For the most part, and as will be apparent when referring to the figures, when an item is used unchanged in more than one figure, it is identified by the same alphanumeric reference indicator in all figures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The present invention is an exercise device and method that strengthens specific muscles of the abdominal cavity or trunk. Referring to the drawings, FIG. 1a and FIG. 1b show a side view of the device in the starting and final positions respectively. A trunk exercise device 10 is shown with a back pad 12 that is movably mounted to the frame 14 by way of an arm 16. This “movable mounting” can include a sliding linear guide, or rollers that guide the pad through a track of any shape. What is shown is the preferred embodiment as a pivot mount 18. This is done by mounting the pad 12 to one end of the arm 16, with the other end of the arm 16 being pivotally mounted to the frame 14. The back pad 12 rests against the lower back of the user, preferably adjacent to the lumbar vertebrae 20 of the user. FIG. 1a shows the natural “S” shape of these vertebrae and the relative position of the pelvis 22. The seat 24 is used to aid in positioning the user in reference to the back pad 12. Resistance handles 26, attached to the frame 14, are provided to allow the user to balance the force applied to the pad 12. The force applied to the pad 12 is optimally done without significantly moving the user's lower body relative to the seat 24.
 The movement of the exercise is to rotate the pelvis 22 by contracting specific trunk muscles (as previously outlined). The lumbar vertebrae 20 are translated in a posterior direction. This is illustrated in FIG. 1b in that the curve of the lumbar spine 20 is greatly reduced from that shown in FIG. 1a. Further illustration is in the posterior rotation of the pelvis 22. The angle of the pelvis is determined here from the femur (upper leg) to the top of the iliac crest. The starting angle (a1) is 5° less than that shown in the final position (a2). As an example, the maximum range of motion of the 5th lumbar and sacral joint in flexion plus extension is reported to be 17° (White, et al, The basic kinematics of the human spine. Spine 3:12-20, 1978). The pelvis 22 is the major load bearing structure of the human body. It is the support for the spinal column on top and the lower limbs under. The center of gravity of the body, when standing, is approximately in the center of the pelvis 22. Range of motion is sacrificed for stability, therefore what might appear a minor movement in comparison to a wrist or shoulder joint, is in fact quite significant. The muscles involved in this rotation and translation are also involved in the support of the body, and are therefore critical to lower back health.
 For a resistance training device to be effective it must have a method of applying resistance. In this case a bias or spring 28 is attached between the arm 16, which is an extension of the pad 12, and the frame 14. The dimension X1 in the starting position and X2 in the final position depicts the lengths of the spring 28. Moving the pad 12 in a posterior direction, as depicted by the arrow 30, stretches the spring 28, increasing the dimension from X1 to X2. This work done strengthens the muscles of the user.
 A slightly modified version of the invention is shown in FIG. 2a and FIG. 2b. An anterior trunk flexion handle bar 32 has been added to the device 10. The handle bar 32 includes a substantially longitudinal extension 34. Here this extension 34 is made in two parts, a male extension 36 and a female extension 38. The male extension 36 is received by the female extension 38 to provide a means for adjustment of the handle bar 32 relative to the user in the plane of movement of the pad 12. In addition, the materials of construction of the extension 34 can be chosen to allow displacement of the handle bar 32 in the plane perpendicular to the movement of the pad 12. This can be done by using round tube for the female extension 38 and a mating round structure for the male extension 36. The handle bar 32 can then be rotated about a long axis of the female extension 38 moving the distal end of the handle bar 32 to the side relative to the user. This lateral placement enables a torsional load on the abdominal muscles during an abdominal flexing movement (as shown in FIG. 2b). This activates the internal and external obliques without requiring a twisting movement of the spine. A twisting movement under load can result in elevated propensity to disk injury (Lu, Y. M., et al., 1996, Spine, 21(22):2570-2579).
 Movement of the handle bar 32 can be accomplished in a number of ways. For manufacturing elegance and cost effectiveness, the handle bar 32 and the pad 12 are both pivotally mounted to the frame 14 at the same pivot 18. This can allow the handle bar 32 and the pad 12 to move together, as shown here, or each could move independent of each other, though sharing the same pivot shaft. If the latter were the case, as the handle bar 32 was moved in an anterior direction during trunk flexion, as shown in the movement from FIG. 2a to FIG. 2b, the pad 12 would remain stationary with respect to the frame 14. In some cases this might be desirable, but to reduce manufacturing costs, another effective version is to make the handle bar 32 and pad 12 move as one unit, as shown here. To perform the abdominal flexion movement the user moves their body forward toward the handle bar 32. This allows room for the pad 12 to rotate toward the user without restricting the range of motion of the handle bar 32 during the flexion movement.
 Using a continuous structure to attach the handle bar 32 and the pad 12 presents an interesting situation regarding the bias members or springs. Since the direction of movement is in opposite directions with the lower back against the pad 12 as opposed to the hands on the handle bar 32, the springs must be made to apply resistance in opposite directions without obstructing movement when not being tensioned. To overcome this issue, a pair of slotted springs 40 was used. The slot 42 allows movement of the arm pin 41 on the arm 16 or the handle bar pin 43 on the handle bar 32 to toward the frame pin 44 without interference, yet provides resistance by providing tension when either part is moved away from the frame pin 44.
