Shoulder muscle activation strategies differ when lifting or lowering a load
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ORIGINAL ARTICLE
Shoulder muscle activation strategies differ when lifting or lowering a load Nicolas A. Turpin1 · Romain Martinez2 · Mickael Begon2 Received: 25 February 2020 / Accepted: 4 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Purpose Lowering a load could be associated with abnormal shoulder and scapular motion. We tested the hypothesis that lowering a load involves different shoulder muscle coordination strategies compared to lifting a load. Methods EMG activity of 13 muscles was recorded in 30 healthy volunteers who lifted and lowered a 6, 12 or 18 kg box between three shelves. Kinematics, EMG levels and muscle synergies, extracted using non-negative matrix factorization, were analyzed. Results We found greater muscle activity level during lowering in four muscles (+ 1–2% MVC in anterior deltoid, biceps brachii, serratus anterior and pectoralis major). The movements were performed faster during lifting (18.2 vs. 15.9 cm/s) but with similar hand paths and segment kinematics. The number of synergies was the same in both tasks. Two synergies were identified in ~ 75% of subjects, and one synergy in the others. Synergy #1 mainly activated prime movers’ muscles, while synergy #2 co-activated several antagonist muscles. Synergies’ structure was similar between lifting and lowering (Pearson’s r ≈ 0.9 for synergy #1 and 0.7–08 for synergy #2). Synergy #2 was more activated during lowering and explained the greater activity observed in anterior deltoid, serratus anterior and pectoralis. Conclusion Lifting and lowering a load were associated with similar synergy structure. In 3/4 of subjects, lowering movements involved greater activation of a “multiple antagonists” synergy. The other subjects co-contracted all shoulder muscles as a unit in both conditions. These inter-individual differences should be investigated in the occurrence of shoulder musculoskeletal disorders. Keywords Electromyography · Eccentric · Ergonomics · Shoulder injuries · Musculoskeletal · intramuscular
Communicated by Toshio Moritani. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00421-020-04464-9) contains supplementary material, which is available to authorized users. * Nicolas A. Turpin [email protected] Romain Martinez [email protected] Mickael Begon [email protected] 1
IRISSE (EA 4075), UFR SHE‑STAPS Department, University of la Réunion, 117 rue du général Ailleret, 97430 Le Tampon, France
School of Kinesiology and Exercise Sciences, University of Montréal, Montreal, QC, Canada
2
Abbreviations BB biceps brachii DeltA deltoid (Anterior part) DeltM deltoid (Middle part) DeltP deltoid (Posterior part) of the deltoid InfE infraspinatus LD latissimus dorsi MVC Maximal voluntary contraction Pect pectoralis major SerrA serratus anterior SubS lower subscapularis SupE supraspinatus TB triceps brachii TraS trapezius (Superior part) TraL trapezius (Lower part) VAF Variance accounted for
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