Exploring walking-dependent co-activation of quadriceps and hamstrings muscles using an isolated leg-swinging...

Muscle co-activation, the simultaneous activation of muscles or muscle groups, is a common strategy to enhance the stability of the musculoskeletal system. However, co-activation can also be the consequence of underlying neurological impairments. To better understand and discern functional co-activation during walking, this study explored the difference in quadriceps-hamstrings co-activation during the swing phase of walking and an isolated leg-swinging movement in healthy adults.

Twelve healthy young adults performed walking and isolated leg-swinging at slow (0.6 m/s) and comfortable speed. Isolated leg-swinging was frequency and amplitude matched to the walk conditions. Electromyography signals from quadriceps muscles: m. vastus lateralis and m. rectus femoris, and hamstring muscles: m. biceps femoris and m. semitendinosus were recorded. Co-activation indices (CI) were determined across the quadricep-hamstrings using two metrics: 1) Pearson correlation coefficient (Pearson-CI) was calculated as to quantify rate of co-activation. 2) Area under the curve (AUC-CI) was calculated as a measure of co-activation magnitude Both metrics were averaged to a single quadriceps-hamstrings CI for each metric.

The results showed a higher Pearson-CI, but not AUC-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 higher during slow speed, compared to comfortable speed.

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.