A121: The Relationship Between Muscle Activation, Co-Activation, and Leg-Spring Stiffness in Recreational Runners

In running, the organism needs to select appropriate muscle activation strategies to modulate leg-spring stiffness to achieve a high level of exercise performance. However, direct evidence regarding the relationship between leg-spring stiffness, muscle activation, and co-activation during running remains limited. Therefore, we aimed to explore the relationship between muscle (co) activation and leg-spring stiffness (leg stiffness and vertical stiffness) in recreational runners. Method: Twenty-nine male recreational runners (age: 21 ± 1.17 yr, height: 1.80 ± 0.06 m, weight: 72.13 ± 9.91 kg) ran at 12 km/h while kinematic data, ground reaction forces, and surface electromyography signals were collected for nine muscles. Statistical Non-Parametric Mapping (SnPM) was utilized to investigate the correlation between leg-spring stiffness time-series and lower limb muscle activation and co-activation time-series. The higher activation of the gastrocnemius lateralis during the mid-stance phase (r = 0.506 to 0.529, 12.75%-17.40% of the gait cycle) and the higher activation of the soleus during the propulsion phase (r = 0.509 to 0.554, 14.60%-21.10% of the gait cycle) were significantly positively correlated with the higher leg stiffness at 12 km/h. In addition, large associations were found between larger vertical stiffness at 12 km/h, higher activation of the lateral gastrocnemius muscle during the mid-stance phase (r = 0.510 to 0.603, 9.80%-20.35% of the gait cycle), and higher activation of the soleus muscle during the propulsion phase (r = 0.511 to 0.625, 16.55%-24.20% of the gait cycle). This study suggests that higher ankle and knee extensor activation contributes to enhanced leg-spring stiffness regulation during the stance phase, with unilateral and distal muscles playing a more important role than bilateral and proximal muscles.