Schindler-Ivens S, Brown DA; Department of Physical Therapy and Human Movement Sciences, Northwestern University Feinberg School of Medicine, Chicago, IL USA. s-schindler-ivens@northwestern.edu.

THEORY: The neuromuscular system of individuals with chronic post-stroke hemiparesis can scale the amplitude and duration of muscle activity but cannot appropriately modify muscle activity phasing to adapt to locomotor task demands. PHENOMENON: This theory may explain why many individuals with hemiparesis can alter their gait velocity volitionally and with practice, but have considerably more difficulty performing demanding locomotor tasks such as walking backwards and turning. PURPOSE: This theory was developed to explain why persons post-stroke are capable of adapting locomotor muscle activity appropriately to increasing speed, workload and antigravity posture, while they are not able to adjust the phasing of muscle activity to adapt to reversal of locomotor direction. EVIDENCE: Abnormal phasing of muscle activity contributes to impaired locomotor performance post-stroke. However, despite abnormal phasing under initial conditions, paretic locomotor muscle activity adapts its amplitude and duration appropriately to changes in speed, workload, and antigravity posture. Experiments using pedaling to study hemiparetic locomotion have shown that when pedaling speed was increased, the duration of vastus medialis activity was reduced appropriately in terms of the absolute time that it was active. When an increase in workload was encountered, the amplitude of vastus medialis activity increased appropriately. In each of these cases task-dependent adaptations in muscle activity can be described best as scaling adjustments. EMG amplitude was scaled upward with increasing workload, and EMG duration was scaled down with increasing pedaling speed. An appropriate task-dependent adaptation to reversal of pedaling direction is for some muscles to modify their spatial pattern of activity and for other muscles to preserve their phasing. In studies of forward and backward pedaling, paretic muscles failed to show appropriate phasing responses when pedaling direction was reversed. TESTABLE HYPOTHESES: The theory described here can be tested further by examining muscle activity scaling and phasing during backward walking and fast walking. We hypothesize that individuals with hemiparesis will have difficulty walking backward because of an inability to alter the phasing of muscle activity in a way that is necessary for this task. However, persons post-stroke will transition successfully from slow to fast walking because they are able to scale the amplitude and duration of muscle activity appropriately. IMPORTANCE: This theory may guide the development of novel treatment interventions that focus on shifting the phasing of muscle activity in response to changing environmental demands. FUNDING SOURCES: NICHD/NCMRR 5 R01HD39406-02, NICHD 2 T32 HD007418-10, 1 F32 HD044299-01.

Copyright 2004 by the American Physical Therapy Association

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