Background: Mechanical instability, neurosensory deficits, and/or modified coactivation strategies following anterior cruciate ligament injury and surgery might compromise the ability of the knee extensors to exert a precise force during isokinetic knee extension with maximal effort. The purpose of this study was to examine the effects of an anterior cruciate ligament deficiency and reconstruction on knee extensor torque-time curve smoothness and to elucidate its association with hamstring (antagonist) activation and physical performance.
Methods: Thirteen subjects who had a unilateral deficiency of the anterior cruciate ligament, twenty-five matched subjects with a unilateral reconstructed anterior cruciate ligament, and thirty-three control subjects performed knee extension and flexion repetitions bilaterally with maximal effort at 180°/sec on a Cybex dynamometer. For the subjects with a deficient or a reconstructed anterior cruciate ligament, hamstring activation was measured electromyographically. Physical performance was measured by requiring subjects to perform single-limb timed hopping on the involved limb.
Results: In the subjects with a reconstructed or a deficient anterior cruciate ligament, wavelet-derived mean instantaneous frequency of the extensor torque-time curves was significantly (p < 0.001) higher in the involved limb than in the noninvolved limb and the limbs of the control subjects. Furthermore, for the subjects with a reconstructed or deficient anterior cruciate ligament, the mean instantaneous frequency of the extensor torque-time curves was positively associated with the level of hamstring antagonist activity (r = 0.580, p < 0.001) and with hopping performance (b = -0.943, p = 0.019), whereas isokinetic peak torque was not (b = -0.001, p = 0.797).
Conclusions: For individuals who have a deficient or a reconstructed anterior cruciate ligament, an increased frequency content of the knee extensor torque is not, as previously and commonly assumed, a deleterious manifestation of the neuromuscular system. Rather, our results suggest that torque smoothness reduces as hamstring activation increases—a positive neuromuscular adaptation that enhances joint stability and, hence, optimizes physical performance. Wavelet-derived measures of extensor torque smoothness may provide valuable clinical information regarding joint function that conventional isokinetic torque measures cannot.
Level of Evidence: Therapeutic Level III. See Instructions to Authors for a complete description of levels of evidence.