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Fiber type differences in O2 cost of force development during fatigue in isolated single fibers

One purported contributor to the VO2 slow component is that some lower threshold motor units may fatigue but continue to utilize O2, resulting in uncoupling of oxidative phosphorylation and force generation. The present study utilized intact isolated single myocytes of differing fatigue resistance to investigate the relationship between fatigue, tension development, and aerobic metabolism (assessed by intracellular PO2; PiO2) during a progressive electrically stimulated tetanic contraction protocol. Single Xenopus fibers were grouped by contraction frequency required to elicit fatigue to 60% of initial force into 0.5 Hz (fast fatiguing; FF) and 1 Hz (slow fatiguing; SF) subgroups. Phosphorescence quenching was used to determine ΔPiO2 (a proxy for VO2) and developed isometric tension was monitored to allow calculation of the time‐tension integral (TxT). Absolute peak force (p = 0.06) and peak ΔPiO2 (p = 0.36) were not different between groups, but time to 60% of peak was significantly (p < 0.05) longer in SF vs. FF. Prior to fatigue, both ΔPiO2 and TxT rose proportionally with contraction frequency in SF and FF ‐ therefore ΔPiO2/TxT was identical between groups. At fatigue, TxT fell dramatically in both groups, but ΔPiO2 decreased a proportionate amount only in the FF group, resulting in an increase in ΔPiO2/TxT in the SF relative to the pre‐fatigue condition. These data show that more fatigue resistant fibers better maintain aerobic metabolism when they fatigue, resulting in an increased O2 cost of contractions that could contribute to the VO2 slow component in whole body exercise.