Abstract
IntroductionAn internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.MethodsWe compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n = 11, age 19–24 years), middle-aged (n = 10, age 37–56 years), and older healthy adults (n = 11, age 63–78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120 ms, 100 ms, 80 ms, 60 ms, 40 ms, 20 ms, and 0 ms before as well as 20 ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.ResultsAll subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.ConclusionBecause APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs.
Highlights
An internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs)
We found that the stereotypical H reflex depression associated with an APA in younger adults was reduced in middle-aged adults and reversed to facilitation in healthy older adults
The average (14%) and peak (29%) depression in the H reflex (Fig. 2) during APAs qualitatively agrees with the pattern of depression reported previously (Kasai and Kawai 1994; Kawanishi et al 1999)
Summary
An internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). Spinal excitability measured by the Hoffmann (H) reflex during this silent period declines sharply with the reflex depression peaking around the onset of EMG activity in the anterior deltoid (Kasai and Kawai 1994; Kawanishi et al 1999). Because the reflex depression starts before the onset of anterior deltoid EMG activity, this anticipatory depression is likely to be the result of a preprogrammed descending motor command acting on the motoneuron directly or via Ia inhibitory interneurons, giving rise to a feedforward control of APAs in standing and during walking (Capaday and Stein 1986; Simonsen et al 1995)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
