Patent Publication Number: US-9895568-B2

Title: Belt-based system for strengthening muscles

Description:
BACKGROUND 
     Field of the Invention 
     The present invention relates generally to exercise systems and, more specifically, to systems for strengthening, e.g., the abdominal muscles and related muscle groups. 
     Discussion of Related Art 
     Exercises designed to strengthen muscles such as the transversus abdominis muscle and related muscle groups have long played an important role in workout routines intended to improve fitness and health. The abdominal muscles come into play in almost every functional movement that involves the body&#39;s “core” components. Also, exercising these muscles can flatten the stomach and minimize the paunchy appearance of abdominal muscular sag or fat deposits, even in otherwise slender, fit individuals. 
     According to some researchers and fitness experts, three of the best exercises for engaging, e.g., the transversus abdominis and toning the core include: ab rollouts (illustrated in  FIGS. 4A and 4B  and described below), reverse ab rollouts (illustrated in  FIGS. 5A and 5B  and described below), and ab planks (illustrated in  FIG. 19  and described below). The first of these exercises, the “ab rollout”, has been the basis of several fitness products in the past: the “Ab Wheel,” the “Ab Slide,” and the “Torso Track.” 
     The “Ab Wheel,” shown in  FIG. 16 , consists of a wheel with two side handles. To use an Ab Wheel, one assumes a kneeling or standing position, grasps the handles, and rolls forward across the floor, then back. The basic principle is this: because gravity tends quickly to propel us forward, one is forced to engage one&#39;s transversus abdominis to slow the forward rate of motion and maintain balance. However, using an Ab Wheel properly requires a relatively high degree of initial abdominal conditioning. Without the proper experience and conditioning, exercising with the Ab Wheel can cause hyperextension. 
     The Ab Slide (shown in  FIG. 17 ) and the Torso Track (shown in  FIG. 18 ) were designed to slow the rate of forward motion, thereby making the exercise easier and less dangerous to perform. However, neither of these products is fully adjustable, allowing the user to freely vary the level of forward resistance. The Ab Slide, implemented using a torsion spring, was designed with a one-size fits all approach and is not at all adjustable. The Torso Track, implemented using rubber bands, had only two or three difficulty settings, and switching among them required the user to manually adjust rubber bands. 
     In addition, both the Ab Slide and the Torso Track allowed only a limited range of motion, and could be used to perform a limited number of exercises. The Ab Slide could only slide a short distance before the torsion spring wound up completely, preventing further movement. The Torso Track could only move as far as its rubber bands could stretch. Also, both the Ab Slide and the Torso Track were designed primarily for an ab rollout-type motion, ignoring reverse ab rollouts and planks, two of the three most important abdominal exercises referred to above. 
     SUMMARY OF THE INVENTION 
     Herein are described systems for performing a variety of abdominal exercises, the systems including a rigid framework, a first belt configured to roll relative to the framework in both clockwise and counter-clockwise directions, a second belt configured to roll relative to the framework in both clockwise and counter-clockwise directions, and a resistance-control mechanism for controlling the amount of force required to roll the first and second belts in one or more of the clockwise and counter-clockwise directions. 
     Herein are further described systems including a rigid framework, a single belt configured to roll relative to the framework in both clockwise and counter-clockwise directions, and a resistance-control mechanism for controlling the amount of force required to roll the belt in one or more of the clockwise and counter-clockwise directions. 
     Embodiments of the present invention may employ one-way resistance-control mechanisms that control the resistance in only the clockwise or the counter-clockwise direction, but not both. Embodiments of the present invention may include belts that run along substantially parallel paths. Embodiments may include detachable or integrated risers that are used to create an incline. Embodiments may include belts that are constructed of modular segments linked by hinged interconnects, or a one-piece fixed-length belt which approximates the look and feel of a modular belt. 
     Embodiments of the present invention may employ one or more rigid or semi-rigid belts that include bottom-facing rollers or sprockets designed to reduce friction. Embodiments of the present invention may employ a framework that includes upward-facing rollers or sprockets designed to reduce friction. Embodiments of the present invention may include belts that rest on glide-strips constructed using a low-friction material such as, e.g., polyethylene. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of various embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: 
         FIG. 1  shows a system with two continuous belts, left and right handles, resistance control knob, and side/center trim, according to some embodiments. 
