Voluntary Activation of the Knee Extensors in Chronic Poststroke Subjects

Objective: To assess the extent to which knee extensor muscle weakness in subjects with chronic mild to moderate poststroke hemiparesis is caused by a decreased voluntary activation. Design: Forty community dwelling and ambulant men and women (mean age, 59.8 +/- 5.5 yrs) with residual hemiparesis (19.2 +/- 8.5 mos poststroke) were tested. Torque measurements were performed on a computerized dynamometer and the superimposed electrical stimulation technique was used to assess voluntary activation of the knee extensors in both the paretic and the nonparetic lower limbs. Results: The mean voluntary activation ratio of the knee extensors in the nonparetic and paretic leg was 0.97 +/- 0.04 and 0.86 +/- 0.13, respectively. Subjects who had a greater relative weakness, implying a more pronounced poststroke impairment, also had lower voluntary activation ratios. The mean percentage difference in total torque between the nonparetic and the paretic knee extensors after the electrical stimulation was 36.4% +/- 17.0%. Conclusions: Paretic knee extensor muscle weakness in chronic poststroke subjects is only partially explained by a reduced voluntary activation ability, indicating that other neuromuscular structural or functional factors contribute to poststroke hemiparetic muscle weakness.