Passive resistance of the human knee: the effect of immobilization

The effects of immobilization on both the circumferential dimensions of the lower limb, and the passive resistance of the knee to sinusoidal motion in the flexion-extension plane, were studied in patients immobilized after tibial fractures or ligamentous injuries of the knee. Immobilization resulted in a decrease of circumferential dimensions of the lower limb, indicating atrophy of thigh and calf musculature. The equilibrium angle of the knee, i.e. the angle at which the net passive moment equals zero, approached the angle of the knee during immobilization. At knee angles of 60 degrees (the maximal angle imposed on the immobilized knee) and 45 degrees, the resistance was increased, and at angles of 30 degrees and 15 degrees, the resistance was decreased. The passive resistances at +/- 22.5, 15 and 7.5 degrees relative to the equilibrium angle were all increased. A simple model, taking into account changes in the length of muscles at the anterior and posterior side of the knee, explains changes in passive resistance. The duration of immobilization for patients with ligamentous injuries of the knee was less than 50% of that for patients with tibial fractures, whose ankle joint was not immobilized, both resulted in a smaller muscular atrophy for patients with ligamentous injuries. Differences in passive resistance between unaffected and immobilized legs were the same in both groups of patients. Changes in passive resistance in the immobilized leg for those with ligamentous injury are probably not the effect of immobilization alone but the combined effect of immobilization and ligamentous lesions.