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Motor systems show an overall robustness, but because they are highlynonlinear, understanding how they achieve robustness is difficult. In manyrhythmic systems, robustness against perturbations involves response of boththe shape and the timing of the trajectory. This makes the study of robustnesseven more challenging.  To understand how a motor system produces robust behaviors in a variableenvironment, we consider a neuromechanical model of motor patterns in thefeeding apparatus of the marine mollusk \textit{Aplysia californica}\citep{shaw2015,lyttle2017}. We established in \citep{WGCT2021} the tools forstudying combined shape and timing responses of limit cycle systems undersustained perturbations and here apply them to study robustness of theneuromechanical model against increased mechanical load during swallowing.Interestingly, we discover that nonlinear biomechanical properties conferresilience by immediately increasing resistance to applied loads. In contrast,the effect of changed sensory feedback signal is significantly delayed by thefiring rates' hard boundary properties. Our analysis suggests that sensoryfeedback contributes to robustness in swallowing primarily by shifting thetiming of neural activation involved in the power stroke of the motor cycle(retraction). This effect enables the system to generate stronger retractormuscle forces to compensate for the increased load, and hence achieve strongrobustness.  The approaches that we are applying to understanding a neuromechanical modelin \textit{Aplysia}, and the results that we have obtained, are likely toprovide insights into the function of other motor systems that encounterchanging mechanical loads and hard boundaries, both due to mechanical andneuronal firing properties.

Variational and phase response analysis for limit cycles with hard boundaries, with applications to neuromechanical control problems