Anterior cruciate ligament injuries and the measurement of muscle stiffness

We sought to investigate the reported changes in angular stiffness of the knee due to the hamstring muscles in the Anterior Cruciate Ligament (ACL) deficient knee using a modification of a recently described method. The stiffness of the knee in isometric flexion and extension was measured by applying weights to a deLorme boot attached to the foot and then measuring the frequency of oscillation of the lower limb after a sharp tap to the boot, with the patient seeking to maintain the position of the lower leg by contraction of the flexor or extensor muscles of the knee depending on the position of the patient in the test. We found that stiffness not only varied between patients but also between legs with the injured leg having significantly stiffer flexor (p < 0.001) and significantly less stiff extensor muscles (p = 0.01). The results contrast quite markedly with published work and whilst it is not readily clear why this should be so, we attempt to offer an alternative explanation to that given by other authors. If the ACL is ruptured one would expect to see a difference in muscle stiffness. From a physiological point of view it makes sense to have the flexors of the knee more stiff and the extensors less so to provide some compensation for the loss of the ACL.