Thesis Defense Meeting: Elizabeth Bell
Title: Muscular Fatigue Influences Motor Synergies During Push-ups
Abstract: Motor redundancy is common in human movements, as a same motor task can be accomplished in an infinite number of ways due to extra degrees of freedom within the body system. The mathematical problem of motor redundancy has recently been redefined as the principle of motor abundance. This principle suggests that having many motor solutions allows the central nervous system (CNS) to create flexible solutions, called synergies, for a given motor task. These synergies are organized with elemental variables (inputs) that co-vary to stabilize a fewer number of task-specific performance variables (outputs). It is unclear how the CNS uses these flexible solutions to overcome challenges imposed during motor tasks. Muscular fatigue decreases the ability or capacity of muscles to produce high force which would inherently change available motor solutions. The conventional push-up is a popular exercise often used to test muscular endurance of the upper extremities against fatigue. Specifically, this motor task requires controlled upward and downward movement of the body. This movement requires a significant amount of power, or energy transfer, to be generated or absorbed by the upper extremities. Energy transfer can theoretically be achieved through an infinite combination of joint actions, which presents the problem of motor redundancy/abundance to the CNS. This research used the push-up action as an experimental paradigm for the study of adaptations in motor synergies throughout muscular fatigue. Eleven young adults were recruited from the University of Maryland, College Park, and were asked to repeatedly perform push-up movements at a controlled pace until he or she failed to perform the next repetition. Spectral analysis and tracking of muscle activation in each repetition indicated that performance of push-ups to volitional fatigue induced muscular fatigue of the bilateral triceps brachii and pectoralis major muscle groups. It was expected that fatigue would lead to a strengthening of motor synergies between and within-limbs to overcome the reduced ability of the fatigued muscles to perform the task. Although the CNS initially performed push-ups using positive between and within-limb joint power synergy, the magnitude of synergies reduced throughout fatigue. Fatigue was expected to lead to differential changes in synergies between eccentric and concentric movement phases. Eccentric actions were expected to not require motor adaptions due to the assistance of gravity and greater available force production. Although synergy reductions were observed in each movement, the between and within-limb synergy reduced at a greater rate throughout concentric movements. The CNS was not able to adapt control strategies, but instead the dominant strategy was affected throughout fatigue.