages of the present invention will be understood by review of the illustrative embodiments about to be described and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which: 
         FIG. 1  is a left side view of a lateral heel release alpine ski binding in accordance with the principles of the present invention. 
         FIG. 2  is a left side view of the heel unit of the ski binding of  FIG. 1 . 
         FIG. 3  is a right side view of the heel unit of  FIG. 2 . 
         FIG. 4  is a cross-sectional top view of the heel unit of  FIG. 2 . 
         FIG. 5  is a detailed cross-sectional top view of the heel unit of  FIG. 2 . 
         FIG. 6  is a cross-sectional left side view of the heel unit of  FIG. 2 . 
         FIG. 7  is a cross-sectional right side view of the heel unit of  FIG. 2 . 
         FIG. 8  is a top rear perspective view of the heel unit of  FIG. 2 . 
         FIG. 9  is a top rear perspective view of another embodiment of an alpine ski binding heel unit in accordance with the principles of the present invention. 
         FIG. 10  is a top rear perspective view of another embodiment of an alpine ski binding heel unit in accordance with the principles of the present invention. 
         FIG. 11  is a top rear perspective view of another embodiment of an alpine ski binding heel unit in accordance with the principles of the present invention. 
         FIG. 12  is a rear view force diagram of the heel unit of  FIG. 2 . 
         FIG. 13  is a rear view diagram of the heel unit of  FIG. 2 . 
         FIG. 14  is a rear view of another embodiment of an alpine ski binding heel unit in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A novel alpine, AT or ‘tech’ ski binding heel unit will be described hereinafter. Although the invention is described in terms of specific illustrative embodiments, it is to be understood that the embodiments described herein are by way of example only and that the scope of the invention is not intended to be limited thereby. 
     Now referring to  FIG. 1 , a ski binding comprising a toe unit  101  and a heel unit  100  is shown. In accordance with the present invention, the top surface of heel pad  13  has been lowered as a result of the lowered support structure  500  that houses the lateral heel release mechanism  340  ( FIG. 2 ) and longitudinal pressure spring  75  ( FIG. 7 ). 
     Now referring to  FIGS. 2 and 3 , the heel unit  100  comprises an upper heel housing  16 , a lower heel housing  27 , a heel pad  13 , and a heel track  330 . The lower heel housing  27  contains at least one lateral release mechanism  340 . The heel pad  13  is connected or fixed to the heel track  330 . The heel track  330  comprises a front portion  330   a  and a rear portion  330   b . In the present embodiment, the heel track front portion  330   a  is preferably anchored to the ski by fasteners (not shown). A rear heel prong guide  331  is typically also anchored to the ski structure (not shown), preferably underneath the rear portion  330   b  of the heel track  330 . As such, the rear heel prong guide  331  provides support to the rear portion  330   b  of the heel track  330  independently of the change in strain that takes place between the fasteners of the heel track&#39;s front portion  330   a  and the fasteners of the rear heel prong guide  331  during bending of the ski—though  330  and  330   b  as well as  330   a  could be attached directly into an under-binding device, an AT-rail, a rail-system, an integral-binding-ski-system or directly to the ski without  331 . 
     In other embodiments, the heel track  330  and/or rear heel prong guide  331  and/or heel track  330  could be moulded into the ski structure. In still other embodiments, the heel track  330  and/or rear heel prong guide  331  and/or heel track  330  could also be fixedly-integrated into the ski. In the latter embodiments, although part of the binding would be integrated within the ski structure, the user would preferably have the ability to remove the integrated part(s) if necessary. 
     Now referring to  FIGS. 4-5 , the lateral release cam  17  is disposed next to mating cam  50  as taught by Bogner et al. in U.S. Pat. No. 5,205,576. Typically, the lateral release mechanism  340  comprises the lateral release cam  17 , the mating cam  50 , at least one lateral heel release spring  35 , at least one tension shaft member  36 , optional washers  40  and  42 , and at least one optional lateral release indicator  53  and an integral opening  44 . In the present embodiment, the heel track  330  comprises a bottom surface  45  ( FIG. 2 ) and slots  31  ( FIG. 3 ). The front portion of bottom surface  45  is typically connected to the top surface of a ski (not shown), to a riser plate (not shown), a lifter (not shown) or to an integral rail-system (not shown). 
