Further evidence against a direct automatic neuromotor link between the ACL and hamstrings

The purpose of this experiment was to determine whether reflexive, or more broadly, automatic hamstrings excitation could be elicited during isometric, maximum effort, step increases in knee extension torque. Eight healthy subjects without lower extremity dysfunction or injury performed maximum effort isometric knee extension at 15 and 85 degrees of knee flexion in minimum elapsed time. Surface electromyography was used to record medial (semimembranosis and semitendinosis) and lateral (biceps femoris--long head) hamstrings excitation that was subsequently normalized to the excitation during maximum isometric knee flexion. To assess whether automatic hamstrings excitation was elicited, the amplitude of the EMG signals was analyzed and compared for 50 ms prior to peak knee extension torque, and three consecutive 50 ms windows following peak knee extension torque. The amplitude of the EMG subsequent to the peak knee extension torque failed to support the contention of an automatic hamstrings excitation mediated by neural circuitry from the ACL. The present results, in conjunction with previous work, suggest that a protective automatic hamstrings contraction is not normally elicited in response to conditions subjecting the anterior cruciate ligament to strain.