         FIG. 2  is a schematic showing a cross-section of a belt looped around front and back axles, forming a continuous loop, according to some embodiments. 
         FIG. 3  is a schematic showing a cross-section of a belt looped around front and back axles supported by a slider deck, according to some embodiments. 
         FIGS. 4A and 4B  show the movements involved in an “ab rollout” exercise, according to some embodiments. 
         FIGS. 5A and 5B  show the movements involved in a “reverse ab rollout” exercise, according to some embodiments. 
         FIG. 6  is a schematic showing a cross-section of the resistance-adjustment mechanism, according to some embodiments. 
         FIG. 7  shows a one-way bearing enclosure with cavities that house rollers and springs, according to some embodiments. 
         FIG. 8A  shows a one-way bearing mechanism in a “free” position, in which the bearing enclosure may rotate independently of the inner axle, according to some embodiments. 
         FIG. 8B  shows a one-way bearing mechanism in a “locked” position, in which the bearing enclosure may not rotate independently of the inner axle, according to some embodiments. 
         FIG. 9  shows a belt segment constructed using multiple modular belt elements, according to some embodiments. 
         FIG. 10  shows a belt segment with an easy-to-grip strip running down its length, according to some embodiments. 
         FIG. 11  shows a belt segment in which alternate belting elements are partially coated with an easy-to-grip surface, according to some embodiments. 
         FIG. 12  shows the trajectories of two curved belts, according to some embodiments. 
         FIG. 13  shows a curved belt segment constructed from modular belt elements that are joined elastically, according to some embodiments. 
         FIG. 14  shows a non-continuous belt segment with stopping elements, according to some embodiments. 
         FIG. 15A  shows a machine with an integrated, deployable riser under the front end of the machine, according to some embodiments. 
         FIG. 15B  shows a machine with an integrated, deployable riser under the back end of the machine, according to some embodiments. 
         FIG. 16  shows the Ab Wheel, an existing abdominal exercise device. 
         FIG. 17  shows the Ab Slide, an existing abdominal exercise device. 
         FIG. 18  shows the Torso Track, an existing abdominal exercise device. 
         FIG. 19  shows an “ab plank” exercise, according to some embodiments. 
         FIG. 20  shows a belt that includes rollers, according to some embodiments. 
         FIG. 21  shows a slider deck including rollers, according to some embodiments. 
         FIG. 22  shows a slider deck including rollers, according to some embodiments. 
         FIG. 23  shows a slider deck including rollers, according to some embodiments. 
         FIG. 24  shows a roller track, according to some embodiments. 
         FIG. 25  shows a track-ball module, according to some embodiments. 
         FIG. 26  shows a slider deck including multi-directional rollers, according to some embodiments. 
         FIG. 27  shows glide-strips, according to some embodiments. 
         FIG. 28  shows parallel glide-strips, according to some embodiments. 
         FIG. 29  shows a system with a unitary plastic belt, according to some embodiments. 
         FIG. 30  shows a standalone, gradated riser, according to some embodiments. 
         FIG. 31  shows a standalone, gradated riser, according to some embodiments. 
         FIG. 32  shows a belt that rolls along three parallel rails, according to some embodiments. 
         FIG. 33  shows a pin/axle component, according to some embodiments. 
         FIG. 34  shows a belt segment with cylindrical holes to accommodate pins and/or axles, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one embodiment of the present invention. Continuous belts  101 ,  102  are looped around axles that allow them to roll in a direction parallel to the long axis of framework  103 . In some embodiments, belts  101 ,  102  may roll bi-directionally, in both “forward” and “backward” directions, depending on the direction of the force that is applied to the top surface of the belt. Applying a force pointing toward front end  104  causes a belt to roll in the “forward” direction, such that points on the top surface of the belt move toward the front end  104  of the framework; applying a force pointing toward back end  105  causes the belt to roll in the “backward” direction, such that points on the top surface of the belt move toward the back end  105  of the framework. 
