The cellular mechanisms for the age-related loss in skeletal muscle contractile function and increased fatigability are unresolved. We previously observed that the depressive effects of fatiguing levels of hydrogen (H + ; pH 6.8, 6.6, and 6.2) and inorganic phosphate (P i ; 12, 20, and 30 mM) did not differ in myofibers from young compared with older adults. However, these studies used saturating Ca 2+ , while fatigue during high-intensity contractions in vivo also likely involves a decrease in