Exercise Specificity

The principle of specificity also applies to therapeutic exercise in rehabilitation settings. The exercises prescribed must match the biomechanical needs of the healing patient. Exercises must effectively train the muscles that have been weakened by injury and inactivity. Biomechanical research on therapeutic exercise is even more critical since therapists need to know when inter nal loadings may exceed the mechanical strengths of normal and healing tissues.

Imagine that you are a physical therapist treating a runner with patellofemoral pain syndrome. Patellofemoral pain syndrome (PFPS) is the current terminology for what was commonly called chondroma-lacia patella (Thomee, Agustsson, & Karls-son, 1999). PFPS is likely inflammation of the patellar cartilage since other knee pathologies have been ruled out. It is believed that PFPS may result from misalignment of the knee, weakness in the medial components of the quadriceps, and overuse. If the vastus medialis and especially the vastus medialis obliquus (VMO) fibers are weak, it is hypothesized that the patella may track more laterally on the femur and irritate either the patellar or femoral cartilage. The exercises commonly prescribed to focus activation on the VMO are knee extensions within 30° of near complete extension, similar short-arc leg presses/squats, and isometric quadriceps setting at complete extension, and these exercises with combined hip adduction effort. While increased VMO activation for these exercises is not conclusive (see Earl, Schmitz, and Arnold, 2001), assume you are using this therapeutic strategy when evaluating the exercise technique in Figure 12.2. What bio-mechanical principles are strengths and weaknesses in this exercise.

Most biomechanical principles are well performed. Balance is not much of an issue in a leg press machine because mechanical restraints and the stronger limb can compensate for weakness in the affected limb. There is simultaneous Coordination, and there appears to be slow, smooth movement (Force-Time).

The principle that is the weakest for this subject is the large knee flexion Range of Motion. This subject has a knee angle of about 65° at the end of the eccentric phase of the exercise. This very flexed position puts the quadriceps at a severe mechanical disadvantage, which results in very large muscle forces and the consequent large stresses on the patellofemoral and tibiofemoral joints. This exercise technique can irritate the PFPS and does not fit the therapeutic strategy, so the therapist should quickly instruct this person to decrease the range of motion. Providing a cue to only slightly lower the weight or keeping the knees extended to at least 120° would be appropriate for a patient with PFPS.

A better question would be: should this person even be on this leg press machine? Would it be better if they executed a different exercise? A leg press machine requires less motor control to balance the resistance than a free-weight squat exercise, so a leg press may be more appropriate than a squat. Maybe a more appropriate exercise would be a leg press machine or a cycle that allows the subject to keep the hip extended (reducing hip extensor contributions and increasing quadriceps demand)

Figure 12.2. The leg press technique of a person trying to remediate patellofemoral pain.

and limit the amount of knee flexion allowed. The differences in muscle involvement are likely similar to upright versus recumbent cycling (Gregor, Perell, Rushatakankovit, Miyamoto, Muffoletto, & Gregor, 2002). These subtle changes in body position and direction of force application (Force- Motion) are very important in determining the loading of muscles and joints of the body. Good therapists are knowledgeable about the biomechanical differences in various exercises, and prescribe specific rehabilitation exercises in a progressive sequence to improve function.

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