Muscle synergies for the investigation of focal and postural movement control during upper limb tasks

The central nervous system (CNS) organizes movements through a modular activation of functionally related muscle groups, called muscle synergies. These synergies have been mainly used to describe the contribution of muscles to the focal component of human movements, while their role in ensuring postural control during voluntary tasks has been poorly explored. This study aimed at investigating and characterizing the spatial and temporal organization of whole-body muscle synergies during upper limb tasks in healthy subjects and describing their variability between the dominant and non-dominant upper limb. Fifteen right-handed volunteers performed upper limb elevation movements along three different planes with the dominant and non-dominant arms and surface electromyographic signals were collected from 20 trunk and upper limbs muscles. Non-negative matrix factorization was used to decompose electromyographic signal envelopes and extract spatial and temporal profile of muscle synergies. Inter-subject variability was assessed using cosine similarity. Based on spatial and temporal profile of 4 extracted synergies, M1 explained arm acceleration during elevation movements, M2 was assumed to ensure postural control during arm acceleration, M3 included muscles responsible for arm deceleration, while M4 contributed to ensuring postural control during arm deceleration. Inter-subject similarity was higher for the dominant (median: 0.70, interquartile range: 0.07) compared to non-dominant arm (median: 0.63, interquartile range: 0.09) (p < 0.001). The organization of muscle synergies follows a biomechanical rationale, reflecting CNS ability to manage the focal and postural movement components during upper limb elevation. Higher variability during movements performed with the non-dominant upper limb may depend on lower neuromuscular control.