Fig. 2 |. Skeletal muscle metabolism during higher-intensity exercise.

a, Exercise-onset metabolic inertia (red area). Acetyl-carnitine (aCarn) abundance^83–85 and the acetyl coenzyme A (aCoA)-producing capacities of carnitine acetyltransferase (CRAT)^83,84 and pyruvate dehydrogenase (PDH)⁸⁵ appear rate-limiting for oxidative adenosine triphosphate (ATP) regeneration at the onset of moderate-high intensity exercise. Contraction-induced calcium (${\text{Ca}}^{2+}$) transients promote mitochondrial ${\text{Ca}}^{2+}$ uptake into the matrix space²⁶⁹ through the inner-membrane mitochondrial calcium uniporter (MCU) complex^270,271. Increased matrix ${\text{Ca}}^{2+}$ can upregulate PDH²⁷² through activation of its phosphatase (PDP)²⁷³, and can upregulate isocitrate dehydrogenase (IDH)²⁷⁴ and 2-oxoglutarate dehydrogenase (OGDC)^272,274,275 directly to fine-tune oxidative metabolism via stimulation of the tricarboxylic acid (TCA) cycle⁸². ${\text{Ca}}^{2+}$ kinetics probably precede an allosteric⁸⁶ rise in the [ADP][${\text{P}}_{\text{i}}$] to [ATP] and [creatine][${\text{P}}_{\text{i}}$] to [creatine phosphate] ratios in part because ADP and creatine (Cr) are buffered by the adenylate kinase (not shown) and creatine phosphate (CrP) shuttle reactions. Thus, synchrony between mechanisms of substrate provision, ${\text{Ca}}^{2+}$-feedforward and metabolite feedback regulation might underlie acute metabolic inertia. This could be particularly prominent in type II fibres, which have lower CRAT⁸³ and MCU abundance²¹ and slower mitochondrial ${\text{Ca}}^{2+}$ import rates²⁷⁶. Furthermore, metabolic inertia is more pronounced in metabolically compromised and older untrained adults, related to the lower CRAT activity and acetyl-carnitine content of muscle in these individuals⁸⁴. b, Carbohydrates outcompete non-esterified fatty acids (NEFAs) for oxidation at higher intensities (yellow area). Muscle glucose uptake⁸⁸ and carbohydrate utilization^71,72,120 increases with exercise intensity. At workloads above maximum fat oxidation (>60–65% of maximal oxygen consumption ($\stackrel{.}{\text{V}}{\text{O}}_{2\text{max}}$)) flux of pyruvate to acetyl-CoA progressively exceeds rates of TCA cycle entry at citrate synthase (CS), leading to depletion of the muscle free-carnitine (Carn_free) pool through CRAT-dependent acetylation to acetyl-carnitine⁸³. After higher-intensity submaximal exercise, acetylation of the free-carnitine pool is greatest in type I fibres¹⁴². Insufficient free-carnitine availability would inhibit NEFA mitochondrial import at the first step of the carnitine shuttle — that is, carnitine palmitoyl transferase 1B (CPT1B) conjugation of carnitine to long-chain acyl-CoA (LCaCoA). Reduced fat oxidation is associated with diminished free-carnitine levels at ~70% of $\stackrel{.}{\text{V}}{\text{O}}_{2\text{max}}$ (ref. 72), whereas medium-chain NEFA metabolism bypasses carnitine shuttling and is maintained at higher submaximal workloads²⁷⁷. Therefore, free-carnitine levels appear rate-limiting for long-chain NEFA utilization at increasing exercise intensities. c, Lactate and pyruvate oxidation and NADH shuttles (blue area). Downstream of glycolysis, pyruvate (Pyr⁻) and/or lactate (La⁻) pass through voltage-dependent anion channels (VDAC), where lactate is converted to pyruvate by mitochondrial lactate dehydrogenase (mLDH) in the intermembrane space¹¹⁹ (step 1). Pyruvate then enters the mitochondrial matrix through the mitochondrial pyruvate carrier (MPC)^81,119. The glycerol-3-phosphate (G3P²⁻) shuttle (G3PS) and malate (Mal²⁻)/aspartate (Asp²⁻) shuttle enables mitochondrial oxidation of lactate and pyruvate through compartmentalized redox shuttling. G3PS and MAS recycle extra-matrix nicotinamide adenine dinucleotide (NAD⁺) (step 2) and transport reducing power from glycolysis to the mitochondrial matrix. This occurs through reactions associated with Mal²⁻ and Asp²⁻ delivery into the matrix space^81,119 (step 3) and G3P²⁻ donation of electrons directly to coenzyme Q (CoQ) of the electron transport chain¹¹⁹ (step 4). As such, saturation of these shuttles increases lactate accumulation and upregulates the lactate-favouring LDHA isoform in vitro²⁷⁸. See Box 3 for discussion of CrP, intermyofibrillar and intramyofibrillar glycogen (glycogen_inter and glycogen_intra, respectively) and intermyofibrillar lipid droplet (IMF_LD) stores, and section ‘Acute exercise muscle metabolism’ for details of their metabolism during acute exercise. $\text{β}$-Ox, $\text{β}$-oxidation; ACSL1, acyl-CoA synthetase long-chain family member 1; AGE, aspartate/glutamate exchanger; ANT, adenine nucleotide translocator; ATGL, adipose triacylglyceride lipase; $b\to a$ denotes posttranslational modification of PYGM from its less-active $b$ form to the constitutively active $a$ form by PHK; C, cytochrome $c$; CACT, carnitine acylcarnitine translocase; mCKM mitochondrial creatine kinase muscle; sCKM, sarcoplasmic CKM; DHAP²⁻, dihydroxyacetone phosphate; DHPR tetrad, four dihydropyridine receptors associated with one ryanodine receptor 1 (RYR1); FABPpm, fatty acid binding protein plasma membrane; FAD, flavin AD; FADH₂, reduced FAD; FATP, FA transporter protein; GLUT4/1; glucose transporters 4 and 1; G-1-P, glucose-1-phosphate; G-6-P, glucose-6-phosphate; HK, hexokinase; HSL, hormone-sensitive lipase; IMS, intermembrane space; LCaCarn, long-chain acyl carnitine; LPL, lipoprotein lipase; sLDH, sarcoplasmic LDH; mMAS, mitochondrial MAS; MCT4/1, monocarboxylate transporters 4 and 1; mG3PDH, mitochondrial glycerol-3-phosphate dehydrogenase; MOE, malate/2-oxoglutarate exchanger; NADH, reduced NAD; NEFA-alb, albumin-bound NEFA; NOX2, NAD phosphate oxidase 2; ${{\text{O}}_2}^{•-}$ superoxide; 2OG²⁻, 2-oxoglutarate; OXPHOS, oxidative phosphorylation and associated respiratory complexes; PHK, muscle phosphorylase kinase; PYGM, glycogen phosphorylase muscle-associated; ROS, reactive oxygen species; SERCA, sarcoplasmic/endoplasmic reticulum ${\text{Ca}}^{2+}$-ATPase; sG3PDH, sarcoplasmic glycerol-3-phosphate dehydrogenase; sMAS, sarcoplasmic MAS; T-tubule, transverse tubule; VLDL1, very low density lipoprotein 1.