As the highest percentage of V̇O2max at which steady state conditions can be achieved, a person’s critical power (PCRIT) strongly influences the metabolic strain of a given exercise. In this study we demonstrate that training-induced changes in endurance are more strongly related to the intensity of an exercise training program, relative to PCRIT than relative to V̇o2max. Thus, exercise may be more homogenously and effectively prescribed in relation to PCRIT than traditional factors like V̇o2max.

Background and Objectives: Muscle blood flow is impeded during resistance exercise contractions, but immediately increases during recovery. The purpose of this study was to determine the impact of brief bouts of rest (2 s) between repetitions of resistance exercise on muscle blood flow and exercise tolerance. Materials and Methods: Ten healthy young adults performed single-leg knee extension resistance exercises with no rest between repetitions (i.e., continuous) and with 2 s of rest between each repetition (i.e., intermittent). Exercise tolerance was measured as the maximal power that could be sustained for 3 min (PSUS) and as the maximum number of repetitions (Reps80%) that could be performed at 80% one-repetition maximum (1RM). The leg blood flow, muscle oxygenation of the vastus lateralis and mean arterial pressure (MAP) were measured during various exercise trials. Alpha was set to p ≤ 0.05. Results: Leg blood flow was significantly greater, while vascular resistance and MAP were significantly less during intermittent compared with continuous resistance exercise at the same power outputs (p < 0.01). PSUS was significantly greater during intermittent than continuous resistance exercise (29.5 ± 2.1 vs. 21.7 ± 1.2 W, p = 0.01). Reps80% was also significantly greater during intermittent compared with continuous resistance exercise (26.5 ± 5.3 vs. 16.8 ± 2.1 repetitions, respectively; p = 0.02), potentially due to increased leg blood flow and muscle oxygen saturation during intermittent resistance exercise (p < 0.05). Conclusions: In conclusion, a brief rest between repetitions of resistance exercise effectively decreased vascular resistance, increased blood flow to the exercising muscle, and increased exercise tolerance to resistance exercise.

Epidemiological data indicate that repeated heat stress improves cardiovascular health, making passive heat therapy (PHT) a potential alternative for those unable to exercise. Few studies to date have examined the potential exercise mimetic effects in humans, and it is unclear how adaptations compare in magnitude to exercise training. OBJECTIVE: To examine the effects of 6 weeks of localized, muscle-focused PHT on resistance artery vascular function, exercise hemodynamics, and exercise performance relative to the adaptations observed following high-intensity aerobic exercise training focused on the same muscles. HYPOTHESIS: 6 weeks of PHT, applied through pulsed shortwave diathermy (2 hr, 3 days/week), would increase resistance artery function, improve exercise hemodynamics, and enhance exercise performance more than a sham treatment, but less than single-leg knee extension (KE) exercise training (EX; 40 min, 3 days/week). We also hypothesized that these functional adaptations would be accompanied by increased skeletal muscle capillarity. METHODS: We randomized 34 sedentary but otherwise healthy, young adults (ages 18–36; n = 17 female, 17 male) to receive PHT, EX, or sham heating sessions (SHAM; 2 hr, 3 days/week) over 6 weeks. Vascular function was determined through the blood flow response during both a passive leg movement (PLM) assessment and a knee extension graded exercise test (GXTmax). Muscle biopsies were taken from the vastus lateralis at baseline and after 6 weeks of intervention. RESULTS: Peak muscle treatment temperature was significantly different between all groups with PHT exhibiting a higher peak temperature (~40.80°C) than those in the EX (~37.75°C, P<0.001) and SHAM groups (~36.10°C, P<0.001). Peak blood flow during PLM increased to the same extent (P=0.625) in both the EX (~10.5% increase, P=0.009) and PHT groups (~8.5% increase, P=0.044); but tended to decrease in the SHAM group (P=0.087). KE peak flow increased in EX (~19%, P=0.005), but did not change in PHT (P=0.523) and decreased in SHAM (~7%, P=0.020). Peak vascular conductance during KE significantly increased by ~25% in EX (P=0.030) and PHT (P=0.012). KE peak power increased in EX by ~27% (P=0.001) but did not significantly change in PHT(P=0.175) and SHAM groups (P=0.111). EX, but not PHT or SHAM increased muscle capillary-to-fiber ratio (P = 0.0003), capillary density (P = 0.0428), and the Capillary to Fiber Perimeter Exchange Index (P = 0.0089). CONCLUSIONS: 6 weeks of localized PHT, when applied to young healthy individuals, improved resistance artery function at rest and during exercise to the same extent as exercise training. However, PHT did not lead to increased KE peak flow, microvascular remodeling, or improved exercise performance. Therefore, PHT mimics many, but not all the vascular benefits of exercise training. Further research is necessary to determine the mechanism by which 6 weeks of PHT led to improved vascular function at rest and during exercise. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

AIM: Mild heat stress can improve mitochondrial respiratory capacity in skeletal muscle. However, long-term heat interventions are scarce, and the effects of heat therapy need to be understood in the context of the adaptations which follow the more complex combination of stimuli from exercise training. The purpose of this work was to compare the effects of 6 weeks of localized heat therapy on human skeletal muscle mitochondria to single-leg interval training. METHODS: Thirty-five subjects were assigned to receive sham therapy, short-wave diathermy heat therapy, or single-leg interval exercise training, localized to the quadriceps muscles of the right leg. All interventions took place 3 times per week. Muscle biopsies were performed at baseline, and after 3 and 6 weeks of intervention. Mitochondrial respiratory capacity was assessed on permeabilized muscle fibers via high-resolution respirometry. RESULTS: The primary finding of this work was that heat therapy and exercise training significantly improved mitochondrial respiratory capacity by 24.8 ± 6.2% and 27.9 ± 8.7%, respectively (p < 0.05). Fatty acid oxidation and citrate synthase activity were also increased following exercise training by 29.5 ± 6.8% and 19.0 ± 7.4%, respectively (p < 0.05). However, contrary to our hypothesis, heat therapy did not increase fatty acid oxidation or citrate synthase activity. CONCLUSION: Six weeks of muscle-localized heat therapy significantly improves mitochondrial respiratory capacity, comparable to exercise training. However, unlike exercise, heat does not improve fatty acid oxidation capacity.

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