Skeletal muscle conveys several of the health-promoting effects of exercise; yet the underlying mechanisms are not fully elucidated. Studying skeletal muscle is challenging due to its different fiber types and the presence of non-muscle cells. This can be circumvented by isolation of single muscle fibers. Here, we develop a workflow enabling proteomics analysis of pools of isolated muscle fibers from freeze-dried human muscle biopsies. We identify more than 4000 proteins in slow- and fast-twitch muscle fibers. Exercise training alters expression of 237 and 172 proteins in slow- and fast-twitch muscle fibers, respectively. Interestingly, expression levels of secreted proteins and proteins involved in transcription, mitochondrial metabolism, Ca2+ signaling, and fat and glucose metabolism adapts to training in a fiber type-specific manner. Our data provide a resource to elucidate molecular mechanisms underlying muscle function and health, and our workflow allows fiber type-specific proteomic analyses of snap-frozen non-embedded human muscle biopsies.

Key pointsr Training with blood flow restriction (BFR) is a well-recognized strategy for promoting muscle hypertrophy and strength. However, its potential to enhance muscle function during sustained, intense exercise remains largely unexplored.r In the present study, we report that interval training with BFR augments improvements in performance and reduces net K + release from contracting muscles during high-intensity exercise in active men.r A better K + regulation after BFR-training is associated with an elevated blood flow to exercising muscles and altered muscle anti-oxidant function, as indicated by a higher reduced to oxidized glutathione (GSH:GSSG) ratio, compared to control, as well as an increased thigh net K + release during intense exercise with concomitant anti-oxidant infusion.r Training with BFR also invoked fibre type-specific adaptations in the abundance of Na + ,K + -ATPase isoforms (α 1 , β 1 , phospholemman/FXYD1).r Thus, BFR-training enhances performance and K + regulation during intense exercise, which may be a result of adaptations in anti-oxidant function, blood flow and Na + ,K + -ATPase-isoform abundance at the fibre-type level.Abstract We examined whether blood flow restriction (BFR) augments training-induced improvements in K + regulation and performance during intense exercise in men, and also whether these adaptations are associated with an altered muscle anti-oxidant function, blood flow and/or with fibre type-dependent changes in Na + ,K + -ATPase-isoform abundance. Ten recreationally-active men (25 ± 4 years, 49.7 ± 5.3 mL kg −1 min −1 ) performed 6 weeks of Danny Christiansen is a researcher based in the Section of Integrative Physiology at the Department of Nutrition, Exercise and Sports in Copenhagen. His research focuses on optimizing strategies that aim to enhance human physical performance and health by understanding the molecular factors that drive skeletal muscle adaptation. His work has involved the use of cold-water immersion, simulated altitude, anti-oxidant infusion and blood flow restriction in combination with exercise to study the regulation of muscle ion transport, blood flow, oxygenation and glucose metabolism in man.This article was first published as a preprint. Christiansen D, Eibye KH, Rasmussen V, Voldbye HM, Thomassen M, Nyberg M, Gunnarsson TGP, Skovgaard C, Lindskrog MS, Bishop DJ, Hostrup M, Bangsbo J. 2018. Cycling with blood flow restriction improves performance and muscle K + regulation and blunts the effect of antioxidant infusion in humans. bioRxiv. https://doi. J Physiol 597.9 interval cycling, where one leg trained without BFR (control; CON-leg) and the other trained with BFR (BFR-leg, pressure: ß180 mmHg). Before and after training, femoral arterial and venous K + concentrations and artery blood flow were measured during single-leg knee-extensor exercise at 25% (Ex1) and 90% of thigh incremental peak power (Ex2) with I.V. infusion of N-acetylcysteine (NAC) or placebo (saline) and a resting muscle biopsy was collected. After training, performance...

