COMPARISON OF ACTIVE KNEE FLEXOR STIFFNESS ACROSS GENDER

Abstract

The literature suggests that the hamstrings supplement the anterior cruciate ligament (ACL), and that increased hamstring stiffness may enhance knee joint stability by a variety of biomechanical and neuromuscular mechanisms. Gender differences in knee flexor stiffness may partially explain discrepancies in ACL injury rates between genders. Therefore, the purpose of the current study was to identify gender differences in knee flexor stiffness. Thirty normal individuals volunteered for participation (15 males, 15 females; age = 22.27 ± 2.21 yrs(M), 21.07 ± 1.91yrs(F); height = 1.80 ± .05m(M), 1.61 ± .05(F); mass = 81.03 ± 8.57kg(M), 59.58 ± 5.58kg(F)). Subjects performed an isometric knee flexion maximal voluntary contraction (MVC) on an isokinetic dynamometer from which torque values were used to determine the mass needed to create moments of inertia representing 5, 10, and 15% of the MVC. Active knee flexor stiffness was assessed at 5, 10, and 15% MVC by modeling the lower extremity as a single degree of freedom mass spring system in the open kinetic chain. Subjects were positioned prone with the thigh stabilized in 25° of hip flexion from the horizontal and the leg parallel to the floor in 25° of knee flexion. Mass was added to the leg to create moments of inertia requiring hastring forces equal to 5, 10, and 15% MVC. Subjects were instructed to contract the hamstrings against gravity so that the leg was parallel to the floor. An L-shaped splint was positioned over the posterior leg and palnatr aspect of the foot to standardize ankle position and the influence of the gastrocnemius at the knee. Activation of the medial and lateral hamstrings, quadriceps, and gastrocnemius were monitored via EMG to ensure that muscle activity other than that needed to support the weight of the leg and the attached mass did not occur. A downward force was placed on the distal segment, initiating oscillation at a resonant frequency. An accelerometer positioned on the distal segment provided a trace of the motion occurring in the leg. Knowledge of peak oscillations with reference to time allowed for a linear stiffness calculation. Data were analyzed using a 3 (hamstring stiffness at 5, 10, and 15% MVC) × 1 (gender) repeated measures analysis of variance (ANOVA). Data analysis revealed a significant (p = .015) main effect for gender, with males displaying greater stiffness than females. When stiffness values were normalized to segment mass, no significant gender differences were present (p = .628). These results indicate that overall active knee flexor stiffness is greater in males than in females, but that gender discrepancies may be a function of muscle mass only. Further research involving muscle mass and passive contributions to muscle stiffness is needed to determine the origin of these differences.