Motor Learning and Control

Robot-mediated therapy for motor rehabilitation following stroke remains a compelling complement to traditional physical therapy. It is well established that best practice for robot-mediated therapy is to engage patients in exercises that belong to either the active or active-assistive modalities. These modalities ensure that robotic assistants provide an appropriate amount of ‘assistance as needed’, helping the patient complete important functional movements while maintaining the patient’s engagement, which is necessary to optimally stimulate neuroplasticity. Working against a patient’s engagement in an exercise is the persistent tendency of their motor system to minimize its own effort—a process that has come to be known as ‘slacking’.

Smith et al. (2018b)

Before engineers can develop new technologies that interact with the human body and support mobility and rehabilitation, the function and behavior or the human body itself must be sufficiently understood. In the REACT Lab, we use video games controlled by human movement and muscle force to study behavior that cannot otherwise be measured.

For instance, in one of our studies, we determined that the human body is fundamentally a ‘slacker’. That is, the body’s muscles always and automatically reduces their own contraction force, without us noticing, even when it feels like muscle force is being held steady.

Our research implies that when humans produce sustained forces, such as when carrying a coffee mug, the body relies on various modes of feedback, such as slip detection, to prompt regular corrections for slacking. Supposing those corrections can be interrupted, such as when we get distracted, this discovery might be the best evidence to date for the importance of a good cellphone case.


Smith, B. W., Rowe, J. B., & Reinkensmeyer, D. J. (2018). Real-time slacking as a default mode of grip force control: implications for force minimization and personal grip force variability. J Neurophysiology, 120(5), 2107–2120.

Smith B. W., Rowe J. B., & Reinkensmeyer D. J. (2018). Directly measuring the rate of slacking as stroke survivors produced isometric forces during a tracking task. 40th Annual International Conference of the IEEE Engineering in Medicine and Biology Society.