Intermuscular coordination adaptations to strength training: Insights from muscle synergy analysis during squats with different loads

Neural adaptations to strength training in complex tasks involve changes in muscle activation patterns. Synergy extraction has been used to assess training-induced changes in intermuscular coordination. However, it remains unclear how intermuscular coordination evolves at different training stages. Thus, this study aimed to examine i) the effects of 6 weeks of squat training in muscle synergies at different intensities of the squat, and ii) long-term coordination adaptations by comparing groups with different training status: 1) 11 untrained individuals, 2) 11 short-term trained individuals who engaged in the 6-week training program (the training consisted in 3x/week squatting sessions, with 5 sets of 10 repetitions using 75% of 1RM, with the absolute load being progressively adapted during the 6-weeks period), and 3) 10 experienced powerlifters and weightlifters with at least 2 years of training experience. Electromyography (EMG) of seven lower limb muscles was collected during squats at four loads between 40 and 90% 1RM, and muscle synergies were extracted at light (~42.5% 1RM), moderate-light (~57.5% 1RM), moderate-heavy (~72.5% 1RM) and heavy loads (~87.5% 1RM). Six weeks of squat training led to increased strength (+24.87 ± 7.36%). Two muscle synergies were enough to explain at least 90% of muscle activation during the squat. After the training period, muscle synergies remained robust, with slight differences in the weighting of each muscle within the synergies as well as slight shifts in some individual EMG patterns. Furthermore, when comparing the groups, the experienced individuals presented a delay of synergy #1 and an anticipation of synergy #2, with different muscle weightings, namely concerning the muscles acting around the ankle. It was shown that muscle synergies are flexibly adapted with strength training, namely in the long-term, and that adjustments in intermuscular coordination are progressive over time.