chapter 2: Biomechanics of the Patellofemoral Joint

Knee Extension Against Resistance

Normal activities (walking, running, climbing and descending stairs, squatting, jumping) all load the knee in a standardized, physiologic way. Under all of these circumstances, with body weight applied from above, the flexor lever arm, quadriceps tension, patellar tendon tension, PFJR force, and patellar contact area are all increasing with increasing knee flexion. However, there is another way in which the patellofemoral joint is loaded, namely, by applying a weight or resistance to the ankle against which the patient must extend. Free weights, many home exercise machines, and isokinetic testing equipment can overload the patellar articular surface substantially unless one uses extreme caution.

Under these circumstances, the weight moves further from the center of rotation as knee extension proceeds. Thus, quadriceps and patellar tendon tension steadily increase with decreasing knee flexion. Because PFJR is a function not only of the absolute quadriceps and patellar tendon forces, but also of the angle between them, the PFJR force under these loading circumstances first increases with progressive knee extension and then decreases. However, this PFJR force is falling on a steadily decreasing contact area.

These loading circumstances were experimentally evaluated by Hungerford and Barry (24). Calculating the absolute values, they used M1 = M2, which only changes the numerical results slightly. The quantitative aspects of their work are still valid and show that resistance against a 9‑kg boot equals contact stress of flexion under body weight at approximately 55° of flexion. From there to 30° of flexion, pressure exceeds the contact stresses of body weight by several orders of magnitude. Thus, extension exercises against even modest weight can generate enormous cartilage contact stress. If the cartilage is already damaged, this can produce further increase in symptoms and accelerate pathologic changes. Understanding these concepts is essential so that methods of rehabilitating the quadriceps that may be damaging to the joint itself are not employed.

For these reasons, it may be safer to avoid isokinetic exercise in the rehabilitation of most patients with patellar imbalance or articular disease. If isokinetic exercise is used, it must be carefully supervised with monitoring of the range‑of‑motion, antishear, and higher speeds (less chance of overloading defective cartilage).

        

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