Cerebral palsy (CP) originates from a brain injury, causing impairments in motor control specific to the individual. These motor impairments manifest themselves differently between individuals, resulting in altered and highly heterogeneous gait patterns. Interventions aimed at improving motor function produce highly variable results between individuals, with some benefiting greatly and others seeing little or even negative impacts. Electromyography (EMG) recordings allow muscle activity to be measured, giving an opportunity to see how the brain is controlling and coordinating movement. Muscle synergies provide a tool to quantify motor control by finding lower-dimensional groups of weighted muscles which are commonly activated during an activity such as walking. In individuals with neurologic impairments such as stoke, spinal cord injury, and CP synergy complexity is reduced, and synergy organization is altered. The goals of this dissertation were to evaluate how motor control is altered in individuals with CP and address some of the challenges in translating muscle synergies into a useful clinical measure.

There are many methodologies for pre-processing EMG data prior to calculating synergies, and these choices can impact clinical interpretations. We found that synergy complexity was sensitive to EMG pre-processing, but that these effects could be mitigated by normalizing to typically-developing (TD) synergies. When a consistent number of synergies was evaluated the muscles recruited within synergies and synergy activations were similar across a range of filtering conditions. In order to use synergies as a measure of motor control, we sought to determine whether muscle synergies were repeatable between days and between centers. We found similar levels of repeatability for both TD children and children with CP, for both synergy complexity and structure of synergy weights when measured across two separate days. We also found similar associations between pre-treatment synergy complexity and post-treatment outcomes in CP at two separate institutions. Across a variety of common interventions, children with motor control closer to that of their TD peers were associated with greater improvements in kinematics and walking speed. These results suggest that synergies provide a repeatable measure of motor control which may be useful in treatment planning.

Common treatments which aim to improve walking function may also impact motor control, either by directly targeting the nervous system or by indirectly targeting the biomechanical constraints of the system. We evaluated changes in muscle synergies after treatment and rehabilitation and found only small changes that were inconsistent between individuals. While current treatments do not consistently alter synergies, we found that those children whose synergy activations that more closely matched TD post-treatment were associated with improved treatment outcomes.

We also applied muscle synergies as an individual model of motor control to musculoskeletal simulation techniques in an effort to improve estimation of muscle activations. Muscle force, and thus muscle activations are used to evaluate a range of clinical questions including muscle contributions to gait and joint loadings. We compared experimental EMG to muscle activations computed using standard musculoskeletal optimization methods and to activations constrained to synergy groupings and for both TD and CP children. We found that constraining modeled activations to muscle synergies did not improve predication of muscle activations for either group. These results suggest that generic musculoskeletal parameters including activation dynamics and musculoskeletal geometry may limit predictions of muscle activations during gait and that constraining muscle activations to synergistic patterns cannot alone improve these estimations.

This dissertation examines methodological considerations pertaining to the calculation of muscle synergies, including demonstrating that muscle synergies can be robustly measured across days and processing decisions. This work provides important evidence for the clinical utility of muscle synergies in CP, demonstrating associations with treatment outcomes across treatments, centers, and muscles measured. This work also suggests that synergies may be a promising target for future treatments in an effort to improve mobility in children with CP.