Abstract
This study aimed to identify muscle synergies of the lower limb during treadmill running on level and inclined ground. Eight subjects ran on a treadmill at three speeds (2.5, 3.3, and 4.1 m/s) and two grades (level and 10% grade). Surface electromyographic (EMG) signals were recorded from 10 muscles of the lower limb, including deeper muscles such as vastus intermedius, adductor magnus, and adductor longus. Muscle synergies were extracted applying a non-negative matrix factorization algorithm, and relative co-activations across muscles and the temporal recruitment pattern were identified by muscle synergy vector and synergy activation coefficient, respectively. The scalar product between pairs of synergy vectors and synergy activation coefficients during level and uphill running conditions were analyzed as a similarity index, with values above 0.8 recognized as similar. Approximately 4 muscle synergies controlled the majority of variability in 10 EMGs during running, and were common between level and uphill conditions. At each running speed, inter-condition similarity was observed in synergy vector (r > 0.83) and synergy activation coefficients (r > 0.84) at each type of synergy. These results suggest that types of synergy are consistent between level and uphill running.
Highlights
Running is one of the modes of human locomotion and is generally considered a distinct locomotor mode with strikingly different mechanics from walking[1]
Peak EMG activity of the vastus medialis (VM) was significantly higher during uphill running than during level running at each speed, and that of the vastus intermedius (VI) was significantly higher during uphill running than during level running at 4.1 m/s (p < 0.05)
Neuromuscular activations of the lower limb including the deeper muscles were recorded during running on level and inclined ground surfaces, muscle synergies were identified by negative matrix factorization (NMF)
Summary
Running is one of the modes of human locomotion and is generally considered a distinct locomotor mode with strikingly different mechanics from walking[1]. Regarding EMG recordings for deeper muscles in the thigh, specific activation patterns were obtained from knee and hip muscles, such as vastus intermedius (VI)[6], adductor magnus (AM)[4], and adductor longus (AL)[5] during running over a wide speed range. Slope-related changes in EMG activity were observed in the initial stance and swing phases of running[9], corresponding to synergies comprising mainly hip and knee extensors, and hip adductors and knee flexors, respectively[10,14]. We recorded neuromuscular activations of the lower limb, including the deeper muscles, during running on level and inclined ground, extracted muscle synergies using NMF. This study aimed to test the hypothesis that muscle synergies mainly comprising activation of knee extensors and/or hip adductors during running are modulated in response to ground elevation
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