Electromyographic analysis of fatigue and synergistic activation in striking and supporting legs during continuous side-kicks in elite Sanda athletes.

This study aimed to investigate the functional roles and fatigue effects of the striking and supporting legs in consecutive stationary side-kick techniques among Sanda fighters. By analyzing muscle activation (root mean square, RMS), this study provides insights for targeted muscle training in Sanda. Surface electromyography (sEMG) was utilized to collect data from 10 Chinese elite-level Sanda athletes (height: 178.5 ± 4.3 cm, weight: 72.1 ± 5.6 kg, training experience: 7.8 ± 1.9 years; 4 international master athletes, 6 national master athletes) performing continuous static side-kick techniques. Vicon synchronization equipment was used to determine the start and end points of the movement. Thirty seconds of raw sEMG data were preprocessed and segmented into 1-second intervals. A sliding window approach with feature averaging was employed to calculate the root mean square (RMS) values for 50-millisecond windows, followed by the computation of mean RMS values per 1-second interval. The fatigue characteristics of major muscle groups in the lower limbs (kicking leg and supporting leg) were evaluated by analyzing the mean RMS values across different phases over time: the initial phase (1-5 s), the middle phase (initial: 1-5 s, middle: 15-19 s, final: 26-30 s). Regarding fatigue accumulation, elite Chinese Sanda fighters exhibited significant RMS reductions in the striking leg's primary muscle groups (gastrocnemius, vastus medialis, and gluteus maximus) over time (change rates: -5.29% to -7.50%, p < 0.01), indicating pronounced fatigue. In contrast, the supporting leg's major muscle groups showed stable RMS changes (change rates: -3.52% to -3.97%, p > 0.05), suggesting minimal fatigue accumulation during the task. For muscle activation, the RMS values of the striking leg's primary muscles were significantly higher than those of the supporting leg (p < 0.01). The tibialis anterior in the supporting leg exhibited greater activation compared to the striking leg (p < 0.05), while hamstring activation showed no significant difference between legs (p > 0.05). Correlation analysis revealed that the striking leg's force production is driven by synergistic activity among the vastus medialis, rectus femoris, vastus lateralis, and gluteus maximus (r = 0.74 ~ 0.78, p = 0.007 ~ 0.014), highlighting their critical role in hip and knee joint force generation. In contrast, the supporting leg relied on a strong correlation between the gastrocnemius and tibialis anterior (r = 0.81, p < 0.01) for ankle joint stability and a moderate correlation between the gluteus maximus and gluteus medius (r = 0.65, p < 0.05) for hip balance, underscoring distinct muscle coordination patterns between the two legs. In Chinese elite Sanda fighters performing continuous in-place side-kick movements, the striking leg's power-generating muscle groups-primarily the quadriceps and gluteus maximus-exhibit significant fatigue. Meanwhile, the supporting leg maintains stability through the tibialis anterior and gastrocnemius. These findings highlight potential strength asymmetries that could signal fatigue onset or even injury risk, offering valuable insights for coaching strategies and rehabilitation protocols. Training recommendations include plyometric exercises to enhance the explosive power of the quadriceps and gluteus maximus, resistance training to improve the endurance of the striking leg, and isokinetic training to strengthen the stability of the tibialis anterior and gastrocnemius in the supporting leg, thereby further optimizing the training regimen.