Forearm muscle synergies during ramp-and-hold gripping in individuals with chronic elbow pain

BACKGROUND AND AIM: It is largely unknown how the CNS regulates multiple muscle systems in in the upper limb in the presence of pain. This study investigated spatiotemporal activation patterns of forearm muscle synergies during ramp-and-hold gripping performed by 11 individuals with chronic elbow pain and 11 healthy age-matched controls.

METHODS: Presence of elbow pain was confirmed using standard clinical tests, pain threshold and ultrasound diagnostic measures. Surface electromyography was obtained from six forearm muscles when participants randomly performed ramp-and-hold gripping to 15% MVC at three different wrist postures. Non-negative matrix factorization was used to extract the forearm muscle synergies from the time-normalized EMG’s.

RESULTS: Dimensionality analyses revealed between group differences in number of muscle synergies, where the elbow pain group had a fewer number of synergies during the ramp phase of gripping and increased number of synergies during the isometric phase of gripping. Further analyses of the spatiotemporal features of forearm muscle synergies showed 1) increased similarity in spatial activation patterns, 2) increased co-contraction of forearm flexors, 3) greater activation magnitude of muscle weightings, and 4) delayed timing of peak activation during ramp phase of gripping in individuals with elbow pain. During the isometric gripping the pain group had lower activation magnitude of muscle weightings.

CONCLUSIONS: These findings indicate that people with chronic elbow pain exhibit a less complex neural strategy for the development of pain-free grip force, which manifests as more similar and simultaneous patterns of forearm muscle activation. In contrast, when maintaining an isometric grip force individual with pain exhibit a more complex neural strategy where a redistribution of muscle activation occurred across synergistic muscles.