     In some embodiments, each belt forms a continuous loop around front and back axles, located respectively at the front  104  and back  105  ends of framework  103 .  FIG. 2  shows a horizontal cross section of this loop for a single belt. Belt  201  is looped around front axle  202  and back axle  203 , so it may roll continuously in both the forward (counterclockwise) and backward (clockwise) directions. Sprockets (not shown) may be placed around the axles to prevent the belt from slipping relative to the front and back axles. Further, referring again to  FIG. 1 , the axles around which belt  101  is looped may be decoupled from the axles around which belt  102  is looped. Thus, in some embodiments, the two belts may roll independently of each other; at any particular time, the two belts may be rolling at different rates, or in different directions. 
     In some embodiments, the force required to roll belts  101 ,  102  relative to framework  103  may be controlled by resistance control knob  106 . For example, turning resistance control knob  106  in a clockwise direction may increase the force required to roll the belts, while turning the knob in a counterclockwise direction may decrease the force required to roll the belts. In some embodiments, resistance control knob  106  controls the force required to roll the belts in the forward direction only, while the belts may be rolled in the backward direction by applying a minimal amount of force. This may be accomplished using mechanisms such as one-way bearings, as described in more detail below. 
     In the embodiment shown in  FIG. 1 , the majority of belts  101 ,  102  are hidden from view at any given time; only the top surfaces of the belts are visible. At both the front  104  and back  105  ends, framework  103  includes “trim” that conceals the axles around which the belts are looped, so the visible portions of the belts form substantially planar surfaces. Trim is also included on the left  107  and right  108  sides of the framework, concealing the outer rims of belts  101 ,  102 , and along a strip  109  in the middle of the framework, concealing the inner rims of the belts. The trim on the four sides and along the center of the framework prevents damage to the machine and minimizes the risk of injury to the user by shielding and protecting the axles, the resistance control mechanism, and other moving parts. Handles  110  are fixed to the left  107  and right  108  sides of the framework. 
     Embodiments like the system pictured in  FIG. 1  may be designed to allow multiple such systems to be stacked vertically for storage. In such embodiments, each machine in a stack may interlock with the machines immediately above and beneath it, preventing it from moving horizontally relative to its neighbors. Embodiments may be designed to allow vertical stacks in which every machine in the stack has the same orientation (i.e., every machine in the stack is face-up, with the front and back ends pointing in the same direction), or stacks in which adjacent machines are oriented differently (e.g., stacks in which every other machine is rotated 180° around a vertical axis so the bottom surface of the front end of one machine is stacked atop the top surface of the back end of another; stacks in which every other machine is flipped upside-down so the top surface faces downward and touches the top surface of the machine immediately beneath it, and the bottom surface faces upward and touches the bottom surface of the machine immediately above it). To allow for the most efficient use of space, stackable embodiments may be designed to minimize the vertical distance between adjacent machines in a stack, thereby minimizing the height of the stack as a whole. 
     Framework  103  may be constructed from a variety of materials, including wood, plastic, rubber, and metal (or some combination of the four). As shown in  FIG. 3 , in some embodiments, a “slider deck”  301  may be located immediately beneath the top portion of a belt  302 . The slider deck prevents a belt from buckling or deforming when downward pressure  303  is applied to the top surface of the belt. When a belt is rolled forward or backward, it slides along the slider deck. Friction between the moving belt and the slider deck may be minimized by applying a lubricant (e.g., silicon or generic Teflon) to the top surface of the slider deck and/or to the inner surface of the belt. In addition, the inner surface of the belt and/or the upper surface of the slider deck may be constructed using materials designed to minimize friction (e.g., plastic, silicon, or generic Teflon slider strips). 
