Humans can adjust their walking patterns in response to both internal and external demands, a process referred to as locomotor adaptation. This process is crucial for walking in complex environments and is thought to be driven by sensory prediction errors. While the involvement of supraspinal structures is known, how the oscillatory coupling between the sensorimotor cortex and spinal motor neurons is involved in locomotor adaptation remains unclear. This study aimed to characterize the modulation of corticomuscular coherence (CMC), an index of this coupling, using a split-belt locomotor adaptation paradigm. We recorded electroencephalogram (EEG) and electromyogram (EMG) from the tibialis anterior muscle and calculated CMC in the alpha (8-12 Hz) and beta (12-32 Hz) bands. Results revealed that immediately following the application and removal of the perturbation, both alpha and beta CMC temporarily decreased compared to normal walking, suggesting a disruption of established corticomuscular coupling. However, during the adaptation process, alpha CMC in the slow leg's heel contact phase significantly increased toward normal walking levels. During de-adaptation, both alpha and beta CMC increased, and finally, CMC in all gait phases returned to normal walking levels. These results suggest that corticomuscular coupling was enhanced during the adaptation and de-adaptation processes. Thus, modulation of corticomuscular coupling may be associated with the adjustment of gait patterns to meet environmental demands. These findings will advance our understanding of neuromuscular control of gait and offer valuable insights for gait rehabilitation.

The triceps surae muscle plays an important role in walking. Previous studies using coherence analysis of surface electromyography (EMG) have indicated that each muscle pair in the triceps surae [i.e., the medial head of gastrocnemius (MG)-the lateral head of gastrocnemius (LG), MG-the soleus (SOL), and LG-SOL] receives common oscillatory input during walking. Although significant intermuscular coherence (IMC) in the triceps surae muscle during normal walking has been observed in the beta and low-gamma frequency bands, it remains unclear whether IMC in these frequency bands depends on the degree of the afferent input. This study aimed to investigate whether attenuating afferent input affects IMC in the triceps surae muscle during walking. Since load-related afferent input plays an important role in neuromuscular control during walking, we focused on walking under body weight support (BWS) to modulate load-related afferent input. Fourteen healthy young adults participated in this study. The four BWS conditions were used: 100% (no unloading), 80%, 60%, and 40% of individual body weight. IMC was calculated from surface EMG in the stance phase during walking in the following three pairs: MG-LG, MG-SOL, and LG-SOL. The results showed no significant changes in IMC levels with increasing BWS levels in any frequency bands in all muscle pairs. These results suggest that common oscillatory inputs to the triceps surae muscle during walking are robust to the attenuation of load-related afferent input. The present results will contribute to a better understanding of neuromuscular control in the triceps surae muscle during walking.

Emotions influence postural control, but previous studies have mainly examined postural dynamics during quiet standing using the center of pressure as a measure. During quiet standing, neuromuscular activities of ankle plantarflexors are crucial for maintaining postural balance, both in terms of individual and synchronized activities. This study aimed to clarify the emotional effects on the individual and synchronized neuromuscular activities of ankle plantarflexors during quiet standing. Twenty-four healthy male participants were instructed to stand and view emotional pictures for 72 s following a fixation cross. The task was repeated four times with four picture conditions, which was composed of two arousals (High and Low) and two valences (Pleasant and Unpleasant). During the task, electromyograms (EMG) of the tibialis anterior (TA), soleus (SOL), and medial (MG) and lateral gastrocnemius muscles (LG) were recorded. The EMG signals were rectified, and mean amplitude was calculated to assess individual neuromuscular activity. Inter-muscular coherence (IMC) between ankle plantarflexors was calculated to assess synchronized neuromuscular activity, with mean IMC calculated at 0-4 Hz, 8-12 Hz, 15-30 Hz, and 30-40 Hz. Results showed that aroused emotions induced a reduction in SOL activity. Aroused emotions also increased IMC at 8-12 Hz, indicating enhanced physiological tremor and reduced capacity for postural adjustment. In addition, IMC at 15-30 Hz was increased while 30-40 Hz was decreased in aroused emotions, implying modulation of cortical common drive to ankle plantarflexors.These findings deepen our understanding of the emotional impacts on standing postural control.

Transcranial magnetic stimulation studies have indicated that action observation (AO) modulates corticospinal excitability. Although a few previous studies have shown that the AO of simple motor movements at a slow playback speed facilitates corticospinal excitability more than that at normal playback speed, it is unclear if this effect occurs during the AO of sport-related complex movements. Therefore, we investigated the changes in the motor evoked potential (MEP) amplitudes of the flexor carpi radialis (FCR) and abductor digiti minimi (ADM) muscles during the AO of a basketball free-throw movement at three different playback speeds (100%, 75%, and 50% speeds). Additionally, we evaluated the effects of stimulus timing (holding the ball vs. releasing the ball for shooting) and motor expertise (expert basketball players vs. novices) on the MEP amplitude during the AO. Our results demonstrated that regardless of motor expertise, the MEP amplitude of the FCR muscle was significantly smaller in the 50% speed condition than in the 100% condition. In the ADM muscle, the MEP amplitude was significantly larger when the ball was held after dribbling than when the ball was released. Therefore, it is suggested that corticospinal excitability in specific muscles during the observation of complex whole-body movements is influenced by video playback speed and stimulus timing.

Humans have great locomotor adaptability to environmental demands, which has been investigated using a split-belt treadmill with belts on both the left and right sides. Thus far, neuromuscular control in split-belt locomotor adaptation has been evaluated by analyzing muscle activities at the individual muscle level. Meanwhile, in the motor control field, the muscle synergy concept has been proposed. Muscle synergies are considered the fundamental building blocks of movement and are groups of coactive muscles and time-varying activation patterns, thereby, reflecting the neurophysiological characteristics of movement. To date, it remains unclear how such muscle synergies change during the adaptation and de-adaptation processes on the split-belt treadmill. Hence, we chronologically extracted muscle synergies while walking on the split-belt treadmill and examined changes in the number, muscle weightings, and temporal activation patterns of muscle synergies. Twelve healthy young males participated, and surface electromyography (EMG) signals were recorded bilaterally from 13 lower-limb muscles. Muscle synergies were extracted by applying non-negative matrix factorization to the EMG data of each leg. We found that during split-belt walking, the number of synergies in the slow leg increased while an extra synergy appeared and disappeared in the fast leg. Additionally, the areas under the temporal activation patterns in several synergies in both legs decreased. When both belts returned to the same speed, a decrease in the number of synergies and an increase in the areas under the temporal activation patterns of several synergies were temporally shown in each leg. Subsequently, the number of synergies and the areas under the temporal activation patterns returned to those of normal walking before split-belt walking. Thus, changes in the number, muscle weightings, and temporal activation patterns of synergies were noted in the split-belt locomotor adaptation, suggesting that the adaptation and de-adaptation occurred at the muscle synergy level.