Muscle synergies in different physiological demands during rowing

Shaharudin, Shazlin
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University of Delaware
Muscles synergy is a strategy of central nervous system (CNS) to improve redundancy at musculoskeletal level. The study of muscle synergy and its association to energy capacity is crucial for rowing as huge muscle mass are recruited during high intensity exercise. Due to close link that exists between the state of energy supply and types of muscle fibers being recruited, the muscle synergy was hypothesized to enhance rowing economy and further improve rowing performance. Although the robustness of muscle synergy has been extensively studied across tasks, mechanical constraints, training effect, and posture, the robustness of muscle synergy across different physiological demands was still an open question. Therefore, this body of work was designed to fulfill the gap in muscle synergy literature. The pilot study was the starting point to evaluate the muscle synergy in two different stretcher mechanisms (i.e fixed and slides ergometer) in untrained subjects during 6 min maximal rowing test. Untrained subjects were chosen to avoid the training effect bias on synergy. The same protocol was further repeated with collegiate rowers. As the slides system provides a close resemblance to on-water rowing, Wingate anaerobic and VO2 max test were conducted on slides ergometer for both untrained subjects and collegiate rowers. Wingate anaerobic test was an assessment of anaerobic power and VO2 max test was applied to evaluate aerobic capacity. As a power endurance sport, both energy pathways (i.e aerobic and anaerobic) were crucial for maximum rowing performance. The 6 min maximal rowing test was a simulation of a typical rowing event where interplay of all energy pathways were highlighted. Muscle synergy was extracted from 16 rowing specific muscles using Principal Component Analysis with varimax rotation. Parallel Analysis (PA) and Minimum Average Partial (MAP) were computed to further enhance the extraction method. Surface electromyography, kinematics, rowing performance, and energy metabolism were quantified from 10 collegiate rowers and 10 physically active untrained (e.g not specifically trained in rowing) subjects. All rowing tests were conducted on Concept 2 rowing ergometer. Appropriate statistical tests were applied to find the association of muscle synergy and rowing economy and its effect on rowing performance. Three muscle synergies were sufficient to explain the majority of variance for both untrained and rowers groups across three rowing tests (e.g. Wingate anaerobic test, VO2 max test, 6 minutes maximal rowing test). Despite small differences in muscle contributions to specific synergy, overall, for both subject groups, Synergy #1 was activated during the first half of the drive phase, Synergy #2 was engaged during the second half of the drive phase and Synergy #3 was predominant during the transition of strokes positions (e.g from recovery to catch). Synergy #1 always gained largest contribution from the leg, back and chest muscles, Synergy #2 was typically made of upper limb muscles and synergy #3 comprised of Rectus Abdominis (AB), Rectus Femoris (RF) and Tibialis Anterior (TA) with minor variations in different experimental tests. Based on these results, we were able to show that muscle synergy is robust across different physiological demands. Through the statistical analysis, we found that Synergy #1 (which comprised about half of total variance accounted for all synergies) was highly correlated to rowing economy. However, rowing economy is not the main predictor of rowing performance as revealed by Multiple Linear Regression. Both collegiate rowers and untrained groups exhibited similar rowing strategy in different stretcher mechanisms. They tended to row faster with shorter strokes when rowing on slides ergometer (SE), but slower with longer strokes when rowing on fixed ergometer (FE). However, when compared across the groups in similar rowing condition (i.e SE rowing), the rowers tended to row slower with longer strokes compared to untrained subjects. This strategy is an indicator of practice-related adaptation that was suggested to reduce energy cost. Due to this rowing technique, the rowers were able to exert more energy with better rowing economy compared to the untrained subjects in any tests. The results proved the flexibility of muscle synergy to adapt to the mechanical constraints (e.g different stretcher mechanisms) and different physiological demands. The findings of this study could guide the rowers and their coaches to enhance the training regime. As there is no difference in muscle synergy pattern and rowing performance during rowing on FE and SE, both ergometers could be utilized by the experienced rowers. Expertise in rowing is not related to sequence of synergies activation but to the ability to adjust the muscle activation economically. As rowers have to sustain high aerobic intensity during a rowing event, they could apply our findings by focusing the training on the synergies refinement (particularly Synergy #1), which will improve their rowing economy.