Purpose Age-related neuromuscular control adaptations have been investigated mainly in untrained populations, where higher antagonist activation in adults was observed with respect to children. In elite athletes age-related differences in neuromuscular control have scarcely been investigated. Therefore, this study aims at investigating differences in co-activation about the knee joint in two groups of karate athletes belonging to the Junior (JK) and Senior (SK) age categories, performing the roundhouse kick (RK). Methods Six SK and six JK performed the RK impacting on a punching bag. Each participant performed three attempts during which kicking limb kinematics and sEMG from the vastus lateralis (VL) and from the biceps femoris (BF) were recorded. Co-activation index during knee flexion and extension (CIF; CIE) and agonist and antagonist activation areas of VL and BF (I AGO-VL; I AGO-BF; I ANT-VL; I ANT-BF) were computed. Hip and knee range of motion, peak angular velocity and minima and maxima of lower limb angular momentum were computed. Results During knee extension, the SK demonstrated higher CIE, higher IANT-BF and higher total angular momentum with respect to the JK. Significant relationships were observed between I ANT-BF and total angular momentum maxima, and between I ANT-BF and age. Conclusions IANT-BF is partially related to the age of the group and to joint protection upon impact. Moreover, given the very brief duration of the task, a feed-forward mechanism modulating antagonist activation partly based on the stress imposed on the knee joint could be hypothesized. This mechanism potentially involves skill dependent re-modelling of the peripheral and central nervous system.
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