Quadriceps-hamstrings muscle co-activation during the swing phase of walking is modulated by task constraints in healthy adults

Background Muscle co-activation, the simultaneous activation of muscle groups, is a common strategy to stabilize walking. However, co-activation can also be the consequence of underlying neurological impairments. This complicates differentiation between functional and pathological co-activation during walking. To better understand and discern functional co-activation during walking, this study investigated the difference between quadriceps-hamstrings co-activation during the swing phase of walking and isolated leg-swinging in healthy adults.

Methods Twelve healthy young adults performed walking and isolated leg-swinging at slow (0.6 m/s) and comfortable speed. Electromyography signals from m. vastus lateralis, m. rectus femoris, m. biceps femoris, and m. semitendinosus were recorded. Co-activation index (CI) was calculated using Pearson correlation coefficient and area under the curve (AUC) and averaged to one quadriceps-hamstrings CI per metric.

Results The results showed a higher Pearson-CI during walking compared to isolated leg-swinging, specifically during mid- and terminal-swing at both speeds. AUC-CI, but not Pearson-CI was significantly different between the two speeds.

Conclusion Quadriceps-hamstrings co-activation towards the end of the swing phase during walking reflects preparation for heel-strike, which is not present in isolated leg-swinging. Therefore, an isolated leg-swinging task could serve as a feasible method to distinguish pathological from functional muscle co-activation during walking.

Competing Interest Statement
The authors have declared no competing interest.