Investigating the impact of external load on muscle synergies during bipedal squats.

Purpose: A broad functional movement repertoire is crucial for engaging in physical activity and reducing the risk of injury, both of which are central aspects of lifelong health. As a fundamental exercise in both recreational and rehabilitative training regimes, the bipedal squat (SQ) incorporates many everyday movement patterns. Crucially, SQ can only be considered functional if the practitioner can meet the coordinative demands. Many factors affect coordinative aspects of an exercise, most notably external load. Since compound movements are assumed to be organized in a synergistic manner, we employed muscle synergy analysis to examine differences in muscle synergy properties between various external load levels during SQ.

Methods: Ten healthy male recreational athletes were enrolled in the present study. Each participant performed three sets of ten SQ on a smith machine at three submaximal load levels (50%, 62.5%, and 75% of 3 repetition maximum) across three non-consecutive days. Muscle activity was recorded from 12 prime movers of SQ by way of electromyography (EMG). Muscle synergies were analyzed in terms of temporal activation patterns, i.e., waveform, as well as the relative input of each muscle into individual synergies, i.e., weight contribution.

Results: Waveforms of muscle synergies did not differ between loads. Weight contributions showed significant differences between load levels, albeit only for the gastrocnemius muscle in a single synergy.

Conclusion: Taken together, our results imply mostly stable spatiotemporal composition of muscle activity during SQ, underlining the importance of technical competence during compound movement performance in athletic and rehabilitative settings.