Abstract
This study aimed to assess modulation of lower leg muscle reflex excitability and co-contraction during unipedal balancing on compliant surfaces in young and older adults. Twenty healthy adults (ten aged 18–30 years and ten aged 65–80 years) were recruited. Soleus muscle H-reflexes were elicited by electrical stimulation of the tibial nerve, while participants stood unipedally on a robot-controlled balance platform, simulating different levels of surface compliance. In addition, electromyographic data (EMG) of soleus (SOL), tibialis anterior (TA), and peroneus longus (PL) and full-body 3D kinematic data were collected. The mean absolute center of mass velocity was determined as a measure of balance performance. Soleus H-reflex data were analyzed in terms of the amplitude related to the M wave and the background EMG activity 100 ms prior to the stimulation. The relative duration of co-contraction was calculated for soleus and tibialis anterior, as well as for peroneus longus and tibialis anterior. Center of mass velocity was significantly higher in older adults compared to young adults (p<0.001) and increased with increasing surface compliance in both groups (p<0.001). The soleus H-reflex gain decreased with surface compliance in young adults (p= 0.003), while co-contraction increased {(p}_{mathrm{S}mathrm{O}mathrm{L},mathrm{T}mathrm{A}}=0.003 mathrm{a}mathrm{n}mathrm{d} {p}_{mathrm{P}mathrm{L},mathrm{T}mathrm{A}}<0.001). Older adults did not show such modulations, but showed overall lower H-reflex gains (p<0.001) and higher co-contraction than young adults {(p}_{mathrm{S}mathrm{O}mathrm{L},mathrm{T}mathrm{A}}<0.001 mathrm{a}mathrm{n}mathrm{d} {p}_{mathrm{P}mathrm{L},mathrm{T}mathrm{A}}=0.002). These results suggest an overall shift in balance control from the spinal level to supraspinal levels in older adults, which also occurred in young adults when balancing at more compliant surfaces.
Highlights
In upright stance, balance is challenged by gravity and the relatively high position of the body center of mass (CoM) over a small base of support
For all independent variables, we evaluated the effect of surface compliance and age using a two-way mixed model ANOVA with age as between-subjects factor and surface compliance as within-subjects factor
There was no significant effect of surface compliance nor an interaction of surface compliance and age group, on H-reflex amplitude (F(3,51) = 0.221, p = 0.881; F(3,51) = 0.420, p = 0.074, respectively, see Fig. 6b)
Summary
Balance is challenged by gravity and the relatively high position of the body center of mass (CoM) over a small base of support. This challenge increases with impairments in neuromuscular control resulting from age Communicated by Winston D Byblow. In balancing on a rigid surface, moments around the ankle joint instantaneously and proportionally change the position of the center of pressure and therewith cause moments that accelerate the body center of mass (Hof 2007). Moments around the ankle joint change the center of pressure by moving or deforming the support surface.
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