Intra-Subject Consistency during Locomotion: Similarity in Shared and Subject-Specific Muscle Synergies

Abstract

Human locomotion is a complex motor task. Previous research hypothesized that muscle synergies reflect the modular control of muscle groups operated by the Central Nervous System (CNS). Despite the high stride-to-stride variability characterizing human gait, most studies analyze only a few strides. This may be limiting, because the intra-subject variability of motor output is neglected. This gap could be filled by recording and analyzing many gait cycles during a single walking task. In this way, it can be investigated if CNS recruits the same muscle synergies consistently or if different strategies are adopted during the locomotion task. The aim of this work is to investigate the intra-subject consistency of muscle synergies during overground walking. Twelve young healthy volunteers were instructed to walk for 5 min at their natural pace. On the average, 181 ± 10 gait cycles were analyzed for each subject. Surface electromyography was recorded from 12 muscles of the dominant lower limb and the trunk. Gait cycles were grouped into subgroups containing 10 gait cycles each. The consistency of the muscle synergies extracted during the gait trial was assessed by measuring cosine similarity (CS) of muscle weights vectors, and zero-lag cross-correlation (CC) of activation signals. The average intra-subject CS and CC were 0.94 ± 0.10 and 0.96 ± 0.06, respectively. We found five synergies shared by all the subjects: high consistency values were found for these synergies (CS = 0.96 ± 0.05, CC = 0.97 ± 0.03). In addition, we found 10 subject-specific synergies. These synergies were less consistent (CS = 0.80 ± 0.20, CC = 0.89 ± 0.14). In conclusion, our results demonstrated that shared muscle synergies were highly consistent during walking. Subject-specific muscle synergies were also consistent, although to a lesser extent.

Muscle Synergies Extracted Using Principal Activations: Improvement of Robustness and Interpretability

“…The relevance of the analysis of the muscle synergies is well known in literature and several studies supporting the importance of this theory have been published in the last years [2], [3], [11], [15], [31], [33]. The approaches previously used in literature ("standard" approach) to extract the muscle synergies generally considered the whole sEMG signals as input of the factorization algorithm [6], [10], [13], [15].…”

“…Considering PAs allow evaluating only the "necessary" muscle activations, discarding those with auxiliary function [22], such as those providing corrections to cyclic motion and body segment posture. According to this approach, only the time-samples in correspondence of PAs are considered as inputs of the factorization algorithm for the extraction of the muscle synergies [11], [21], while the remaining time-samples are set to zero.…”

“…First, the foot-switch signals were used to segment the gait cycles, keeping only the gait cycles with a normal sequence of gait phases (Heel contact (H), Flat foot contact (F), Push off (P) and Swing (S)), while discarding the atypical gait cycles [25]. Only the gait cycles belonging to the rectilinear path were analyzed, removing those corresponding to direction changes at the beginning and at the end of the walkway (including deceleration before and acceleration after the U-turn) [11]. Then, each segmented gait cycle was time-normalized to 1000 samples [15] and all gait cycles selected were concatenated in a single vector [26].…”

“…These biomechanical functions were generally assigned to each muscle synergy by observing the prevailing muscles contribution ( > 0.5) and the profile of the activation coefficients ( ) [4], [5], [11], [32]. Rimini et al [11] discovered 5 biomechanical function common to all the analyzed subjects during the walking task. TABLE II reports the description of each biomechanical function and the muscles involved in each function.…”

“…The number of muscle synergies extracted and their biomechanical functions may vary depending on the type and the number of muscles acquired [10]. On the average, five muscle synergies are needed to properly describe human locomotion [6], [11].…”