Figure 2. Associations between the accumulation of metabolites and the reductions in mechanical power during a fatiguing knee extension exercise.

Young (22.7 ± 1.2 yrs) and old participants (76.4 ± 6.0 yrs) performed a high-intensity fatiguing knee extension exercise consisting of 120 maximal velocity contractions (1 contraction per 2-s) lifting a load equivalent to 20% of the individual specific maximal voluntary isometric contraction. The reduction in power during the 4-min dynamic fatiguing exercise was closely associated with the [P_i] (A), pH (B), and concentration of diprotonated phosphate [H₂PO₄⁻] (C) measured over the final 64-s of the exercise. The greater fatigue in the old compared with young adults was accompanied by an ~30% greater increase in the [H⁺] (pH 6.61 vs. 6.73) and an ~42% greater increase in the [P_i] (32 vs. 23 mM P_i). The combination of the greater decrease in pH and increase in [P_i] resulted in an ~59% greater increase in the [H₂PO₄⁻] in the old compared with the young adults. The figure is reprinted with permission from [31].