     Still referring to  FIG. 4 , the release binding assembly  100  for securing the heel portion of a ski boot to a ski having a top surface, the release binding assembly generally comprises a lower heel assembly comprising a heel track  330  typically attachable to the top surface of the ski, a lower heel housing  27  having two laterally disposed portions and a central longitudinal axis (A-A), a surface having at least a pair of cams  50  disposed on either side of said central longitudinal axis (A-A), wherein the heel track  330  is configured to slidingly engage the lower heel housing  27 , wherein the portions of the lower heel housing  27  typically engaging the heel track  330  are asymmetric relative to said central longitudinal axis (A-A), an upper heel assembly  47  coupled to the lower assembly, and a lateral release assembly for applying lateral securing pressure to the ski boot, the lateral release assembly comprising a surface having at least a second pair of cams  17 , complementary to and facing the first pair of cams  50 , disposed on either side of said central longitudinal axis (A-A). In the present embodiment, the lower heel assembly comprises at least one laterally off-center longitudinal pressure spring  75  relative to said central longitudinal axis (A-A). In addition, the lower heel housing  27  is generally secured to at least one side wall of the heel track  330 . As such, the heel track is generally substantially asymmetric and matches the lower heel housing  27 . 
     The lower heel housing  27  generally slides longitudinally within heel track  330 , which sliding is typically controlled by the compressive force of the longitudinal pressure spring  75  against the lower heel housing  27 . The lower heel housing is generally limited in maximum aftward longitudinal travel by the coil-to-coil compression of spring  75  and also typically limited in forward longitudinal travel by the shoulder  49  of worm gear  29  acting against lower heel housing  27  on wall  43  ( FIG. 7 ). In addition, lower heel housing  27  is generally attached to and positioned on heel track  330  by the selective engagement of teeth  30  of worm gear  29  into slots  31  ( FIG. 3 ) of the heel track  330 . Turning worm gear  29  changes the position of the lower heel housing  27  in relation to heel track  330 . The ratchet plate  51  serves to semi-fix the rotation of worm gear  29  during incremental rotation of worm gear  29 . 
     Now referring to  FIGS. 6 and 7 , a spring biased force is supplied between lower heel housing  27  and heel track  330  by the compression of longitudinal pressure spring  75  on one end of the spring at worm gear  29 , which worm gear  29  is selectively positioned and fixed against heel track  330 , while the other end of longitudinal pressure spring  75  is compressed by the recessed cavity  32  within lower heel housing  27  when the ski flexes. The longitudinal pressure spring  75  provides longitudinal pressure to the ski boot that is held between the upper heel housing  47  and the alpine binding toe piece  101  to ensure proper fixation of the boot during flex and counter-flex of the ski. When the ski flexes in the presence of an incompressible ski boot sole, the change in strain along the top surface of the ski located between the binding&#39;s heel cup and toe cup becomes relieved by the longitudinal translation of the lower heel housing  27  relative to the heel track  330 . The heel unit, through the spring-bias action of the longitudinal pressure mechanism, supplies a longitudinal contact force through the incompressible ski boot sole to the toe piece  101  of the binding when the ski flexes. This longitudinal force is proportional to the spring constant of the at least one longitudinal spring  75  and the amount of its compressive displacement. Surface  34  or surface  98  of the heel cup  47  maintain the longitudinal position of the heel unit  100  on the heel projection of the ski boot sole or, respectively, on the upper of the boot (not shown) as a consequence of the longitudinal pressure exerted by the lower heel housing  27 . Typically, a shoulder element  49  of worm gear  29  captures worm gear  29  within the aftmost end of the longitudinal pressure cavity of lower heel housing  27 . As the longitudinal pressure typically increases during ski flex, the longitudinal pressure spring biasing means, comprising the elements  75 ,  32 ,  29 ,  30 ,  31  and  51  located within the lower heel housing  27 , adapt accordingly. 
