HIP JOINT POSITION AFFECTS VOLITIONAL KNEE EXTENSOR ACTIVITY POST-STROKE.

Abstract

Purpose/Hypothesis: Individuals post-stroke often exhibit significant functional limitations due to either inadequate or inappropriate volitional muscle activation. In the lower extremity this is particularly apparent during ‘spastic paretic stiff-legged gait’, which has been attributed to excessive quadriceps activity during the stance to swing transition. The mechanism underlying the altered volitional activation of the knee extensors has yet to be elucidated; however, evidence from animal and human models has demonstrated the importance of hip angle sensors in modulating lower extremity muscle activity. Although the hip's position has become a focus for retraining ‘normal’ walking patterns following stroke, there are as of yet, no quantitative studies which examined the influence of hip angle afferent information during isolated volitional muscle activation after stroke. The purpose of this study was therefore to quantify the role of hip joint positioning on the volitional activation of the uniarticular knee extensor muscles after stroke. It was hypothesized that greater hip flexion would yield greater quadriceps activity. Number of Subjects: Fourteen subjects with chronic (> 1 yr) stroke were recruited for testing. Materials/ Methods: Subjects were positioned on a Biodex dynamometer to record the electromyographic (EMG) activity from the uniarticular knee extensor muscles [vastus lateralis (VL) and vastus medialis (VM)] during maximum voluntary isometric knee extension contractions. The knee was fixed at 60°, while hip position was fixed by altering the subject's posture in random order to include: sitting upright (90°), semi-reclined (45°), and supine (0°). EMG data was full wave rectified and low pass filtered to produce a linear envelope. The outcome variable of interest was the integrated EMG activity around peak torque production. A two-way (muscle and hip position) repeated-measures ANOVA (repeated for hip position) was used to determine the effect of varying hip posture on knee extensor activity. Results: Hip position had a substantial effect on the amount of volitional VL and VM muscle activity produced (p=0.07). With the hip positioned in full extension, both muscles were volitionally activated significantly less than when the hip was positioned in flexion (p=0.03). Conclusions: The angle of the hip joint appears to play a significant role in the volitional activation of the uniarticular knee extensors, although more data is required. Patients with stroke tend to walk with the hips in greater flexion, which may facilitate activation of the knee extensors contributing to ‘stiff knee gait’ pattern. Clinical Relevance: Hip joint angle contributes to the modulation of volitional quadriceps activation and likely influences reflexive/spastic activation contributing to inappropriate muscle activity during functional activities. A strong emphasis should therefore be placed on hip joint posturing during gait retraining to assist with appropriate muscle activation patterns.