Force variability is a potential biomarker of motor impairment in hemispheric stroke survivors

During voluntary isometric contractions of upper extremity muscles in individuals with chronic stroke, the magnitude of force variability appears to increase consistently as force increases. However, research on how such force variability changes with increasing motor impairment remains limited. This study aims to determine whether force variability is increased on the paretic side during either index finger abduction or elbow flexion in the same group of stroke survivors, and whether these changes are consistent across different muscles. Force variability was assessed using the standard deviation of force during sustained isometric contractions. Linear mixed-effects models were implemented to test whether force variability is changed on the paretic side post stroke, and whether such alterations show dependence on force level and on the degree of impairment. The results demonstrated a significant increase in force variability on the paretic side across force levels during finger abduction, while force variability for elbow flexion was increased only at high force levels. In addition, the force variability appears to increase as isometric elbow flexion force increases, whereas no clear trend was found during index finger abduction. The increase in force variability demonstrated moderate-strong dependence on the reduction in maximum muscle strength on the paretic side during elbow flexion, suggesting that monitoring force variability could potentially serve as a quantitative diagnostic tool for assessing severity of impairment in motor control and for raising potential mechanisms at the motor unit level.