When faced with acute musculoskeletal injuries, the human neuromuscular control system adapts to the imposed restrictions on motion in order to achieve desired kinematic outcomes. In some cases, these adaptations result in alternate compensation strategies that linger after the injury heals. By combining dynamical-systems modeling and clinical experimentation, this work attempts to explore and explain discontinuous changes in neuromuscular control, as well as the possibility of coexisting longitudinal neuromuscular control paths, during recovery from injury. The specific aims are to explore the origins of such nonlinear phenomena in the context of broken symmetry in simple dynamical-systems models, as well as to examine the clinical progression of adaptive changes in neuromuscular control throughout incremental recovery from a simulated ankle injury.

First, two studies are presented which deal with simple dynamical-systems models in which the purposeful introduction of a broken symmetry is interpreted as an injury. In the first study, the dynamics and adaptive compensations of a two-degree-offreedom nonlinear oscillator under harmonic excitation are investigated. The adaptive strategy involves a frequency-dependent adjustment to the excitation amplitudes and phases on each degree of freedom in an attempt to maintain symmetric oscillations. The analysis shows the coexistence of distinct branches of control strategies, including a jump from one branch to the other while healing from a symmetry fault. In the second study, a more general periodic excitation is applied to the same oscillator. The frequency-dependent adjustments made to the harmonic excitation break the shape of the more general periodic excitation, making it impossible to retain symmetry with a global adjustment to amplitude and phase. Nevertheless, the analysis shows that simple adjustments to timing and amplitude commands may suffice when attempting to walk in the presence of injury. Both studies discuss clinical implications to injury rehabilitation and the control of movement during injury.

The final two studies employ a clinical experiment to produce and examine neuromuscular control adaptations with a controlled recovery from ankle motion resistance. The first of these studies examines the compensations developed in response to a reduction in ankle range of motion via increased stiffness of an ankle orthosis. The second study analyzes the progression of compensation strategy changes during a systematic reduction in this stiffness until returning to normal. These studies demonstrate that subjects successfully maintain whole-limb motion during ankle perturbation through a combination of adaptations to kinematic and kinetic strategies. Most of these adaptations return to normal during recovery, but not at the same rate, suggesting a change of neuromuscular control strategies during recovery.

The modeling aspects of this dissertation provide in-depth analyses of broken symmetries in mechanical oscillators, bringing to light the effects of small nonlinearities and multi-harmonic excitations on compensation strategies during recovery from the symmetry fault. Further, the experimental studies extend these abstract findings into clinical relevance by detailing compensations during a simulated recovery from injury. In each case, results suggest that clinicians should consider the possibility that multiple compensation strategies can achieve the same kinematic motor goals, and further that these compensations can follow different paths during recovery.