Collaborative Research: Elucidating the contributions of nonlinearities in musculotendon properties to enabling locomotion in unpredictable environments.

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For animals that locomote on uneven and diverse terrains, the ability to negotiate obstacles that can impede their forward progression and can cause instability is critical to survival. In many cases, the musculoskeletal system handles these obstacles by itself, independent of control by the nervous system, but the exact mechanisms by which this occurs are not thoroughly understood. The overall objective of this proposal is to elucidate how specific features within the musculoskeletal system achieve the appropriate locomotor behaviors without needing reflex interventions from the nervous system. To achieve this objective, the researchers will study bipedal hopping by kangaroo rats within the context of abrupt changes in slope that can disrupt forward velocity and/or cause unwanted rotational pitch of the body. Novel experiments using a specially designed rotational treadmill will make it possible to expose kangaroo rats to controlled perturbations that mimic ecologically relevant obstacles. The conceptual framework of the studies is that the components of the musculoskeletal system represent an embedded intelligence that can ease the computational burden of centralized controllers for complex dynamic systems. We advance this idea by investigating specific examples in which specialized properties link functional demands, conditional properties, and system state. Translation of the findings can solve long-standing challenges for robots moving stably over variable terrains. Furthermore, the project will involve local high school teachers, who will develop, test, implement, and assess teaching modules based upon the ongoing research to encourage pre-college students to appreciate how the study of basic biology and engineering applications are complementary. The objective of this project is to elucidate how nonlinearities within the musculoskeletal system express contextually appropriate system properties to achieve desired motor function without needing neural feedback. The approach is to study bipedal hopping by kangaroo rats with abrupt changes in slope that can disrupt forward velocity and/or cause unwanted pitch. More specifically, the research will examine how the actions of ankle extensors are coupled to the mechanics of the feet, and how this coupling is modulated by the nonlinear features of tendon. This coupling is hypothesized to provide the functional benefit of automatically making the foot compliant upon landing to mediate perturbations and then stiff at takeoff to enable propulsion. To test this hypothesis, the research team will use an integrated framework of in vivo, in situ, and in silico methods to obtain behavioral data, test mechanistic hypotheses, and manipulate component properties, respectively. A novel custom-designed rotational treadmill will make it possible to have controlled perturbations that mimic ecologically relevant obstacles. More generally, nonlinearities expressed by a system’s components can expand a system’s range of operating conditions and contexts. The findings will provide guidance for the design of nonlinearities, namely, their type and parameterization, so that the nonlinearities will be advantageous for achieving function while interacting with unpredictable environments. By collaborating with local high school teachers, the PIs will build a network of STEM teachers who will develop, test, implement, and assess teaching modules based upon the research. In addition, the project will provide interdisciplinary research training and mentoring for two graduate students and one postdoctoral fellow.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.