Introduction Motor control during human grip is complex, due to the seemingly high redundancy of muscles and numerous degrees of freedom.2 Muscle synergies, are defined as a group of muscles that work in synchrony to complete a task, and have been proposed as a way of simplifying the complex neuromuscular control of human grip.2 Lateral epicondylalgia (LE) is pain provoked at the lateral elbow during gripping that is associated with motor system impairments.1 This study investigated whether coordination (muscle synergies) of forearm muscles during dynamic gripping differ between people with and without unilateral LE.
Methods Intramuscular electromyography (EMG) was used to record activity from extensor carpi radialis brevis (ECRB) and longus (ECRL), extensor digitorum communis (EDC), flexor digitorum superficialis (FDS) and profundus (FDP), and flexor carpi radialis (FCR) in 12 participants with LE and 14 age-matched painfree controls. Participants performed 15 consecutive repetitions of painfree grip on the affected side at 20% of the maximum grip strength (recorded for the unaffected side in the LE group) in four positions; shoulder neutral with elbow flexed to 90° and forearm pronated or neutral, and shoulder flexed to 90° with elbow extended and forearm pronated or neutral. Muscle synergies were extracted using non-negative matrix factorisation for each position and participant, independently. Muscle synergies for the control group were used to reconstruct EMG patterns for each LE participant in each limb position to enable comparison of synergies between groups. A mixed effects repeated measures ANOVA was used to compare the Variance Accounted For (VAF), which reflects the variance between the control synergies and that of the LE participants.
Results Independent analysis performed on each position and participant demonstrated that two muscle synergies accounted for > 97% of the VAF values for the LE and control group. The degree to which FDS muscle activity contributed to synergy #1 and #2 differed between groups. The VAF values obtained when reconstructing EMG patterns of patients with LE from the synergies of control participants were lower (main effect group: p < 0.001) than when reconstructing the EMG patterns of control participants. However, VAF values remained high (range: 83.3 ± 6.8–89.2 ± 5.3%) depending on the position. Upper limb position did not influence VAF values in people with LE or control participants (interaction group x position p = 0.41).
Discussion Overall there are differences between the muscle synergies of people with LE and control participants, but the difference is small. The FDS muscle exhibited different activation patterns between people with LE and control participants for the two identified muscle synergies, which were not influenced by differences in upper limb position.
References 1 Alizadehkhaiyat, et al. J Orthop Res. 2007;25:1651–57
2 Castellini, et al. Bio Cybern. 2013;107:233–45