ABSTRACT: Intramuscular hydrogen ion (H+ ) and inorganic phosphate (Pi) concentrations were dissociated during exercise to challenge their relationships with peripheral and central fatigue in vivo. Ten recreationally active, healthy men (27 ± 5 years; 180 ± 4 cm; 76 ± 10 kg) performed two consecutive intermittent isometric single-leg knee-extensor trials (60 maximal voluntary contractions; 3 s contraction, 2 s relaxation) interspersed with 5 min of rest. Phosphorus magnetic resonance spectroscopy (31 P-MRS) was used to continuously quantify intramuscular [H+ ] and [Pi] during both trials. Using electrical femoral nerve stimulation, quadriceps twitch force (Qtw ) and voluntary activation (VA) were quantified at rest and throughout both trials. Decreases in Qtw and VA from baseline were used to determine peripheral and central fatigue, respectively. Qtw was strongly related to both [H+ ] (β coefficient: -0.9, P < 0.0001) and [Pi] (-1.1, P < 0.0001) across trials. There was an effect of trial on the relationship between Qtw and [H+ ] (-0.5, P < 0.0001), but not Qtw and [Pi] (0.0, P = 0.976). This suggests that, unlike the unaltered association with [Pi], a given level of peripheral fatigue was associated with a different [H+ ] in Trial 1 vs. Trial 2. VA was related to [H+ ] (-0.3, P < 0.0001), but not [Pi] (-0.2, P = 0.243), across trials and there was no effect of trial (-0.1, P = 0.483). Taken together, these results support intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents in the interstitial space, as a contributor to central fatigue during exercise. KEY POINTS: We investigated the relationship between intramuscular metabolites and neuromuscular function in humans performing two maximal, intermittent, knee-extension trials interspersed with 5 min of rest. Concomitant measurements of intramuscular hydrogen (H+ ) and inorganic phosphate (Pi) concentrations, as well as quadriceps twitch-force (Qtw ) and voluntary activation (VA), were made throughout each trial using phosphorus magnetic resonance spectroscopy (31 P-MRS) and electrical femoral nerve stimulations. Although [Pi] fully recovered prior to the onset of the second trial, [H+ ] did not. Qtw was strongly related to both [H+ ] and [Pi] across both trials. However, the relationship between Qtw and [H+ ] shifted leftward from the first to the second trial, whereas the relationship between Qtw and [Pi] remained unaltered. VA was related to [H+ ], but not [Pi], across both trials. These in vivo findings support the hypotheses of intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents, as a contributor to central fatigue.
