Knee joint angle: its relationship to quadriceps femoris activity in normal and postarthrotomy limbs

The relationship of knee joint angle to maximum effort quadriceps femoris activity, as measured by smoothed and rectified EMG, was systematically examined in 30 normal and 18 post-meniscectomy limbs. Our results show that motor unit activity depends not only upon joint angle, but also upon peripheral receptor feedback, which is altered in the post-arthrotomy limbs, producing the characteristic "extensor lag" or inability to maintain the knee at 0 degrees while flexing the hip. The significant interaction of joint position and limb condition must therefore be taken into account when evaluating or rehabilitating pathologic joint conditions. Current motor control theory does not account for the findings presented; an alternative model, proposing integration of the central motor program and peripheral receptor feedback, is presented, which may explain the "extensor lag."