Effect of a reduced base of support in standing and balance training on the soleus H-reflex.
The present study provides evidence that a reflex at the segmental level can adapt over a two hour experiment in a functionally appropriate manner in response to a balance training task. Subjects (N=9) received soleus (S) H-reflexes in blocks of seven trials while free standing on a normal base-of-support (NBOS) and while standing on a plafform with a reduced base-of-support (RBOS) in the sagittal plane. During the RBOS condition, the H-reflex served as a postural perturbation. Subjects were instructed to suppress the H-reflex when it was evoked, as an attempt to maintain a balanced state. Background EMG from the S and tibialis anterior muscles, the S M-wave, and stimulus current were maintained at a constant level during the experiment. Subjects initially received a block of NBOS trials, followed by 4 RBOS blocks (training), a second NBOS block, four additional RBOS blocks, and a third NBOS block with the protocol repeated on three different days (D1, D2 and D3) within the same week. The S H/M ratio was depressed 9% upon standing on the RBOS. With training the S H/M ratio decreased by 22% on Dl, 18% on D2 and 6% on D3. The ratio between the H-reflex and background S EMG (H-reflex gain) decreased 10% on D1, 40% on D2 and 23% on D3 when the first NBOS and first RBOS blocks were compared. Due to a slight increase in the S EMG across blocks, the H-reflex gain decreased considerably more across blocks than the H/M ratio. Although the S H/M ratio underwent an 7% decrease from D1 to D3, the differences were not significant. Individually, however, six of the nine subjects decreased their H/M ratios from 12-42% across days. The results may reflect the inception of longer-term adaptations of the segmental stretch reflex system.
University of Florida, Department of Physical Therapy, Gainesville, FL 32610-0154. mtrimble@hp.ufl.edu
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D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D004576:Electromyography; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology; D013663:Teaching; D013997:Time Factors