Post-activation depression of the soleus H-reflex in the elderly.
OBJECTIVE
This study examined the effect of age on the spinal mechanism post-activation depression (PD) and its relationship with postural sway.


METHODS
Two groups, young (n = 10, 25.2 +/- 5.2 yr) and elderly (n = 10, 74.6 +/- 6.3 yr) participated in this study. Soleus H-reflex amplitude (peak-to-peak EMG) was measured prior to and after a passive dorsiflexion about the ankle (from 120 degrees plantarflexion to 110 degrees plantarflexion at 15 degrees/s). All H-reflex measurements were recorded at an ankle joint angle of 110 degrees. For each group, stimulus intensity was set at 25% of maximal motor response (M-max) and PD measurement intervals were randomly assigned at 0, 1, 2, 3, 4, 5, 10, 15, and 18 seconds post-movement. To ascertain whether PD was related to postural sway, standing sway area (mm2) was also measured. Subjects stood motionless (20 second duration) on a Kistler force platform during two conditions: with and without vision.


RESULTS
A split-plot ANOVA (Group x Interval) indicated significant differences (F(1.18) = 14.80, p < 0.05) in PD between young and elderly, as well as a Group x Interval interaction (F(10, 180) = 16.02, p < 0.05). Simple main effects identified significant (p < 0.05) intervals at 0, 1, 2, and 3 seconds. The elderly had on average 16.1% less H-reflex depression during the first three seconds after muscle activation. Additionally, a split-plot ANOVA (Group x Condition) indicated significant differences (F(1, 18) = 10.72, p < 0.05) in sway area between young and elderly, but regression analysis indicated post-activation depression and postural sway were not significantly correlated.


CONCLUSIONS
It is concluded that the integrity of the Ia-motoneuron synapse as measured with a PD protocol, is different between young and elderly subjects, but that this difference is not related to postural sway.
Robertson
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CT
Motor Control Laboratory, Department of Kinesiology, Program in Neural Science, HPER 112, Indiana University, Bloomington, IN 47405, USA.
Koceja
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D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology*; D012016:Reference Values; D012026:Reflex, Stretch / Q000502:physiology*; D013569:Synapses / Q000502:physiology