Effects of Fatigue on Motor Unit Activation
Shannon Toy1, Christopher De Leon1, Mirai Manatad1, Tiffany Lubrino1, Armond Gray1, Christopher Hoang1&2, Michael Shiraishi1, Rahul Soangra1&3
1Crean College of Health and Behavioral Sciences, Chapman University, Orange, CA 92866
2Schmid College of Science and Technology, Chapman University, Orange, CA 92866
3Fowler School of Engineering, Chapman University, Orange, CA 92866
Introduction: Muscle fatigue is the temporary decline of force and power skeletal muscles can produce (Potvin and Fuglevand, 2017). A motor unit (MU) comprises of a neuron and the group of fast and slow-twitch skeletal muscle fibers it innervates. This study uses the Delsys Trigno EMG to identify MU recruitment before and after muscle performance to observe the effects of fatigue through gait tasks. Due to the body’s natural negative feedback system to protect the muscles, this study predicts the decrease of MU activation as the body experiences muscle fatigue.
Materials and Methods: Eight healthy college students (age 20.5±2.2, 61.9±8.49 kg, 168.9±5.8 cm) participated in this IRB-approved fatigue study. Four Delsys NeuroMap EMGs were placed on the right and left tibialis anterior and gastrocnemius to obtain normal walking data. The functional trials were separated into pre- and post- fatigue tests. The subjects were fatigued using the Biodex Dynamometer by repeating 22 reps/min of unilateral plantar/dorsal-flexion ankle movement until 60% of maximum voluntary contractions (MVC) is achieved. The contribution of muscle fatigue to the anterior tibialis and the gastrocnemius was calculated by decomposing high/low EMG frequencies to separate motor unit activation.
Results and Discussion: Figure B presents a 7.00% and 6.52% decrease in the MU firing rate of fast and slow-twitch muscle fibers of the right gastrocnemius, respectively. The right tibialis anterior displays a 32.22% decrease in the activation of slow-twitch fibers post fatigue, but a 3.96% increase for fast-twitch fibers. As the brain recognizes muscle fatigue within the right lower limb, more motor units are activated in the left lower limb due to muscle compensation. Figure C displays a 2% increase in the interpulse interval (IPI) from pre- to post-fatigue of the right gastrocnemius – time in between action potentials. This verifies that skeletal muscles generally lead to a reduction in motor unit activation with muscle fatigue.
Figure 1 A: Placement of EMG on the tibialis anterior. Figure B: Change in average firing rates of the right gastrocnemius and tibialis anterior. Figure C: Change in IPI in the right and left gastrocnemius
Conclusions: The decrease in MU firing rates and IPI can result in decreased limb coordination – this can increase the risk of injury because of the body’s expectation of the pre-fatigue level of functionality but can only produce limited motions. The expansion of this study can further investigate the relationship between fatigue and risk of injury in athletes as they approach a fatigued state.