The studies included in this dissertation address the role of blood flow and cellular oxygenation in skeletal muscle fatigue, with an underlying interest in age-related differences in muscle fatigue. In the first study, I found that blood flow immediately post-contraction was tightly coupled to contraction intensity throughout an incremental isometric dorsiflexor contraction protocol in healthy young men. This relationship was independent of fatigue, and the onset of fatigue occurred before any reduction or plateau in blood flow. These data suggest that fatigue during intermittent incremental contractions does not result from a blood flow limitation. The second study used magnetic resonance spectroscopy of deoxymyoglobin to examine the role of intracellular oxygenation in muscle fatigue during slow and rapid incremental contractions in healthy young men and women. Data from this study showed that the rate of deoxygenation relative to force during contractions was predictive of fatigue, and this was true for slow and rapid contractions. Further, intracellular oxygenation was lower, and fatigue greater, during more metabolically demanding rapid contractions. These data support a role for intracellular oxygenation in the development of fatigue. The final study investigated whether differences in intracellular oxygenation could explain age-related differences in muscle fatigue. During slow contractions, intracellular oxygenation was higher, but fatigue lower, in older compared to young subjects. After matching a subset of subjects by muscle strength, age-group differences in oxygenation were eliminated, but fatigue was still less in older subjects, suggesting that while differences in oxygenation exist between young and older subjects, they cannot explain the age-group differences in fatigue. This conclusion was supported by a similar oxygenation, but greater fatigue, in young compared to older subjects during rapid contractions. Based on these findings, intracellular oxygenation does not appear to play a role in the age-related differences in fatigue observed in this and other studies. Together, data from this dissertation suggest that intracellular oxygenation plays a role in the development of muscle fatigue, despite the ability of blood flow to match contraction intensity. Differences in oxygenation, however, cannot explain age-related differences in muscle fatigue.