     The embodiment shown in  FIG. 1  may be used to perform a variety of exercises, such as the “ab rollout” illustrated in  FIGS. 4A and 4B , and the “reverse ab rollout” illustrated in  FIGS. 5A and 5B . An “ab rollout” is a three-step exercise: (1) a user begins in position  401  ( FIG. 4A ); (2) the user moves his/her hands forward to reach position  402  ( FIG. 4B ); and (3) the user moves his/her hands backward, returning to position  401 . The forward/backward movement of the hands may be facilitated by belts  101 ,  102  (shown in  FIG. 1 ): first, in the starting position  401 , the user&#39;s left hand is placed atop left belt  101 , and the user&#39;s right hand is placed atop right belt  102 ; then, the user&#39;s hands apply a forward force (toward front end  104 ), causing the belts to roll forward, until position  402  is reached; finally, the user&#39;s hands/torso apply a backward force (toward back end  105 ), causing the belts to roll backward, returning to position  401 . The embodiment of  FIG. 1  may also be used to perform a “single-hand ab rollout,” which is similar to an “ab rollout,” except that the user is supported by only one hand throughout the exercise. 
     The embodiment shown in  FIG. 1  may also be used to perform a “reverse ab rollout,” illustrated in  FIGS. 5A and 5B . Like the “ab rollout,” the “reverse ab rollout” is a three step exercise: (1) a user begins in position  501  ( FIG. 5A ); (2) the user moves his/her feet backward to reach position  502  ( FIG. 5B ); and (3) the user moves his/her feet forward, returning to position  501 . The forward/backward movement of the user&#39;s feet may be facilitated by belts  101 ,  102  (shown in  FIG. 1 ): first, in the starting position  501 , the user&#39;s left foot is placed atop left belt  101 , and the user&#39;s right foot is placed atop right belt  102 ; then, the user&#39;s feet apply a backward force (toward back end  105 ), causing the belts to roll backward, until position  502  is reached; finally, the user&#39;s feet apply a forward force (toward front end  104 ), causing the belts to roll forward, returning to position  501  (alternatively, reverse ab rollouts may be performed facing in the other direction, in which moving the feet backward causes the belts to roll forward, and vice versa). The embodiment of  FIG. 1  may also be used to perform a “single-foot reverse ab rollout,” which is similar to a “reverse ab rollout,” except that the user is supported by only one foot throughout the exercise. 
     Additional exercises may be performed with the embodiment shown in  FIG. 1 , including, but not limited to, push-ups, lunges, planks, and manual treadmilling. Push-ups are facilitated by handles  110 , which may be used as push-up handles. Lunges are performed by (1) starting in a push-up position, with head pointing toward front end  104  and feet on belts  101 ,  102 ; and (2) sliding the feet forward, bringing the knees toward the chest, for as long as possible. Planks are performed by (1) kneeling on belts  101 ,  102  and grasping handles  110 ; and (2) lifting the knees off the belts, such that the user&#39;s weight is supported entirely by the hands and toes, while keeping the hips in-line with the body. Manual treadmilling is performed by (1) placing the hands on belts  101 ,  102  in a push-up position; and (2) “walking” either forward or backward, using the left and right hands alternately to roll the left and right belts (manual treadmilling may be performed with the user&#39;s head facing front end  104 , or back end  105 ). The descriptions above do not constitute an exhaustive list of the exercises that may be performed with embodiments of the present invention, but serve as illustrative examples only. 
     As described above, resistance control knob  106  may be used to control the amount of force required to roll belts  101 ,  102 . In some embodiments, this is accomplished using a mechanism like the one illustrated in  FIG. 6 , which is a schematic diagram showing a horizontal cross-section of the resistance-control mechanism. Resistance control knob  601  is connected to screw  602 , which passes through axle housing  603 , and presses against the top half of annular friction element  604 . Annular friction element (including top half  604  and bottom half  605 ) is constructed using an elastic material with a high kinetic friction coefficient, such as rubber; thus, when screw  602  is moved downward, resistance axle  606  is compressed between the upper  604  and lower  605  halves of the annular friction element, increasing the inward normal force on the outer surface of resistance axle  606 , which increases the amount of torque that is required to rotate resistance axle  606 . In some embodiments, resistance axle  606  runs parallel to the front axle of a belt, and is connected to the front axle (e.g., front axle  202  of belt  201 , as shown in  FIG. 2 ), so the force required to move the belt in the forward and/or backward direction is proportional to the torque required to rotate resistance axle  606 . Thus, turning resistance control knob  601  clockwise increases the belt&#39;s resistance, and turning resistance control knob  601  counter-clockwise decreases the belt&#39;s resistance. 
