Preswing knee flexion assistance is coupled with hip abduction in people with stiff-knee gait after stroke

Abstract

Background and purpose: Stiff-knee gait is defined as reduced knee flexion during the swing phase. It is accompanied by frontal plane compensatory movements (eg, circumduction and hip hiking) typically thought to result from reduced toe clearance. As such, we examined if knee flexion assistance before foot-off would reduce exaggerated frontal plane movements in people with stiff-knee gait after stroke.

Methods: We used a robotic knee orthosis to assist knee flexion torque during the preswing phase in 9 chronic stroke subjects with stiff-knee gait on a treadmill and compared peak knee flexion, hip abduction, and pelvic obliquity angles with 5 nondisabled control subjects.

Results: Maximum knee flexion angle significantly increased in both groups, but instead of reducing gait compensations, hip abduction significantly increased during assistance in stroke subjects by 2.5 degrees , whereas no change was observed in nondisabled control subjects. No change in pelvic obliquity was observed in either group.

Conclusions: Hip abduction increased when stroke subjects received assistive knee flexion torque at foot-off. These findings are in direct contrast to the traditional belief that pelvic obliquity combined with hip abduction is a compensatory mechanism to facilitate foot clearance during swing. Because no evidence suggested a voluntary mechanism for this behavior, we argue that these results were most likely a reflection of an altered motor template occurring after stroke.

Figure 1

The experimental setup consisted of a subject walking on a split-belt force treadmill used to measure individual ground reaction forces. Kinematics was measured using motion capture cameras and reflective markers attached to both legs. Each subject wore a powered knee brace to provide knee flexion torque.

Figure 2

The experimental protocol consisted of 3 treadmill-walking stages. The torque calibration test determined the magnitude of the torque used during assistance. During this stage, a range of knee flexion torques was applied to determine an appropriate level of assistance for each subject. After the baseline stage (no assistance), the assistance stage provided constant, open-loop flexion torque assistance during the preswing phase of each step. There were 4 catch trials without assistance in this stage.

Figure 3

Kinematics from a representative control subject. Each trace is an average and 95% CI for 4 steps. Knee flexion torque increases peak knee flexion, hip flexion angle, and toe clearance, but has no apparent effect on frontal plane characteristics.

Figure 4

Kinematics from a representative stroke subject. Each trace is an average and 95% CI for 10 steps. At peak toe height, knee flexion, and hip flexion angle, when compensations are needed the least, more hip abduction occurs.

Figure 5

Key kinematic outcome measures show increase knee flexion of both control (left) and stroke (right) subjects, but a significant increase in hip abduction only for stroke subjects. Horizontal dashed line at zero represents baseline and is used for reference.