Interaction between Muscle Activation, Co-Activation, and Lower Extremity Stiffness in Recreational Runners: A Statistical Parametric Mapping Approach

PURPOSE: To investigate the relationship between muscle (co-) activation time series and lower extremity stiffness (leg stiffness [ Kleg ], vertical stiffness [ Kvert ], and joint quasi-stiffness) during running in male recreational runners using statistical parametric mapping (SPM). METHOD: Twenty-nine male recreational runners (age: 21 ± 1.17 yr, height: 1.80 ± 0.06 m, weight: 72.1 ± 9.91 kg) ran at 10 and 12 km·h -1 , while kinematic data, ground reaction forces, and surface electromyography signals from nine muscles were collected. SPM analyzed the temporal correlation between muscle (co-) activation time series and lower extremity stiffness across gait cycles. RESULTS: Higher soleus activation during the initial propulsion phase ( r = 0.506-0.552, 14.5%-21.2% of the gait cycle) and higher gastrocnemius lateralis activation during the mid-stance phase ( r = 0.504-0.527, 12.5%-17.5% of the gait cycle) showed large positive correlations with greater Kleg at 12 km·h -1 . Large correlations were also found between greater Kvert at 12 km·h -1 and higher soleus activation during the initial propulsion phase ( r = 0.508-0.622, 16.5%-24.3% of the gait cycle) and higher gastrocnemius lateralis activation during the mid-stance phase ( r = 0.507-0.601, 9.70%-20.5% of the gait cycle). Vastus lateralis activation during the propulsion phase was moderately to largely negatively correlated with ankle joint quasi-stiffness ( r = -0.595 to -0.464, 30.1%-37.9% of the gait cycle). CONCLUSION: Higher ankle plantar flexor activation contributes to greater lower extremity stiffness regulation during stance, particularly during higher speeds. Furthermore, unilateral and distal muscles exhibited a more important role than bilateral and proximal muscles in regulating lower extremity stiffness. The knee extensors primarily regulated ankle joint quasi-stiffness during propulsion at lower speeds.