Central and peripheral fatigue and changes in oxygenation in healthy older adults

The decline in ability of a muscle group to produce force following a contraction can be described as muscular fatigue. Muscular fatigue has been generalized into two categories: peripheral fatigue and central fatigue. Peripheral fatigue has been attributed to metabolic buildup within the muscular tissues, while central fatigue is attributed to down-regulation of motor unit firing rate to avoid systemic failure. A lack of oxygen at the contractile tissue and brain has both been linked to peripheral and central fatigue, respectively. The purpose of this study was to evaluate oxygen changes that occur at the soleus muscle and prefrontal cortex during contractions following a plantar flexor fatigue protocol. Eleven participants were assessed for volitional and non-volitional peak torque output before and after a fatigue protocol. Resting twitch (RT), electromyography (rEMG), percent of voluntary muscle activation (ITT), soleus muscle oxygenation (StO2), and pre-frontal cortex oxygenation (HbO) levels were observed in a non-fatigued and fatigued state. The dynamic concentric fatigue protocol was utilized to elicit plantar flexion fatigue across individuals. A difference in ITT and rEMG were used to indicate central fatigue, while change in RT indicated peripheral fatigue. Change in central and tissue oxygenation due to a contraction was observed and compared during and prior to the fatigued state. All participants reached the fatigue threshold. Significant decreases were found for ITT, rEMG, StO2, and HbO following the fatigue protocol. Participants were also grouped based on repetitions to fatigue (> or < 286 reps). A significant main effect of group was found for soleus tissue oxygenation, which decreased in the fast fatigue group and increased in the slow fatigue group. These results indicated that central fatigue was the primary contributor to the fatigue protocol. Although no peripheral fatigue was observed, the ability to remove metabolites quickly may have reduced the amount of peripheral fatigue observed, but still contributed to central fatigue. Individual muscle fiber type may have been related to tissue oxygenation and the onset of fatigue.