     Prior to the boot being placed between toe piece  101  ( FIG. 1 ) and heel unit  100 , the lower heel housing  27  is selectively positioned relative to the heel track  330  by turning worm gear  29  such that the distance between toe piece  101  and heel unit  100  is typically a few millimeters less than the boot sole length. Such shorter distance than the boot sole length is chosen so that during counter-flex of the ski, whereby the change in strain along the top surface of the ski where the toe piece and heel unit are attached, the relative longitudinal position of the toe piece  101  and the heel unit  100  is expanded and thus the lower heel housing  27  displaces forward typically a few millimeters on heel track  330  to maintain contact with the non-expanding boot sole. 
     The increased compression then decreased compression of the longitudinal pressure mechanism of heel unit  100  can take place among typical ski-boot-binding systems. Therefore, in order for the toe piece and heel piece of the binding to provide and maintain proper contact of surface  33  with the boot sole during the dynamic rhythm of fore-and-aft longitudinal displacement action of the lower heel housing  27  within the heel track  330 , the compressive force supplied by the longitudinal pressure mechanism must not be significantly impeded, dissipated or absorbed by the force of friction arising between the mating surfaces of lower heel housing  27  and heel track  330 . Such force of friction generally acts cyclically-opposite in direction to the cyclical force supplied by the longitudinal pressure mechanism. Further, the adverse effects of friction have the potential of becoming amplified by the leverage supplied by any distance over which the force of friction is eccentric to the counter-resistive force supplied by the longitudinal pressure mechanism, unless the leverage-effect is minimized. 
     The outermost surfaces of upper surface  28  ( FIG. 4 ) comprise the lateral-most surfaces  28   a  and  28   b  of the top sides of lower heel housing  27  together with surfaces  80 ,  81 ,  82  ( FIG. 10 ) and  83  ( FIG. 12 ) are uniquely configured to integrate with the cross-sectional orientation of heel track  330 , generally to refract the respective forces of friction that would otherwise become adverse to the compressive force supplied by longitudinal pressure spring  75 . 
     Alternatively, heel track  330  could be partially-connected, or fully-connected (not shown) across the top of lower heel housing  27 , touching on surfaces  28   b  or  28   a  and/or touching on top surface  28 . 
     Referring again to  FIGS. 6-12 , another non-centered spring may be contained within lower heel housing  27 . The other non-centered spring may be embodied as a lateral heel release spring  35  and respectively be contained within a non-centered opening  71  within lower heel housing  27 . As such, the combination of the non-centered lateral heel release spring and the non-centered forward pressure mechanism allow the height of the lower heel housing  27  and of the corresponding heel track  330  to be reduced to allow the upper heel housing to become respectively lowered, thereby lowering heel cup surface  33  so that the top surface  15 , together with optional element  14 , of heel pad  13  can become lowered relative to the prior art to mitigate valgus torque through the knee. These means and functions are new and useful over prior art lateral heel release bindings. For clarity purposes,  FIG. 12  depicts one example of a force vector centroid and an example of a superpositioned centroid. 
     Still referring to  FIGS. 8-12 , the surfaces that are preferably off-center, eccentric to the longitudinal pressure mechanism, and which form the interface between the lower heel housing  27  and heel track  330  are resistive “force-of-friction surfaces” that generate corresponding instant forces of friction that act to oppose the force of the longitudinal pressure spring. Such instant forces of friction at each surface that are eccentric to the longitudinal pressure mechanism can have the perpendicular bisectors of their force-vectors  91  and  92  refracted within the lower heel housing  27  in order to place the virtual loci of their vectors—their superpositioned resultant  93 —closer to and less eccentric from the counter-force  90  supplied by the longitudinal pressure spring in order to mitigate the amplification of the leverage  94  that acts between the opposing forces—the force of the longitudinal pressure spring and resultant force of friction. Thus, at least two “force of friction surfaces” at the interface between lower heel housing  27  and heel track  330  supply a “refracted resultant vector force” with a virtual centroid  93  that acts closer to the opposing force  90  supplied by the longitudinal pressure mechanism to mitigate the leverage  94  between the opposing forces in order to maximize the force supplied by the longitudinal pressure mechanism to maintain proper contact of the toe piece  101  and the heel unit  100  on the boot sole. 
