Gender specific strategies for adaptation of leg stiffness in demanding hopping conditions

Purpose: Difference in leg stiffness between females and males is considered to be a cause of higher rates of Anterior Cruciate Ligament injury in female athletes. Females are believed to have lower stiffness and as a consequence different recruitment strategies to adapt functional tasks. The purpose of the present study was to elucidate how females reacted different than males to adapt their leg stiffness to higher demanding task as faster hopping conditionsMethods: 21 healthy subjects (11 Male-10 Female; age: 18-35 years) participated in this study. Subjects performed two-legged hopping tasks (at their preferred rate, metronome rate at 3.0 Hz and as fast as possible) on a force platform. Kinetic data (ground reaction force, frequency of hop, ground contact time, aerial time, hop cycle were obtained from force platform. Changes in leg length, knee and ankle joints moments and stiffness were calculated by using MATLAB 6.5.1 version. Kinematics (touchdown angles and changes in joint angles) were determined by VICON (3D motion analyses system).4-channel telemetry EMG system (pasaq) was used to assess muscle dynamics (preactivation time, preloading and loading responses) of lower extremity (vastus medialis, biceps femoris, gastrocnemius medialis and tibialis anterior).Results: Males had significantly greater ground reaction forces than females at all hopping conditions (p<0.005). After normalizing the values with subjects? body weight the differences between men and women disappeared. Males had greater knee and ankle moments than females at preferred and fastest hopping rates however, at 3.0 Hz, females had significantly greater knee moment even for the normalized values (p<0.005). Knee stiffness at preferred and fastest hopping rates is similar between females and males while at 3.0 Hz females indicated greater knee stiffness values than males. Normalized values by body weight of subjects indicated that females had significantly greater leg stiffness than males at faster hopping conditions as metronome and fastest rate hopping (p<0.005). Males preferred to touch the ground with significantly greater plantar flexion degrees than females did (p<0.005). At the fastest hopping rate it was seen that males had significantly increased their activation ratio of Gastrocnemius medialis to Tibialis anterior muscle before ground contact indicating greater Gastrocnemius medialis activity than Tibialis anterior (p<0.005) while Tibialis anterior muscle preloading and loading responses did not significantly differ between males and females.Conclusion: The main differences between males and females for regulation of their leg stiffness is that males chose more plantar flexed posture to touch the ground and activated their gastrocnemius muscle more than tibialis anterior just before ground contact which represents pre-activation period of muscles. Thus there is a significant difference between males and females in the movement strategy they used. EMG results showed that females have no limiting factor in their musculoskeletal system to recruit their leg stiffness as high as males under condition of maximum effort. The relationship between observed gender differences in leg stiffness and increased Anterior Cruciate Ligament injury rate in females requires further study.