     Two independent belts may be adjusted using the same friction tensioner: for example, the front axle of one belt may be connected to a resistance axle that is inserted into one end of annular friction element  604 ,  605 , and the front axle of the other belt may be connected to a resistance axle that is inserted into the other end of annular friction element  604 ,  605 . In this case, both resistance axles may rotate independently, but a single resistance control knob  601  may be used to adjust the torque required to turn both resistance axles. Alternatively, other embodiments may employ separate resistance-control mechanisms for each belt. 
     Various alternative techniques may be used to adjust the force required to roll a belt in one or both directions (e.g., torsion springs, magnetic or hydraulic tensioners, rubber bands, etc.). The resistance-control mechanism described above with reference to  FIG. 6  is provided as an illustrative example only. 
     In some embodiments, the user may adjust the force required to move the belt in the forward direction without changing the force required to move the belt in the backward direction. As explained above, resistance in the forward direction is desirable for “ab rollouts” and similar exercises to counterbalance the tendency of gravity to push the body forward, but movement in the backward direction goes against the force of gravity, so additional resistance is unnecessary. Such embodiments may use a variety of mechanisms to accomplish this unidirectional resistance-control, such as the one-way bearing mechanisms described below. 
     One-way bearings are used in a variety of mechanical devices in order to allow an object to be rotated in one direction, but not the other. As shown in  FIG. 7 , bearing enclosure  701  is lined with grooved cavities, each of which contains a roller  702  and a spring  703 . An inner axle (not shown in  FIG. 7 ) runs through the middle of bearing enclosure  701 . As explained below, the configuration of rollers and springs allows bearing enclosure  701  to rotate independently of the inner axle only in the clockwise direction. To rotate bearing enclosure  701  in the counter-clockwise direction, the inner axle must rotate with it. 
       FIGS. 8A and 8B  show a portion of the one-way bearing mechanism in greater detail. These figures show a cross-section of a single grooved cavity in bearing enclosure  701 . Roller  801  occupies the right side of the cavity, and spring  802  occupies the left side. In  FIG. 8A , the mechanism is in the “free” position: because the roller  801  is not touching inner axle  803 , the bearing enclosure is free to rotate independently of the inner axle. As long as the bearing enclosure rotates in the clockwise direction, roller  801  will remain in the “free” position. 
     However, if bearing enclosure  701  rotates in the counter-clockwise direction, roller  801  will move to the right until it is touching bearing cage  804 , as shown in  FIG. 8B  (in which the mechanism is in the “locked” position). In this position, roller  801  is pushed inward by bearing cage  804  until it is tangent to inner axle  803 , which prevents the roller from rotating. Because the roller is not free to rotate, bearing enclosure  701  is also not free to rotate and becomes locked; rotating the bearing enclosure further in the counter-clockwise direction is only possible if inner axle  803  rotates with it. 
     Bearing enclosure  701 , shown in  FIGS. 7, 8A, and 8B , may be coupled to the front axle of a belt (e.g. front axle  202  of belt  201 , shown in  FIG. 2 ), and inner axle  803 , shown in  FIGS. 8A and 8B , may be coupled to a friction element (e.g., annular friction element  604 ,  605 , shown in  FIG. 6 ). In this configuration, the belt may roll in the backward direction freely, because the bearing enclosure  701  may rotate in the clockwise direction independently of inner axle  803 . However, the belt encounters increased resistance when rolling in the forward direction, because rotating the bearing enclosure in the counter-clockwise direction requires rotating inner axle  803  as well. Embodiments of the present invention may use this mechanism to increase the force required to roll a belt in one direction (e.g., forward), while allowing it to roll freely in the other (e.g., backward). This result may also be achieved with a variety of alternative techniques (e.g., ratchet-based assemblies, etc.); the one-way bearing mechanism described above with reference to  FIGS. 7, 8A, and 8B  is provided as an illustrative example only. 
