Unexpected Elements of Human Force Control and Instances of Preservation after Severe Stroke: Implications for Optimality, Variability and Rehabilitation Technology
Feedback control of human force production involves optimally distributing available computational resources between dual priorities of reliability and energetics. This balance enables the diversity and extent of our daily motor function, but given the neuromuscular system’s susceptibility to damage, such as following stroke, the demands on force control can change dramatically. It is not well understood which elements of force control are preserved after stroke or whether, and with what timescale, the motor system adapts this essential tuning.
This dissertation studied force control by young adults, older adults, and stroke survivors using a visuomotor grip force paradigm and a novel, lever drive wheelchair. We show for the first time that slacking, defined as an unprompted and persistent tendency to reduce muscle force, is a fundamental aspect of force control ideally posed to govern the tuning between efficiency and accuracy. Slacking prompts regular feedback corrections that explain an elusive nonlinearity observed in the noise-to-signal ratio of human force via a novel stochastic model. This explanation yielded key evidence that indeed the motor system dynamically adapts its tuning, both immediately to environmental demands, and in response to long term modulations including aging. Surprisingly, slacking and grip force control are largely preserved following stroke, and appear to preserve this crucial adaptability.
Next, we show for the first time that people with severe stroke retain sufficient force control with their shoulder and elbow to propel lever drive wheelchairs. They exhibit altered elbow muscle activation patters but avoid unhealthy compensation with the trunk or shoulder—supporting the likelihood of therapeutic benefit.
This work has several important implications. It elevates slacking as a major design consideration for machines that physically interact with humans, including rehabilitation exoskeletons, teleoperation systems and devices that employ human force production for control input. It implies that slacking, a hitherto unrecognized but major contributor to human motor variability, might be manipulated to promote movement optimization and rehabilitation. Finally, it motivates devices, such as lever drive wheelchairs, that leverage force control and other capacities preserved following stroke to empower individuals seeking to incorporate therapeutic, functional exercise into their daily lives.