Neuromuscular shoulder activity during exercises with different combinations of stable and unstable weight mass

Omar Baritello; Mina Khajooei; Tilman Engel; Stephan Kopinski; Andrew Quarmby; Steffen Mueller; Frank Mayer

doi:10.1186/s13102-020-00168-x

Neuromuscular shoulder activity during exercises with different combinations of stable and unstable weight mass

BMC Sports Sci Med Rehabil. 2020 Mar 26:12:21. doi: 10.1186/s13102-020-00168-x. eCollection 2020.

Authors

Omar Baritello¹, Mina Khajooei¹, Tilman Engel¹, Stephan Kopinski¹, Andrew Quarmby¹, Steffen Mueller², Frank Mayer¹

Affiliations

¹ 1Clinical Exercise Science, Department Sports and Health Science Medicine, University Outpatient Clinic Potsdam, University of Potsdam, Am Neuen Palais 10, House 12, 14469 Potsdam, Germany.
² 2Computer Science and Therapy Science, Trier University of Applied Science, Schneidershof, 54293 Trier, Germany.

Abstract

Background: Recent shoulder injury prevention programs have utilized resistance exercises combined with different forms of instability, with the goal of eliciting functional adaptations and thereby reducing the risk of injury. However, it is still unknown how an unstable weight mass (UWM) affects the muscular activity of the shoulder stabilizers. Aim of the study was to assess neuromuscular activity of dynamic shoulder stabilizers under four conditions of stable and UWM during three shoulder exercises. It was hypothesized that a combined condition of weight with UWM would elicit greater activation due to the increased stabilization demand.

Methods: Sixteen participants (7 m/9 f) were included in this cross-sectional study and prepared with an EMG-setup for the: Mm. upper/lower trapezius (U.TA/L.TA), lateral deltoid (DE), latissimus dorsi (LD), serratus anterior (SA) and pectoralis major (PE). A maximal voluntary isometric contraction test (MVIC; 5 s.) was performed on an isokinetic dynamometer. Next, internal/external rotation (In/Ex), abduction/adduction (Ab/Ad) and diagonal flexion/extension (F/E) exercises (5 reps.) were performed with four custom-made-pipes representing different exercise conditions. First, the empty-pipe (P; 0.5 kg) and then, randomly ordered, water-filled-pipe (PW; 1 kg), weight-pipe (PG; 4.5 kg) and weight + water-filled-pipe (PWG; 4.5 kg), while EMG was recorded. Raw root-mean-square values (RMS) were normalized to MVIC (%MVIC). Differences between conditions for RMS%MVIC, scapular stabilizer (SR: U.TA/L.TA; U.TA/SA) and contraction (CR: concentric/eccentric) ratios were analyzed (paired t-test; p ≤ 0.05; Bonferroni adjusted α = 0.008).

Results: PWG showed significantly greater muscle activity for all exercises and all muscles except for PE compared to P and PW. Condition PG elicited muscular activity comparable to PWG (p > 0.008) with significantly lower activation of L.TA and SA in the In/Ex rotation. The SR ratio was significantly higher in PWG compared to P and PW. No significant differences were found for the CR ratio in all exercises and for all muscles.

Conclusion: Higher weight generated greater muscle activation whereas an UWM raised the neuromuscular activity, increasing the stabilization demands. Especially in the In/Ex rotation, an UWM increased the RMS%MVIC and SR ratio. This might improve training effects in shoulder prevention and rehabilitation programs.

Keywords: EMG; Instability; Overhead athlete; Rotator cuff; Unstable resistance training; Water pipe.