Modulation of EMG-EMG Coherence in a Choice Stepping Task

Abstract

The voluntary step execution task is a popular measure for identifying fall risks among elderly individuals in the community setting because most falls have been reported to occur during movement. However, the neurophysiological functions during this movement are not entirely understood. Here, we used electromyography (EMG) to explore the relationship between EMG-EMG coherence, which reflects common oscillatory drive to motoneurons, and motor performance associated with stepping tasks: simple reaction time (SRT) and choice reaction time (CRT) tasks. Ten healthy elderly adults participated in the study. Participants took a single step forward in response to a visual imperative stimulus. EMG-EMG coherence was analyzed for 1000 ms before the presentation of the stimulus (stationary standing position) from proximal and distal tibialis anterior (TA) and soleus (SOL) muscles. The main result showed that all paired EMG-EMG coherences in the alpha and beta frequency bands were greater in the SRT than the CRT task. This finding suggests that the common oscillatory drive to the motoneurons during the SRT task occurred prior to taking a step, whereas the lower value of corticospinal activity during the CRT task prior to taking a step may indicate an involvement of inhibitory activity, which is consistent with observations from our previous study (Watanabe et al., 2016). Furthermore, the beta band coherence in intramuscular TA tended to positively correlate with the number of performance errors that are associated with fall risks in the CRT task, suggesting that a reduction in the inhibitory activity may result in a decrease of stepping performance. These findings could advance the understanding of the neurophysiological features of postural adjustments in elderly individuals.

Keywords: EMG-EMG coherence; anticipatory postural adjustment; corticospinal excitability; elderly; risk of fall.

Figure 1

(A) Visual imperative stimuli used in each task (SRT, simple reaction time and CRT, choice reaction time). The CRT task consisted of a right arrow on the right side and a left arrow on the left side. (B) Examples of force plate data for forward step execution with the right leg. The top figure shows a non-erroneous CRT (CRT_noerr) trial, and the bottom shows an erroneous CRT (CRT_err) trial. The vertical line indicates timing of the visual imperative stimulus. The erroneous CRT trials were identified by an initial incorrect center of pressure (COP) shift toward the stance leg.

Figure 2

Mean reaction time (RT), anticipatory postural adjustment (APA) amplitude, and baseline sway are shown for each condition (SRT, CRT_noerr and CRT_err).

Figure 3

Representative time frequency analyses of coherence (intramuscular tibialis anterior (TA), intermuscular TA and intermuscular soleus (SOL)) are shown for SRT and CRT.

Figure 4

(A) Rectified electromyography (EMG) signals during a stepping task in a representative subject. Each EMG signal was segmented into epochs of 1000 ms that corresponded to the fixed period of paired muscle activity prior to the imperative stimulus. The EMG activities of proximal and distal TA muscle and SOL muscle were used in each coherence analysis. The vertical and shaded dashed box indicate the analysis periods. (B) A representative example of EMG-EMG coherence in the muscle pairs (intramuscular TA, intermuscular TA and intermuscular SOL). The horizontal dashed lines indicate the 95% confidence limit of coherence. The calculation of phase spectrum was implemented as the argument of the cross-spectrum for the estimation of the timing relationships between the EMG signals, and this figure showed the subplot that is inserted in the coherence plot. The phase spectrum is defined over the range, and the y axis is showed in radians. The phase spectrum is valid in the frequency band in which the coherence shows significantly increase, and hence, only those regions are illustrated in the phase plots.

Figure 5

Mean coherence of each paired EMG signal (intramuscular TA, intermuscular TA and intermuscular SOL) in each task (SRT and CRT). The 95% confidence level is presented with horizontal dashed lines.

Figure 6

Comparison of average alpha (A) and beta (B) coherence across three paired muscles (intramuscular TA, intermuscular TA and intermuscular SOL) in each task (SRT and CRT). For the alpha band, the average coherence of all paired muscles in the SRT task was significantly higher than the CRT task. For the beta band, there were also significant differences in the average coherence for all paired muscles in the SRT task compared with the CRT task.

Figure 7

Relationship between beta intramuscular TA coherence and the number of APA errors (Spearman’s R² = 0.443, p > 0.1), although a trend was present.