On the role of skeletal muscle acidosis and inorganic phosphates as determinants of central and peripheral fatigue: A 31 P-MRS study

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 (Q_tw ) and voluntary activation (VA) were quantified at rest and throughout both trials. Decreases in Q_tw and VA from baseline were used to determine peripheral and central fatigue, respectively. Q_tw 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 Q_tw and [H⁺ ] (-0.5, P < 0.0001), but not Q_tw 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 (Q_tw ) 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. Q_tw was strongly related to both [H⁺ ] and [Pi] across both trials. However, the relationship between Q_tw and [H⁺ ] shifted leftward from the first to the second trial, whereas the relationship between Q_tw 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.

Keywords: exercise; intramuscular metabolic by-product; magnetic resonance spectroscopy; neuromuscular fatigue.

Figure 1.. Exercise-induced changes in neuromuscular function.

A: maximal voluntary contraction force (MVC). B: Quadriceps twitch force (Q_tw). D: Voluntary quadriceps activation (VA) during 60 knee-extensor MVCs. Trial 1 and 2 were separated by 5 min of rest. Data are presented as mean ± SD. Significant time, trial, and time × trial interaction effects are indicated on the graphs. N = 10.

Figure 2.. Exercise-induced changes in intramuscular [H⁺], [Pi] and [H₂PO₄⁻].

Intramuscular hydrogen ion (H⁺), inorganic phosphate (Pi) and the acidic diprotonated form of Pi (H₂PO₄⁻) were measured using phosphorus magnetic resonance spectroscopy (³¹P-MRS) during 60 knee-extensor MVCs. Trial 1 and 2 were separated by 5 min of rest. Data are presented as mean ± SD. Significant time, trial, and time × trial interaction effects are indicated on the graphs. N = 10.

Figure 3.. Relationship between quadriceps twitch force (Q_tw) and intramuscular [Pi] during exercise.

Q_tw and Pi were measured using electric femoral nerve stimulation and phosphorus magnetic resonance spectroscopy (³¹P-MRS), respectively, during two 60 knee-extensor MVCs trials. The data were fit with a two-segment piecewise linear function to test for a break point between two linear segments (i.e., critical [Pi]). The Pi break point was 15.1 ± 3.3 mM, r² = 0.72 ± 0.13. N = 10.

Figure 4.. Relationship between quadriceps twitch force (Q_tw) and intramuscular [H⁺], [Pi], and [H₂PO₄⁻] during exercise.

Q_tw, H⁺, Pi, and H₂PO₄⁻ were measured using electric femoral nerve stimulation and phosphorus magnetic resonance spectroscopy (³¹P-MRS) during 60 knee-extensor MVCs. Trial 1 and 2 were separated by 5 min of rest. Individual participant data are presented in panels A,C, & E (these data are fit with linear regressions for illustrative purposes only). The linear mixed effects model results on the standardized (Z-score) Q_tw, H⁺, Pi, and H₂PO₄⁻ are presented in panels B, D, & F. N = 10.

Figure 5.. Relationship between quadriceps voluntary activation (VA) and intramuscular [H⁺] and [Pi] during exercise.

VA, H⁺, and Pi were measured using electric femoral nerve stimulation and phosphorus magnetic resonance spectroscopy (³¹P-MRS) during 60 knee-extensor MVCs. Trial 1 and 2 were separated by 5 min of rest. Individual participant data are presented in panels A & C (these data are fit with linear regressions only to illustrate trends). The linear mixed effects model results on the standardized (Z-score) VA, H⁺, and Pi are presented in panels B & D. N = 10.

Figure 6.. Relationship between maximal voluntary quadriceps force (MVC) intramuscular [H⁺] and [Pi] during exercise.

H⁺ and Pi were measured using phosphorus magnetic resonance spectroscopy (³¹P-MRS) during 60 knee-extensor MVCs. Trial 1 and 2 were separated by 5 min of rest. Individual participant data are presented in panels A & C (these data are fit with linear regressions only to illustrate trends). The linear mixed effects model results on the standardized (Z-score) MVC, H⁺, and Pi are presented in panels B & D. N = 10.