Muscle fatigue is defined as the reduced capacity to produce torque or power in response to contractile activity. Previous work has observed greater fatigue in older than young adults in response to high-velocity contractions, but similar fatigue with age in response to moderate-velocity contractions. The mechanisms for these age-related differences is not clear. One potential explanation is a greater accumulation of putative agents of fatigue (e.g., Pi, H2PO4-, and H+) in older than young muscle during high- but not moderate-velocity contractions. Older muscle may accumulate these metabolites to greater extent because of deficits in muscle metabolic economy (ME). This dissertation sought to address these knowledge gaps using 31P-MRS to evaluate the role of bioenergetics in two studies: 1) age-related differences in knee extensor muscle fatigue in response to two distinct bouts of contractions chosen to permit high- (MVDC20%; 20% MVIC) and moderate-velocity (MVDC120; 120°.s-1) contractions, and 2) in vivo muscle ME during three 24-s bouts of maximal isometric, isokinetic or isotonic contractions (MVIC, MVDCIsoK, MVDCIsoT). We observed similar fatigue in young and older adults in response to the MVDC20% protocol. Surprisingly, this was coincident with lower pH and greater [H2PO4-] in young than older muscle. In response to the MVDC120 protocol, fatigue, [Pi], and [H2PO4-] were greater, and pH lower, in young than older muscle. During both protocols, oxidative ATP production increased from baseline to the end of the fatiguing contractions in older, but not young, muscle. Collectively, these results indicate relatively greater oxidative ATP production in older than young quadriceps during in vivo contractions at two different loads (20% MVIC (MVDC20%) versus ~50% (young) and 35% MVIC (older) at the start of the MVDC120 task). Further, these results suggest the contractile machinery is more sensitive to changes in pH in older than young muscle in vivo. In Study 2, we observed similar in vivo muscle ME during the MVIC, but lower ME in older than young muscle during the MVDCIsoK and MVDCIsoT tasks. These results show an age-related deficit in ME that is evident only during dynamic contractions, potentially due to the higher energy demand of these contractions.