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Author ORCID Identifier



Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program


Year Degree Awarded


Month Degree Awarded


First Advisor

Graham E. Caldwell

Second Advisor

Richard van Emmerik

Third Advisor

Brian R. Umberger

Fourth Advisor

Nicholas G. Reich

Subject Categories

Biomechanics | Exercise Science | Motor Control | Neurosciences


Learning many daily life motor skills is critical for survival and the quality of living in humans. As children, we develop walking and running patterns to move the body from point A to B without falling, and we learn to grasp a wide variety of objects during activities of daily living. Motor skills can be properly performed by appropriate muscle activations which are controlled by the central nervous system. How does the central nervous system develop and fine-tune its control strategy to learn a new motor skill? The aim of this dissertation was to better understand how human participants alter muscle activities in learning to direct pedal force toward a specified target while performing one-legged pedaling. In Study 1, participants improved their targeting performance after practice with visual feedback. We speculated that post-practice (Ret-I) changes in multiple leg muscle activities compared to before practice (Pre-L) were associated with task constraints and improved performance. To quantify this idea, Study 2 identified non-muscular and muscular contributions to pedal force direction by induced acceleration analysis. Consistent task and mechanical constraints in Pre-L and Ret-I limited how much the direction of individual muscle-induced pedal force contributions could change. Therefore, the CNS could only alter individual muscle contribution magnitude to improve pedal force direction, suggesting the importance of muscle coordination. Taken together, Study 1 and 2 suggest that proper directional end-point force production requires the CNS to implement an appropriate muscle coordination strategy. Pre-L versus Ret-I differences in muscle coordination were identified in Study 3 by muscle synergy analysis. Four muscle synergies were identified, with changes in synergy muscle weighting and synergy activation levels explaining how altered muscle coordination led to better targeting performance. In this case, the use of muscle synergies may offer the CNS an efficient and flexible way to alter pedal force direction quickly in a short-term learning environment.