     Embodiments of the present invention may include one or more belts constructed using a variety of techniques and materials. In some embodiments, belts may be constructed using modular belt elements as illustrated in  FIG. 9 . Modular belt elements  901 ,  902 ,  903  interlock to form belt segments of arbitrary length. These belt segments may be formed into a continuous belt (like, e.g., belt  201  shown in  FIG. 2 ) by attaching the modular belt element at one end of the belt segment to the modular belt element at the other end of the belt segment. Such belts are durable, relatively easy to repair, and modifiable. Repairing belts constructed using modular elements is easy and relatively inexpensive; when such a belt is damaged, it can often be fixed by replacing only the damaged belting elements, and rarely requires replacing the entire belt. Modular belts can also be lengthened or shortened as desired by adding or removing modular belt elements. 
     In some embodiments, each modular belt element is totally or partially coated in a surface designed to prevent a user&#39;s hands and feet from slipping, and to provide a satisfying tactile experience.  FIG. 10 , shows a belt segment in which the middle upper portion of each modular belt element is coated with a rubberized, easy-to-grip surface  1001 . These belting elements are assembled to form a belt with a strip of rubberized material  1002  running down its center. Other embodiments may include belts in which every other belting element is partially coated with a rubberized surface, as shown in  FIG. 11 . Embodiments of the present invention may use a variety of belt materials and configurations (e.g., belts made of rubber, plastic, fabric, some combination of these materials, etc.). Also, embodiments may include a one-piece fixed-length belt which approximates the look and feel of a modular belt. The descriptions of modular belts above, with reference to  FIGS. 9-11 , are provided as illustrative examples only. 
     The embodiment illustrated in  FIG. 1  has two parallel belts, but in other embodiments, one or more belts may be partially curved. For example, as illustrated in  FIG. 12 , a system may include two belts  1201  and  1202  that follow curved trajectories. At the back end  1203  of the device, the two belts are not parallel to each other. As the belts run along the device, they curve symmetrically away from each other, so that when they reach the front end  1204 , they are parallel to each other. While the back axles of these two belts are not collinear, the front axles are, and may be adjusted using a single resistance-control mechanism as described above with reference to  FIG. 6 . 
     To allow a belt to follow a curved trajectory, modular belt elements may be attached to each other elastically. In a curved belt segment, the outside of the curve is longer than the inside of the curve. As illustrated in  FIG. 13 , elastic connections allow increased separation between belting elements near the outside of the curve  1301 , while maintaining close spacing between the belting elements near the inside of the curve  1302 . 
     Some embodiments may include belts that allow users to attach hand-grips or other attachable modules to the surface of the belt. For example, a user may attach hand-grips to belts  101 ,  102 . In such embodiments, exercises that involve placing hands on one or both belts may instead be performed by gripping one or both of the attached hand-grips. Analogous foot-grip modules may be attached to belts  101 ,  102  for performing exercises that involve placing feet on one or both belts. Some embodiments include belts that are designed to roll continuously even when one or more attachment modules are present; in such embodiments, the attachment module rolls with the belt along the underside of the machine until it is once again on the top surface of the belt. 
     In other embodiments, when an attachment module reaches the front or back end of the machine, it prevents the belt from rolling forward or backward, respectively. Such modules can be used to prevent injury, e.g., overextending the arms when performing an “ab rollout.” Before beginning the ab rollout, the user positions the attachment module far enough from the front end of the machine to allow a suitable range of forward motion, but close enough to stop the forward motion of the belt before the user extends their arms too far. Such modules can also be used to limit the belt&#39;s range of backward motion. Similarly, two attachment modules may be attached to the belt—one near the front end of the machine, the other near the back end—to limit the belt&#39;s movement in both the forward and the backward directions. 
