The purpose of this investigation was to determine differences between pre- and post-pubescent males and females in quadriceps (vastus medialis; VM) and hamstrings (medial hamstrings and biceps femoris; HAMS) muscular activation patterns via the root mean square of the surface electromyography (SEMG) during self-initiated vertical jump landing. Fifty-eight subjects, divided into age and gender groupings, were compared on kinematic variables during pre-landing (100 msec preceeding initial ground contact), post-landing (100 msec following initial ground contact), and initial-contact-to-maximum-knee-flexion stages.  Kinematic variables investigated were (1) SEMG values during each stage of the vertical-jump landing; (2) Co-contraction ratios (CCR), which represented the ratio of normalized hamstring activity to normalized quadriceps activity; and, (3) knee angle at initial contact. Results indicated that post-pubescent subjects displayed greater HAMS acitivity and CCR values in the pre-landing stage relative to post-landing stages, indicating that post-pubescent subjects had a greater level of HAMS co-contraction prior to landing than pre-pubescent subjects.  Conversely, pre-pubescent subjects displayed greater post-landing and initial contact-to-maximum knee flexion ratios, indicating a greater level of HAMS co-contraction during post-landing stages than post-pubescent subjects. There were no significant differences in knee angle at initial contact.  The greater degree of HAMS co-activation prior to landing by post-pubescent subjects indicates that they used a strategy of pre-tuning the hamstrings prior to landing (more CNS pre-activation) to control ground reaction forces and anterior tibial displacement experienced by the knee during landing.  On the other hand, pre-pubescent subjects controlled ground reaction forces of landing by having a greater co-activation of the hamstrings during landing, which represents more of a reflexive activation in response to ground impact.

2004 Croce RV, Russell PJ, Swartz EE, Decoster LC: Knee Muscular Response Strategies Differ by Developmental Level but Not Gender During Jump Landing.  J Electromyography and Clinical Neurophys., 2004, 44, 339-348.