Effects of movement speed and joint position on knee flexor torque in healthy and post-surgical subjects

Coactivation of knee flexors during knee extension assists in joint stabili ty by exerting an opposing torque to the anterior tibial displacement induc ed by the quadriceps. This opposing torque is believed to be generated by e ccentric muscle actions that stiffen the knee, thereby attenuating strain t o joint ligaments, particularly the anterior cruciate ligament (ACL). Howev er, as the lengths of knee muscles vary with changes in joint position, the magnitude of flexor/extensor muscle force coupling may likewise vary, poss ibly affecting the capacity for active knee stabilization. The purpose of t his study was to assess the effect of changes in movement speed and joint p osition on eccentric/concentric muscle action relationships in the knees of uninjured (UNI) and post-ACL-surgery (INJ) subjects (n = 14). All subjects were tested for maximum eccentric and concentric torque of the contralater al knee flexors and extensor muscles at four isokinetic speeds (15 degrees- 60 degrees . s(-1)) and four joint position intervals (20 degrees-60 degree s of knee flexion). Eccentric flexor torque was normalized to the percentag e of concentric flexor torque generated at each joint position interval for each speed tested (flexor E-C ratio). In order to estimate the capacity of the knee flexors to resist active knee extension. the eccentric-flexor:con centric-extensor ratios were also computed for each joint position interval and speed (flexor/extensor E-C ratio). The results revealed that eccentric torque surpassed concentric torque by 3%-144% across movement speeds and j oint position intervals. The magnitude of the flexor E-C ratio and flexor/e xtensor E-C increased significantly with speed in both groups of subjects ( P < 0.05) and tended to rise with muscle length as the knee was extended: p eak values were generated at the most extended joint position (20 degrees-3 0 degrees). Although torque development patterns were symmetrical between t he contralateral limbs in both groups, between-group comparisons revealed s ignificantly higher flexor/extensor E-C ratios for the INJ group compared t o the UNI group (P < 0.05), particularly at the fastest speed tested (60 de grees . s(-1)). The results indicate that joint position and movement speed influence the eccentric/concentric relationships of knee flexors and exten sors. The INJ subjects appeared to accommodate to surgery by developing the eccentric function of their ACL and normal knee flexors, particularly at h igher speeds and at more extended knee joint positions. This may assist in the dynamic stabilization of the knee at positions where ACL grafts have be en reported to be most vulnerable to strain.