Stroke increases motor output variability during maximal voluntary isometric contractions of knee extensor muscles in adults

Greater motor output variability (MOV, inconsistency and unsteadiness) during submaximal tasks post stroke is associated with poorer motor performance. However, little is known about MOV during maximal tasks and relationships with clinical measures of motor performance among stroke survivors. This study determined inconsistency (across discrete attempts) and unsteadiness (within a single attempt) during knee extension maximal voluntary isometric contractions (MVICs) in stroke survivors and neurotypical controls. Forty-three stroke survivors (28 female) and 31 age-matched neurotypical controls (15 female) performed a minimum of five knee extension MVICs with paretic and non-paretic legs (stroke) or dominant leg (controls). Inconsistency was calculated as coefficient of variation between maximal torque values of each MVIC. Unsteadiness was calculated as the average from coefficient of variation of the torque values during each MVIC. The paretic leg of stroke survivors had greater MVIC inconsistency (7.61 ± 4.31% vs. 4.94 ± 2.77%, p = 0.014) and unsteadiness (7.00 ± 3.16% vs. 3.44 ± 1.68%, p < 0.001) than the dominant leg of neurotypical controls. Greater MVIC inconsistency (r_s = -0.374, p = 0.016) and unsteadiness (r_s = -0.445, p = 0.004) of stroke survivors' paretic leg were associated with lower Fugl-Meyer Assessment-Lower Extremity motor scores. Stroke increases MOV during a single session of MVIC measurements with multiple attempts and is associated with poorer clinical measures of motor performance.