     In some embodiments, a belt&#39;s movement may also be limited by using a linear belt segment as illustrated in  FIG. 14 , instead of a belt that forms a continuous loop. In these embodiments, the belt is not designed to roll continuously in the forward or backward directions; instead, it has a finite range of motion. In some embodiments, a belt segment  1401  is terminated by stopping elements  1402  and  1403  that prevent further motion when they reach the front or back ends of the machine, respectively. 
     As described above, some embodiments of the present invention include two independent belts, which may follow a curved or a parallel trajectory, or a combination of the two. However, other embodiments may include only one belt. Because they include only one belt instead of two, single-belt embodiments are faster and less expensive to manufacture than dual-belt embodiments. While single-belt machines do not allow the flexibility and range of motion possible with dual-belt machines, they may still be used for a wide variety of exercises. 
     In some embodiments, a riser may be placed beneath one end of the machine to create an inclined plane, which may be desired to adjust the difficulty of an exercise. If a riser is placed under the front end of the machine, the belts will slope upward as they roll in the forward direction. If a riser is placed under the back end of the machine, the belts slope downward as they roll forward. In some embodiments, the underside of the machine is designed to accommodate a riser under either the front end or the back end. 
     In some embodiments, risers may be gradated, as shown in  FIGS. 30 and 31 , providing multiple, adjustable incline options. This allows users to control the force with which gravity pulls them forward when performing exercises. Placing a riser under the front of the system creates an “uphill” slope that makes ab-rollouts and other exercises less strenuous, while placing a riser under the back of the system creates a “downhill” slope that increases the difficulty of various exercises. 
     Risers may be separate, attachable modules, or may be integrated into the machine itself and deployable as desired. For example, an integrated riser may be implemented as an fold-out panel that is attached to the underside of the machine using a hinge, as illustrated in  FIG. 15A . When deployed, the riser  1501  swings out on hinge  1502  and locks into position, protruding from the underside of the machine, and causing framework  1503 , supporting belt  1506  looped around front axle  1505  and back axle  1504 , to become inclined. When not deployed, the riser  1501  is flush with the underside of the machine. Embodiments may include two integrated risers, one at the front end of the machine and the other at the back, allowing the user to create an upward or a downward incline. Similarly, an integrated riser may be deployed under the back end of the machine, creating a downward incline, as shown in  FIG. 15B . 
     Risers may also be configured to be adjustable resistance-control systems, providing steeper or shallower inclines as desired. For example, fold-out riser  1501  may be deployed at various angles, each associated with a locking mechanism allowing the riser to be fixed in place at a particular angle. Alternatively, some embodiments may be provided with multiple riser attachments, each one providing an incline of a different slope. 
     Some embodiments may use a grooved runner system that works without axles or sprockets. In such embodiments, the side edges of full-length belts or hand- or foot-sized “treadpads” consisting of left and right belt segments would be inserted into two parallel grooves running in either an oval-shaped path (e.g., the path of the belt in  FIG. 2 ) in the case of full-length belts, or in a planar path representing the top running along the top surface of the machine, in the case of treadpads. These belts or treadpads could then be moved in both the forward and the backward direction along the grooves, which in some embodiments could be lined with rollers to facilitate movement of the belts or treadpads. The foregoing embodiments may include a resistance-control mechanism for controlling the force needed to move the belts or treadpads forward or backward. For example, they may employ a friction-based control mechanism similar to the one described above with reference to  FIG. 6 . 
     Some embodiments may include one or more rigid or semi-rigid plastic or rubber belts that include downward-facing rollers that allow the belt to roll over the surface of the slider deck, reducing the friction between the belt and the slider deck. This allows the belt to roll more smoothly in both the forward and backward directions, and further reduces wear. A view of the underside of a belt that includes rollers is shown in  FIG. 20 . The upper side of a belt as shown in  FIG. 20  may feature a flat or flat/contoured surface suitable for hand- or foot-placement. 
     Some embodiments may include rollers or sprockets arranged on the slider deck, under the belt. The rollers may be arranged on the slider deck in horizontal, vertical or staggered groups, as illustrated in  FIGS. 21-23 . Including rollers or sprockets in the slider deck reduces the friction between the belt and the slider deck, allowing the belt to roll more smoothly in both the forward and backward directions, and reducing wear. 
