Intracortical inhibition in the soleus muscle is reduced during the control of upright standing in both young and old adults

Abstract

Purpose: In a previous study, we reported that a short-interval intracortical inhibition (SICI) decreases in old but not in young adults when standing on foam vs. a rigid surface. Here, we examined if such an age by task difficulty interaction in motor cortical excitability also occurs in easier standing tasks.

Methods: Fourteen young (23 ± 2.7 years) and fourteen old (65 ± 4.1 years) adults received transcranial magnetic brain stimulation and peripheral nerve stimulation, while they stood with or without support on a force platform.

Results: In the soleus, we found that SICI was lower in unsupported (35 % inhibition) vs. supported (50 %) standing (p = 0.007) but similar in young vs. old adults (p = 0.591). In the tibialis anterior, SICI was similar between conditions (p = 0.597) but lower in old (52 %) vs. young (72 %) adults (p = 0.030). Age and standing with or without support did not affect the Hoffmann reflex in the soleus.

Conclusions: The current data suggest that the motor cortex is involved in standing control, and that its role becomes more prominent with an increase in task difficulty.

Keywords: Aging; Balance; Peripheral nerve stimulation; Short-interval intracortical inhibition; Transcranial magnetic stimulation.

Figures

Fig. 1

Illustration of a unsupported and b supported standing conditions

Fig. 2

Group data for young and old adults’ center of pressure velocity in the anteroposterior direction when standing supported (sup) and unsupported (unsup), showing a significant condition effect (p < 0.001). The horizontal line within the box indicates the median value, the box covers the 25th–75th percentiles, and the whiskers represent the range

Fig. 3

Group data for young and old adults of background EMG in a the soleus muscle (condition effect, p < 0.001) and b the tibialis anterior muscle (interaction effect, p = 0.035). MVC maximum voluntary contraction. The horizontal line within the box indicates the median value, the box covers the 25th–75th percentiles, and the whiskers represent the range

Fig. 4

Representative responses to transcranial magnetic brain stimulation in the soleus muscle of one 23-year-old male and one 62-year-old male subject while standing supported (sup) and unsupported (unsup). Waveforms represent the average of 10 motor evoked potentials in response to an unconditioned test pulse (thin gray line) and to a conditioned test pulse (thick black line) given at an interpulse interval of 2.5 ms. Black arrows indicate the time when the test pulse is given. Note the down-modulation of short interval intracortical inhibition (SICI) when standing unsupported vs. supported in both young and old subjects

Fig. 5

Group data for a short interval intracortical inhibition (SICI) in the soleus during supported and unsupported standing (condition effect, p = 0.007), b SICI in the soleus in young and old adults (no age effect), c intracortical facilitation (ICF) in the soleus, d SICI in the tibialis anterior during supported and unsupported standing (no condition effect), e SICI in the tibialis anterior in young and old adults (age effect, p = 0.030), and f ICF in the tibialis anterior muscle. Greater values for SICI and ICF represent more inhibition and facilitation, respectively. Sup supported and unsup unsupported. The horizontal line within the box indicates the median value, the box covers the 25th–75th percentiles, and the whiskers represent the range