INTRODUCTION: High neuromuscular activity before and during ground contact (GC) are the prerequisites to benefit from energy saving mechanisms during stretch-shortening type actions (1). Especially time critical disciplines such as long jump (2) require immediate transition from eccentric to concentric muscle action (1). To meet such criteria athletes are highly involved in plyometric training. Therefore they have an exclusive neuromuscular control and stiffness regulation during the eccentric phase (3). During excessive stretch loads the eccentric activity is negatively affected possibly due to inhibitory influences (2,4). By directly comparing experts and novices the present study investigate the effect of expertise on the neuromuscular control and leg spring behavior in vertical jumps. METHODS: By combining 3D motion capture system with electromyography of the shank and thigh muscles differences in biomechanical and neuromuscular control were elaborated between 13 professional youth elite track and field athletes and 13 sportive novices. All participants performed bipedal drop jumps from 25, 37, 49 and 61 cm. The seven trials with the highest reactive strength index (RSI) were selected for statistical analysis. Two-way Mixed-ANOVA (group x condition) was performed. RESULTS: Significant interaction effect reveal higher RSI and leg stiffness in experts compared to novices. In both groups RSI is significantly reduced in 61 cm. Experts realize highest leg stiffness during low stretch loads; leg stiffness in novices remains constant. Overall experts demonstrate significant lower ankle and knee joint flexion compared to novices; both groups increase ankle and knee flexion with increasing stretch load. Significant interaction is evident for soleus (SOL), gastrocnemius medialis (GM) and vastus medialis (VM) with overall higher pre-activity in experts. During GC SOL activity is significant higher in experts; activity diminishes with increasing stretch load only in experts. During GC GM and TA activity is similar in both groups; GM activity diminishes whereas TA and VM activity increases with increasing stretch load. Further analysis revealed that RSI is positively correlated with leg stiffness (r = .57, p < .01) and eccentric SOL activity (r = .51, p < .01). CONCLUSION: Our results reveal higher neuromuscular activity before GC in experts to achieve an immediate transition and high ballistic performance (2). Compared to previous experiments higher eccentric activity in experts is only evident for SOL (4). Although both groups compensate highest stretch loads by increasing joint flexion and VM activity, experts are able to achieve high stiffness which is associated with RSI. Immediate eccentric-concentric transition is more dominant in low stretch loads.
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