Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Kinesiology

Supervisor

Garland, S. Jayne

Abstract

The ankle plantarflexor muscles (soleus, medial, and lateral gastrocnemius) exhibit direction-specific regional modulation of muscle activity during external perturbations. This study sought to investigate the effect of medial gastrocnemius muscle fatigue on plantarflexor muscle activation patterns. Unipedal balance was tested during external perturbations that challenged standing balance in different directions before and after low-frequency fatigue was induced to the medial gastrocnemius via electrical stimulation. High-density surface electromyography was used to determine the amplitude and barycenter of muscle activation. It was hypothesized that the central nervous system would compensate for a loss of force-generating capacity in the medial gastrocnemius by modulating the activity of the soleus and lateral gastrocnemius to maintain balance. The soleus experienced an increase in muscle-activation amplitude and a proximal barycenter shift when the medial gastrocnemius was fatigued. However, the direction-specific regional modulation of plantarflexor muscle activity was not significantly affected by medial gastrocnemius fatigue.

Summary for Lay Audience

The triceps surae is a group of three ankle plantarflexor muscles (soleus, medial gastrocnemius, and lateral gastrocnemius) located in the calf. Together, these three muscles play a fundamental role in the maintenance of standing balance. Although it was previously believed that the three muscles function as a collective group, it has been discovered that the central nervous system (CNS) can modulate the activity in each muscle during balance tasks, a phenomenon known as regional modulation. It is understood that the CNS can modulate the activity of the triceps surae so that muscles located in mechanically advantageous regions are preferably activated during tasks that challenge standing balance. However, it remains unknown how muscle fatigue affects regional modulation of muscle activity in the triceps surae. To answer this question, participants performed a single-leg balance test before and after fatigue was induced in the medial gastrocnemius via electrical stimulation. During the balance test, participants were pulled in different directions and instructed to maintain their balance on one leg. The results showed that muscle activity was modulated depending on the direction of pull so that muscles located in mechanically advantageous regions were preferably activated. This result was observed before and after the medial gastrocnemius was fatigued, suggesting that muscle fatigue in one of the ankle plantarflexor muscles did not change the regional modulation of muscle activation. However, there was also a marked increase in the activation level of the soleus muscle during the balance testing when the medial gastrocnemius was in a state of fatigue. This suggests that the soleus muscle may compensate to help maintain balance when the medial gastrocnemius is compromised. In summary, not only has this study demonstrated a persisting ability to modulate muscle activity in the triceps surae despite fatigue, but it has also revealed the compensatory role of the soleus muscle in maintaining standing balance when the medial gastrocnemius is not functioning at full capacity.

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