The Step Response in Isometric Grip Force Tracking: A Model to Characterize Aging-and Stroke-Induced Changes

This paper aimed to construct a model to represent dynamic motor behavior to quantitatively investigate aging-and stroke-induced changes and, thus, to explore the underlying mechanisms of grip control. Grip force tracking tasks were conducted by stroke patients, age-matched healthy controls, and healthy young adults at 25%, 50%, and 75% maximum voluntary contractions (MVC), respectively. Sensorimotor control of the tracking task was modeled as the step response of a second-order system. The results revealed that aging had no significant effect on the parameters of the model, whereas significant differences were found between the age-matched control and stroke groups. Target force level significantly affected the damping ratio and natural frequency in the young group, and significantly affected the damping ratio in the stroke group. Significant correlations were found between the Wolf motor function test score and damping ratio, natural frequency, and settling time at 25% MVC. The model could describe the stroke-induced oscillation and slow response in dynamic grip force control and has the potential to be a quantitative evaluation of motor disabilities in clinic.