| Summary: | Abstract We evaluated whether task‐dependent, age‐related differences in muscle fatigue (contraction‐induced decline in normalized power) develop from differences in bioenergetics or metabolic economy (ME; mass‐normalized work/mM ATP). We used magnetic resonance spectroscopy to quantify intracellular metabolites in vastus lateralis muscle of 10 young and 10 older adults during two maximal‐effort, 4‐min isotonic (20% maximal torque) and isokinetic (120°s−1) contraction protocols. Fatigue, inorganic phosphate (Pi), and pH (p ≥ 0.213) differed by age during isotonic contractions. However, older had less fatigue (p ≤ 0.011) and metabolic perturbation (lower [Pi], greater pH; p ≤ 0.031) than young during isokinetic contractions. ME was lower in older than young during isotonic contractions (p ≤ 0.003), but not associated with fatigue in either protocol or group. Rather, fatigue during both tasks was linearly related to changes in [H+], in both groups. The slope of fatigue versus [H+] was 50% lower in older than young during isokinetic contractions (p ≤ 0.023), consistent with less fatigue in older during this protocol. Overall, regardless of age or task type, acidosis, but not ME, was the primary mechanism for fatigue in vivo. The source of the age‐related differences in contraction‐induced acidosis in vivo remains to be determined, as does the apparent task‐dependent difference in the sensitivity of muscle to [H+].
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