     In some embodiments, the belts may roll along tracks consisting of strips of freely-moving rollers or sprockets, as illustrated in  FIG. 24 . Such strips or tracks can also be installed along inner and outer edges of each slider deck, thereby rolling along the belt edges to reduce friction thereon. 
     In some embodiments, the belts may roll over an array of multi-directional rollers, as shown in  FIG. 25 . In other embodiments, the belts may roll over an array of track-ball modules, as shown in  FIG. 26 . In these embodiments, the multi-dimensional rollers or track-ball modules would support the belt above them and reduce the friction between the belt and the framework. These embodiments include belts having the rigidity necessary to travel smoothly over rollers or track-balls while providing a stable, planar tread surface. 
     In some embodiments, the belts may glide across glide-strips constructed using a low-friction material such as, e.g., polyethylene.  FIG. 27  shows individual glide-strips, according to some embodiments. These glide-strips may be arranged in parallel segments under the belt, as shown in  FIG. 28 , or in other patterns (e.g., serpentine segments, chevrons, etc.). 
     Some embodiments do not include front and back axles. For example, embodiments in which the belt rolls in the forward and/or backward direction over rollers or track-balls, or glides over glide-strips may not require axles, as the belt is able to roll smoothly around the front and back ends of the framework. In these embodiments, the belts do not need to include sprocket-holes for interlocking with sprockets attached to the front and back axles. Sprocket-less plastic belts may allow for a thinner, lighter device with a tighter turning radius and slimmer, lighter slider deck capability. Such belts could the take various forms, including, e.g.: (a) plastic modular belts with pivot pin inserts; (b) pin-less plastic modular belts with snap-together modules that serve as the shafts for the small rollers or wheels; (c) plastic belts of unitary construction with individual module-like segments molded or machined thereon (as shown in  FIG. 29 ); or (d) a semi-rigid plastic compound exhibiting both the flexibility to wrap around anterior and posterior nosebars and the rigidity to allow for hand- or foot-placement thereon without excessive friction or user discomfort. 
     In some embodiments, the belts may roll in the forward and/or backward direction across a set of a parallel rails, as shown in  FIG. 32 . Relative to framework  3201 ,  FIG. 32  shows a longitudinal bisection of framework  3201 &#39;s oval-shaped track, along with a partial segment of belt  3206 . Framework  3201  is shown in a configuration that includes three rails: side rails  3202  and center rail  3203 . Side wheels  3205  and center wheels  3204  rotate or glide along side rails  3202  and center rails  3203 , respectively, which form curvilinear tracks. Center wheels  3204  and rails  3203  provide additional support to the center of the belt while still allowing the belt to roll freely over the rails. In this embodiment, the belts are held together by pins that also serve as axles. Belt  3206  is composed of a plurality of modular belt segments  3207 , communicating along front  3208  and rear curvilinear edges  3209 , where they are connected by pin  3210  inserted through cylindrical hinge member  3206 . Pin  3210  is configurable to allow an addition of side wheels  3205  and/or center wheel  3204  to provide belt  3206  with a rotational interface with side and center rails  3202 ,  3203 , instead of a wheel-free configuration thereof.  FIG. 33  shows a single pin used in this embodiment. The pin includes a shaft  3301  that is inserted into the belt segments to hold them together, and a wheel  3302 , that sits in one of the side rails  3202  and rolls as the user of the device moves the belt backward or forward. Wheels  3302  may be included at one or both ends the of the pin/axle component, and the pin/axle component may also accommodate as many center wheels as necessary to stabilize the belt.  FIG. 34  shows a belt segment  3401  that is compatible with this embodiment. Axles like the one shown in  FIG. 33  fit through cylindrical openings  3402 , simultaneously holding the belt segments together and providing wheels that allow the belt to roll along the three parallel rails  3202  and  3203 . 
     Additional embodiments of the present invention are further described in U.S. patent application Ser. No. 13/835,066, filed on Mar. 15, 2013, which is incorporated herein by reference in its entirety.