On the functional connectivity between heart, muscle, and frontal brain cortex during exercise fatigability

We analyze the functional connectivity of the cortico-cardiac-muscular network during muscle fatigability due to exercise. For our experiments, we recruited ten volunteers who performed two cycling routines. Our volunteers were classified according to physical activity level as active or sedentary. First, we measured the electromyography (EMG) signal of the vastus lateralis muscle to find the volunteer's individualized physical workload capacity at the fatigue threshold. Two to three days later, we measured the volunteers' EMG, electrocardiography, and electroencephalography (EEG) data during the development of muscle fatigue with the individualized pedaling load previously obtained. We analyzed the EEG to identify the brain rhythm with highest power, then functional connectivity was assessed by computing the pairwise partial directed coherence of our measurements at such rhythm. Finally, we assessed significant changes in functional connectivity from start to finish of the exercise. Our results indicate that the sedentary group showed a significant decrease in connectivity, while the active group showed an increase in left-frontal connectivity. This might be explained in terms of frontal asymmetry phenomena which, for the case of active volunteers, may be a reflection of positive emotional states. Additionally, the active group showed increased self-regulatory processes in heart and muscle, while the sedentary group showed a decreased connectivity from heart to muscle together with a reduction in the muscle's self-regulation. These last results may be explained by a preference of the body in the sedentary group to attend vital requirements instead of exercise.