Gender differences in both active and passive parts of the plantar flexors series elastic component stiffness and geometrical parameters of the muscle-tendon complex

Men are reportedly at higher risk of plantar flexor muscle injury and Achilles tendon ruptures than women. Biomechanical parameters are thought to play a role in the higher frequency of injury to males. One parameter is the stiffness of tissues; a stiff tissue cannot absorb sufficient energy with loading, and subsequently may be more likely to be injured. Thus, our purpose was to investigate the gender difference in the geometrical parameters of plantar flexor's muscletendon complex and the stiffness of both active and passive parts of the series elastic component (SSEC1 and SSEC2, respectively). Using the alpha method on data obtained from quick stretches to the plantar flexors performed during isometric contractions, SSEC1 and SSEC2 were assessed. Plantar flexor muscles and Achilles tendon cross-sectional areas (CSATS and CSAAT, respectively) were determined in young healthy men (n=49) and women (n=31). The findings showed that SSEC2 was higher in men (p<0.001), but this difference was not apparent when SSEC2 was normalized to CSAAT (p>0.05). In contrast, SSEC1 was lower in men (p<0.001) and remained so after normalization to CSATS. Higher joint stiffness observed in men was notably influenced by lever arm length. Thus, the results of this study have implications for performance and injury. (C) 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:707712, 2012