Abstract
Muscular fatigue is the decline in ability of a muscle group to produce force following a contraction. Although older adults are less susceptible to fatigue, they have decreased muscle mass, strength, and contractile ability. Peripheral fatigue is attributed to metabolic buildup, while central fatigue is linked to down-regulation of motor unit firing. Decreased oxygen in the tissue and brain is linked to peripheral and central fatigue, respectively. The purpose of this study was to evaluate the onset of fatigue in older adults and assess alterations in oxygenation within the soleus muscle and prefrontal cortex following a plantar-flexor fatigue protocol. Eleven participants (56.3 ± 7.6 years, 169.5 ± 6.5 cm, 75.3 ± 6.7 kg) were assessed before and after a fatigue protocol consisting of repeated maximal concentric contractions completed until <50% of isometric one-repetition maximum was reached. Peripheral fatigue was assessed via resting twitch (RT) and central fatigue was assessed via electromyography (rEMG) and percent of voluntary muscle activation (ITT). Soleus muscle oxygenation (StO
2
) was recorded with an oximeter and central oxygenation (HbO) of the prefrontal cortex was measured via functional near infrared spectroscopy (fNIRS). Significant decreases were found for ITT (p<0.01), rEMG (p=0.03), StO
2
(p<0.01), and prefrontal cortex oxygenation (p=0.04). The results indicated that central fatigue was the primary contributor to the observed fatigue. Fewer type II muscle fibers in older adults would cause less metabolic buildup and less peripheral fatigue. A decrease in oxygenation in either the prefrontal cortex or muscular tissue may be related to the faster onset of fatigue in populations with disease.