The Nonlinear Effects of Walking Speed on Calf Muscle Activation During the Ankle Power Generation Phase

Background/Objectives: The calf muscles are vital for generating propulsive force during walking. This power is produced from calf muscle contractions and elastic strain energy release. However, the impact of walking speed on these power-generation mechanisms is understudied. This study aimed to investigate how different walking speeds affect calf muscle activation and ankle power generation. Methods: In this study, we analyzed electromyography (EMG) signals from the gastrocnemius (GAS) and soleus (SOL) muscles of 55 healthy individuals walking at various speeds. C1: household ambulators (0–0.4 m·s−1), C2: limited community ambulators (0.4–0.8 m·s−1), C3: community ambulators (0.8–1.2 m·s−1), C4: self-selected usual speed, and C5: self-selected fast speed. Results: Deviating from a participant’s self-chosen pace led to increased cumulative muscle activity and prolonged plantar flexor activation. Optimal muscle activation was observed at speeds between 0.8–1.2 m·s−1. A second-degree polynomial mixed model best captured the relationship between muscle activation duration and integrated EMG in the ankle power generation phase in late stance, demonstrating the nonlinear relationship between walking speed and calf muscle activation in this phase. Statistically significant models (p < 0.001) explained over 50% of the variability in GAS activation duration (R2 = 0.55) and integrated EMG (R2 = 0.56), as well as SOL activation duration (R2 = 0.52) and integrated EMG (R2 = 0.72). Conclusions: The nonlinear relationship between walking speed and calf muscle activation indicates that normal walking speed optimizes the utilization of elastic strain energy in the ankle power generation phase.