Key points Endurance‐type training with blood flow restriction (BFR) increases maximum oxygen uptake (V̇O2max) and exercise endurance of humans. However, the physiological mechanisms behind this phenomenon remain uncertain. In the present study, we show that BFR‐interval training reduces the peripheral resistance to oxygen transport during dynamic, submaximal exercise in recreationally‐trained men, mainly by increasing convective oxygen delivery to contracting muscles. Accordingly, BFR‐training increased oxygen uptake by, and concomitantly reduced net lactate release from, the contracting muscles during relative‐intensity‐matched exercise, at the same time as invoking a similar increase in diffusional oxygen conductance compared to the training control. Only BFR‐training increased resting femoral artery diameter, whereas increases in oxygen transport and uptake were dissociated from changes in the skeletal muscle content of mitochondrial electron‐transport proteins. Thus, physically trained men benefit from BFR‐interval training by increasing leg convective oxygen transport and reducing lactate release, thereby improving the potential for increasing the percentage of V̇O2max that can be sustained throughout exercise. Abstract In the present study, we investigated the effect of training with blood flow restriction (BFR) on thigh oxygen transport and uptake, and lactate release, during exercise. Ten recreationally‐trained men (50 ± 5 mL kg−1 min−1) completed 6 weeks of interval cycling with one leg under BFR (BFR‐leg; pressure: ∼180 mmHg) and the other leg without BFR (CON‐leg). Before and after the training intervention (INT), thigh oxygen delivery, extraction, uptake, diffusion capacity and lactate release were determined during knee‐extensor exercise at 25% incremental peak power output (iPPO) (Ex1), followed by exercise to exhaustion at 90% pre‐training iPPO (Ex2), by measurement of femoral‐artery blood flow and femoral‐arterial and ‐venous blood sampling. A muscle biopsy was obtained from legs before and after INT to determine mitochondrial electron‐transport protein content. Femoral‐artery diameter was also measured. In the BFR‐leg, after INT, oxygen delivery and uptake were higher, and net lactate release was lower, during Ex1 (vs. CON‐leg; P < 0.05), with an 11% larger increase in workload (vs. CON‐leg; P < 0.05). During Ex2, after INT, oxygen delivery was higher, and oxygen extraction was lower, in the BFR‐leg compared to the CON‐leg (P < 0.05), resulting in an unaltered oxygen uptake (vs. CON‐leg; P > 0.05). In the CON‐leg, at both intensities, oxygen delivery, extraction, uptake and lactate release remained unchanged (P > 0.05). Resting femoral artery diameter increased with INT only in the BFR‐leg (∼4%; P < 0.05). Oxygen diffusion capacity was similarly raised in legs (P < 0.05). Mitochondrial protein content remained unchanged in legs (P > 0.05). Thus, BFR‐interval training enhances oxygen utilization by, and lowers lactate release from, submaximally‐exercising muscles of recreationally‐trained men m...

Mechanisms underlying fatigue development and limitations for performance during intense exercise have been intensively studied during the past couple of decades. Fatigue development may involve several interacting factors and depends on type of exercise undertaken and training level of the individual. Intense exercise (½-6 min) causes major ionic perturbations (Ca , Cl , H , K , lactate and Na ) that may reduce sarcolemmal excitability, Ca release and force production of skeletal muscle. Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise. Regular speed endurance training (SET), i.e. exercise performed at intensities above that corresponding to maximum oxygen consumption (V̇O2, max ), enhances intense exercise performance. However, most of the studies that have provided mechanistic insight into the beneficial effects of SET have been conducted in untrained and recreationally active individuals, making extrapolation towards athletes' performance difficult. Nevertheless, recent studies indicate that only a few weeks of SET enhances intense exercise performance in highly trained individuals. In these studies, the enhanced performance was not associated with changes in V̇O2, max and muscle oxidative capacity, but rather with adaptations in muscle ion handling, including lowered interstitial concentrations of K during and in recovery from intense exercise, improved lactate -H transport and H regulation, and enhanced Ca release function. The purpose of this Topical Review is to provide an overview of the effect of SET and to discuss potential mechanisms underlying enhancements in performance induced by SET in already well-trained individuals with special emphasis on ion handling in skeletal muscle.