Motor Modules Are Largely Unaffected By Pathological Walking Biomechanics: A Simulation Study

BackgroundMotor module (a.k.a. muscle synergy) analysis has frequently been used to provide insight into changes in muscle coordination associated with declines in walking performance, to evaluate the effect of different rehabilitation intervention, and more recently, to control exoskeletons and prosthetic devices. However, it remains unclear whether changes in muscle coordination revealed via motor module analysis stem from pathological walking biomechanics or pathological neural control. This distinction has important implications for the use of motor module analysis for rehabilitation interventions and device design. Thus, this study aims to elucidate the extent to which motor modules emerge from pathological walking biomechanics. MethodsWe conducted a series of computer simulations using OpenSim Moco to simulate abnormal biomechanics by manipulating speed, asymmetry, and step width in a three-dimensional musculoskeletal model. We extracted motor modules using nonnegative matrix factorization from the muscle activation from each simulation. We then examined how alterations in walking biomechanics influenced the number and structure of extracted motor modules and compared the findings to previous experimental studies. ResultsThe motor modules identified from our simulations were similar to those identified from previously published experiments of non-pathological walking. Moreover, our findings indicate that the same motor modules can be used to generate a range of pathological-like waking biomechanics by modulating their recruit timing over the gait cycle. These results contrast with experimental studies in which pathological-like walking biomechanics are accompanied by a reduction in motor module number and alterations in their structure. ConclusionsThis study highlights that pathological walking biomechanics do not necessarily require pathological motor modules. In other words, changes in number and structure of motor modules can be a valuable indicator of alterations in neuromuscular control and may therefore be useful for guiding rehabilitation interventions and controlling exoskeletons and prosthetic devices in individuals with pathological walking function.