Effects of velocity on maximal torque production in poststroke hemiparesis

Abstract

Impaired torque production is a major physical impairment following stroke, and has been studied extensively in isometric conditions. However, functional use of a limb requires torque production during movement, and the effects of velocity on maximal torque production may be abnormally enhanced in the paretic limb. The purpose of this study was to quantify the effects of movement velocity on maximal torque production during isokinetic, concentric flexion and extension of the elbow in poststroke subjects. Three speeds were tested (30, 75, 120 deg/s) over a 100-deg range of motion. To control for strength variations between subjects and limbs, isokinetic torques were normalized by peak isometric torque. As flexion velocity increased, paretic limb torque decreased at a greater rate than in the unaffected limb. During extension, paretic limb torque was much lower than torque in the unaffected limb at all speeds. In both flexion and extension, the disparity between limbs in the constant-velocity torque-angle curves became more pronounced as velocity increased. Torque decreased 44% +/- 7% in flexion and 63% +/- 9% in extension as velocity increased from 30 to 120 deg/s, whereas the corresponding decreases in the unaffected limb were only 9% +/- 5% in flexion and 16% +/- 4% in extension. No electromyographic (EMG) abnormalities were observed during flexion. During extension, EMG data provided evidence for abnormally increased antagonist coactivation in brachioradialis and markedly reduced activation in triceps as potential contributors to the decreased extension torques. The finding that movement velocity produces large deficits in maximal torque might explain why functional use of the paretic limb is often impaired even though isometric strength appears adequate.

Factors That Influence Muscle Weakness Following Stroke and Their Clinical Implications: A Critical Review

“…18 At low velocity (30 /s), no force deficits were found in the paretic biceps or quadriceps muscles relative to the non-paretic side or controls, but force deficits were evident in the paretic hamstrings and triceps muscles. 57,[63][64][65] Similar decreases in force as velocity increased from 30 /s to 120 /s or 180 /s were found in the paretic and nonparetic quadriceps and triceps, 57,64 indicating that the force-velocity relationship was maintained for these muscles or, at least, was maintained at these velocities. Although no difference in the force-velocity relationship was found in the elbow extensors, greater deficits as velocities increased were found in the elbow flexors.…”

“…Although no difference in the force-velocity relationship was found in the elbow extensors, greater deficits as velocities increased were found in the elbow flexors. 57 Lum and colleagues 57 also found that when the elbow flexors were moved from extension to flexion, there were no deficits when the muscle was lengthened; torque deficits were found only late in flexion, when the elbow flexors were shortened. In some cases, the extent of the impairment following stroke was so severe that movement velocities of 300 /s could not be produced, especially in knee flexion.…”

“…Muscle weakness is characteristic of people following stroke, but greater force deficits are evident when the muscle fibres are shortened. 12,[56][57][58] Force deficits have been observed in a shortened position in the elbow flexors, elbow extensors, knee flexors, and knee extensors 12,57 (see Table 2). It has been suggested that the reason for the deficits in a shortened position 12,56-58 is a preference to use the muscle in the strongest position, halfway between extension and flexion, which results in relative disuse of the muscle in other positions.…”

“…Similar trends have been found after stroke, such that as velocity increases, less force is generated in the paretic muscles. 57,63,64 After stroke, the studies reviewed found greater force deficits in concentric contractions 65,66 than in isometric contractions, 32,50,63 and smaller force deficits in eccentric contractions than in concentric and isometric contractions 65,66 (see Table 2). There is an indication that less neural drive is used for eccentric contractions, which may explain why greater force was preserved; similar results have been found in older adults.…”

Retraining of interjoint arm coordination after stroke using robot-assisted time-independent functional training

“…Reaching and grasping movements are components of many activities of daily living (ADLs) and are often impaired after stroke. Studies have reported a variety of impairments related to reaching and grasping, including decreased muscle activation and weakness [9][10][11][12], disrupted interjoint coordination [13][14][15][16], decreased smoothness of movement [17][18], and dyscoordination between reach and grasp movements [19].…”