 Another alternative is shown in FIG. 3. Here an exploded view of the arm 16, pad 12 and handle bar 32 as it would be mounted to the frame 14. In this embodiment the female extension of the handle bar 32 and the pivot of the arm 16 use the same pivot mount 18 on the frame 14. This is not necessary to the novelty of the invention but as compared to two pivot mounts, this design is more elegant and would in most cases be less expensive to manufacture. The arm 16 includes a pair of pivot tubes 46 that are spaced apart. The female extension 38 has a center pivot tube 48 that nests between the pivot tubes 46, creating a collinear arrangement. This assembly is illustrated by the centerline 50. The shaft 52 connects these tubes (46 and 48) and is mounted to the pivot mount 18 on the frame 14. The mounting can be any form including a threaded fastener, sheer pin or any other fastening means known in the art.
 The distal end of the handle bar 32 includes a handle that may take a variety of forms. A handle ball 52 is a preferable embodiment of the handle, the ball 52 being positioned on the distal end of the handle bar 32 and preferably able to rotate about a central axis of the ball 52 or sphere. A washer 54 can be mounted on the male extension 36 to support the ball 52, yet allowing for rotation of the ball 52. The rotation of the ball 52 adds another instability to the user interaction that necessitates trunk muscle coordination and muscular balance. An alternative handle is a T-handle 56. This is functionally similar to the ball 52 in that both are rotatably mounted on the end of the handle bar 52. With the T-handle 56, the handle tube 58 is assembled onto the end of the handle bar 32. The user grasps the cross bar 60 to apply force to move the handle bar 32. Again, the instability caused by the freedom to rotate adds an additional conditioning stress to the trunk muscles.
 The handle is adjustable to the user by way of the male extension 36 in its relative placement in the female extension 38. The male extension 36 includes a spring pin 59 in the lower end. This pin has a protruding shaft that is received by a hole 61 in the female extension 38. Here it can be seen that there are more than one row of holes 61. The set of holes 61 along the longitudinal axis of the extension 38 provides reach adjustment. This gives the user the capability to adjust the handle closer to or further away from the user. The multiple rows (two are shown, but there are preferably three or more rows) allow the male extension 36 to be rotated within the female extension 38 with the pin 59 being able to lock in a hole 61. This rotation of the male extension 36 provides a lateral displacement of the handle to the side of the user. Since 10 the axis of rotation of the handle bar 32 has not changed (still pivots on the frame at the same place) the movement of the user is still true flexion, with no trunk rotation. The side placement of the handle provides additional stress to the muscles of the trunk that would typically produce trunk rotation without actually causing trunk rotation during the exercise. This is extremely desirable, especially to those recovering from a lower back injury.
 The device as assembled and without independent movement of the arm 16 and female extension 38 is shown in FIG. 4a. The rear view is shown in FIG. 4b and is included to show parts and perspective of the device in this embodiment regarding width. As such, the specific features of the device will be noted in both views as is appropriate. It is preferable to have two resistance handles 26, where the user can be positioned between the handles 26. The shape of the pad 12 can take any number of forms without sacrificing the function of the device 10. A convex profile in the side view (FIG. 4a) may be desirable in that this provides a more comfortable fit into the lumbar curve of the lower back. The seat 24 is wide enough to accommodate the user with the female extension 38 of the handle bar 32 extending between the legs of the user. This allows the handle to be positioned in front of the user when seated on the device 10. The location of the pivot mount 18 can be below the seat 26, as shown in this disclosure, or above the seat 24. Because of the movement of the pad 12, providing a pivot above the seat 24 has been shown to be desirable. Do to a more complex frame design in order to support a higher pivot, the pivot mount 18 has been determined to be low on the frame 14. The applicant asserts that both versions are disclosed in this application.
 One version of an assembled ball 52 on the distal end of the male extension 36 of the handle bar 32 is shown in FIG. 5. The spring pin 59 is shown near the end of the male extension 36 opposite to the ball 52. As previously disclosed, the handle may not only take the form of a ball 52 as shown here, but a “T-handle” or any other device suitable for grasping by the user.
 Two versions of a slotted spring 40 are shown in FIG. 6a and FIG. 6b. A composite or molded spring is shown in FIG. 6a. A coil spring is shown in FIG. 6b. Each spring includes a slotted portion 42 and a pin mount 62. Either the arm pin 41 or the handle bar pin 43, as previously shown, receives the pin mount 62. The slot 42 is received by the frame pin 44, thereby allowing tension to be applied by movement of either the arm pin 41 or handle bar pin 43 away from the frame pin 44. Suitable materials of construction are synthetic rubber, natural rubber, spring steel and carbon fiber, though this is not intended to be limiting. The rubber materials would be better suited for the design of FIG. 6a and the steel and carbon fiber would be better suited for the design as shown in FIG. 6b.
 Another variation of the device is shown in FIGS. 7a and 7b. This version eliminates the seat, whereby the user stands in the device and performs an abdominal flexion movement by posterior pelvic rotation and lumbar translation. As previously disclosed, the back pad 12 is seated adjacent to the lumbar vertebrae of the user. The arm 16 supports the pad 12 and is pivotally mounted to the standing frame 64 at the pivot mount 18. A simple spring 66 is used to oppose the movement of the pad 12 away from the user as depicted by the arrow 68. No slot is needed in the spring 66 in all versions that do not require movement in both directions. In the absence of an anterior trunk flexion handle bar that is in fixed communication with the arm 16, the slotted spring is not needed. Therefore in all versions of the device that do not include an anterior trunk flexion handle bar, the slotted spring is also not necessary. Frame handles 70 are included to offset the reaction force provided by the pad 12 against the user when in use. The movement of the pad, noting the rotation of the pelvis, movement of the vertebrae and stretching of the spring 66 is depicted in the variation between the starting position (FIG. 7a) and the final position (FIG. 7b). This is consistent with that which has been previously disclosed in this application.