     Such refractive effects comprise generally positioned geometry of the interfacing surfaces of the lower heel housing  27  and heel track  330  to take into account the need to superposition the virtual vector centroid of the overturning moment generated by the ‘force of friction surfaces’ in order to minimize the jamming effect of the overturning moment and to thus maximize the force supplied by the longitudinal pressure mechanism to maintain contact of the toe piece and heel unit with the boot sole during flex and counter-flex of the ski. 
     Preferably, the novel geometry of the opposing surfaces that refract a virtual vector centroid,  93 , of the overturning moment, comprise a series of flat surfaces  80 ,  81 ,  82 ,  83 , that can be non-co-planar to each other; alternatively, a curved surface (not shown); or a series of curved surfaces (not shown); a series of flat and curved surfaces (not shown) or any combination thereof, any of which surfaces generating a confluent locus-point that shifts the virtual centroid of the resultant vector closer to the longitudinal pressure spring in order to minimize the effective lever-arm of the overturning moment that arises between the opposing forces of friction and the longitudinal pressure spring. 
     Now again referring to  FIGS. 8-12 , in the embodiment wherein the longitudinal pressure spring (or longitudinal springs) is eccentrically positioned, the lateral heel release alpine, AT, or tech, ski binding heel unit  100  provides a ski binding having a lower heel pad  13  ( FIG. 7 ). This new lateral heel release binding with its lower heel housing  27  and heel track  330  comprise unique surfaces that refract forces of friction and transpose vectors in a way that allows the use of a longitudinal pressure spring  75  that supplies minimally-friction-dissipated, longitudinal pressure forces to the boot that act between the toe piece and heel unit during flex and counter-flex of the ski. 
     Now referring to  FIG. 13 , the center  103  of the worm gear  29  and its mating longitudinal pressure spring  75  is off-center of the vertical-center axis  101  of the lower heel housing  27 . Likewise, the center  104  of the lateral heel release spring  35  is off-center of the vertical-center axis  101  of the lower heel housing  27 . 
     In another embodiment, now referring to  FIG. 14 , the center  103  of the worm gear  29  and its mating longitudinal pressure spring  75  is off-center of the vertical-center axis  100  of the lateral heel release spring  35 . Such disposition of the springs also allows the stacking height to become favorably reduced. Preferably, the off-center orientation should provide the possibility of refracting the forces of friction in a way that allows their superpositioned centroid to off-set the eccentric effects of friction stemming from the off-center worm gear and its mating longitudinal pressure spring  75 , so that smooth longitudinal translation is maximized between the heel track and the lower heel housing. 
     In still another embodiment, the two springs, the longitudinal pressure spring  75  and the lateral heel release spring  35 , could be configured to be located concentrically, one inside the other. Such configuration would also enable the heel pad  13  top surface  15 , and/or its optional element  14 , to be lowered while still providing longitudinal pressure compensation, smooth longitudinal movement between a lower heel housing  27  and a heel track  330 , and a lateral heel release mechanism  340 . 
     The heel pad  13  can be made of plastic or metal. The top surface  15  of heel pad  13  can be of low coefficient of friction plastic, PTFE, a PTFE-phosphorus-nickel compound (such as NIFLOR) plated into top-surface  15 . The optional means  14  can be made of rubber or a rubber-like material (such as low flex-modules Nylon-12). The heel track  330  can be made of stamped metal (stainless steel, cold rolled steel, aluminum or the like), and be optionally plated or be of plastic (such as Nylon-12). The lower heel housing  27  and cam  17  can be made of plastic such as, for example, injection molded, non-hydroscopic, Nylon-12, or the like. 
     While the illustrative and presently preferred embodiment of the invention is described in detail hereinabove, it is to be understood that the inventive means and function may be otherwise variously embodied and employed and that the claims are intended to be construed to include such variations except insofar as limited by the prior art.