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This study investigated the impact of dietary nitrate supplementation on peripheral hemodynamics, the development of neuromuscular fatigue, and time to task failure during cycling exercise. Eleven recreationally active male participants (27 ± 5 yr, V̇o(2max): 42 ± 2 mL/kg/min) performed two experimental trials following 3 days of either dietary nitrate-rich beetroot juice (4.1 mmol NO(3)(−)/day; DNS) or placebo (PLA) supplementation in a blinded, counterbalanced order. Exercise consisted of constant-load cycling at 50, 75, and 100 W (4 min each) and, at ∼80% of peak power output (218 ± 12 W), to task-failure. All participants returned to repeat the shorter of the two trials performed to task failure, but with the opposite supplementation regime (iso-time comparison; ISO). Mean arterial pressure (MAP), leg blood flow (Q(L); Doppler ultrasound), leg vascular conductance (LVC), and pulmonary gas exchange were continuously assessed during exercise. Locomotor muscle fatigue was determined by the change in pre to postexercise quadriceps twitch-torque (ΔQ(tw)) and voluntary activation (ΔVA; electrical femoral nerve stimulation). Following DNS, plasma [nitrite] (∼670 vs. ∼180 nmol) and [nitrate] (∼775 vs. ∼11 μmol) were significantly elevated compared with PLA. Unlike PLA, DNS lowered both Q(L) and MAP by ∼8% (P < 0.05), but did not alter LVC (P = 0.31). V̇O(2) across work rates, as well as cycling time to task-failure (∼7 min) and locomotor muscle fatigue following the ISO-time comparison were not different between the two conditions (ΔQ(tw) ∼42%, ΔVA ∼4%). Thus, despite significant hemodynamic changes, DNS did not alter the development of locomotor muscle fatigue and, ultimately, cycling time to task failure. NEW & NOTEWORTHY This study sought to characterize the impact of dietary nitrate supplementation on the hemodynamic response, locomotor muscle fatigue, and time to task failure during cycling exercise. Although nitrate supplementation lowered mean arterial pressure and exercising leg blood flow, leg vascular conductance and oxygen utilization were unaffected. Despite significant hemodynamic changes, there was no effect of dietary nitrate on neuromuscular fatigue development and, ultimately, cycling time to task failure.

We examined the interactive influence of the muscle reflex (MR) and the chemoreflex (CR) on the ventilatory response to exercise. Eleven healthy subjects (5 women/6 men) completed three bouts of constant-load single-leg knee-extension exercise in a control trial and an identical trial conducted with lumbar intrathecal fentanyl to attenuate neural feedback from lower-limb group III/IV muscle afferents. The exercise during the two trials was performed while breathing ambient air ([Formula: see text] ~97%, [Formula: see text] ~84 mmHg, [Formula: see text] ~32 mmHg, pH ~7.39), or under normocapnic hypoxia ([Formula: see text] ~79%, [Formula: see text] ~43 mmHg, [Formula: see text] ~33 mmHg, pH ~7.39) or normoxic hypercapnia ([Formula: see text] ~98%, [Formula: see text] ~105 mmHg, [Formula: see text] ~50 mmHg, pH ~7.26). During coactivation of the MR and the hypoxia-induced CR (O(2)-CR), minute ventilation (V̇e) and tidal volume (V(T)) were significantly greater compared with the sum of the responses to the activation of each reflex alone; there was no difference between the observed and summated responses in terms of breathing frequency (f(B); P = 0.4). During coactivation of the MR and the hypercapnia-induced CR (CO(2)-CR), the observed ventilatory responses were similar to the summated responses of the reflexes (P ≥ 0.1). Therefore, the interaction between the MR and the O(2)-CR exerts a hyperadditive effect on V̇e and V(T) and an additive effect on f(B), whereas the interaction between the MR and the CO(2)-CR is simply additive for all ventilatory parameters. These findings reveal that the MR:CR interaction further augments the ventilatory response to exercise in hypoxia. NEW & NOTEWORTHY Although the muscle reflex and the chemoreflex are recognized as independent feedback mechanisms regulating breathing during exercise, the ventilatory implications resulting from their interaction remain unclear. We quantified the individual and interactive effects of these reflexes during exercise and revealed differential modes of interaction. Importantly, the reflex interaction further amplifies the ventilatory response to exercise under hypoxemic conditions, highlighting a potential mechanism for optimizing arterial oxygenation in physically active humans at high altitude.

This review discusses evidence suggesting that group III/IV muscle afferents affect locomotor performance by influencing neuromuscular fatigue. These neurons regulate the hemodynamic and ventilatory response to exercise and, thus, assure appropriate locomotor muscle O2 delivery, which optimizes peripheral fatigue development and facilitates endurance performance. In terms of central fatigue, group III/IV muscle afferents inhibit motoneuronal output and thereby limit exercise performance.

Key points Although the exercise pressor reflex (EPR) and the chemoreflex (CR) are recognized for their sympathoexcitatory effect, the cardiovascular implication of their interaction remains elusive. We quantified the individual and interactive cardiovascular consequences of these reflexes during exercise and revealed various modes of interaction. The EPR and hypoxia-induced CR interaction is hyper-additive for blood pressure and heart rate (responses during co-activation of the two reflexes are greater than the summation of the responses evoked by each reflex) and hypo-additive for peripheral haemodynamics (responses during co-activation of the reflexes are smaller than the summated responses). The EPR and hypercapnia-induced CR interaction results in a simple addition of the individual responses to each reflex (i.e. additive interaction). Collectively, EPR:CR co-activation results in significant cardiovascular interactions with restriction in peripheral haemodynamics, resulting from the EPR:CR interaction in hypoxia, likely having the most crucial impact on the functional capacity of an exercising human. Abstract We investigated the interactive effect of the exercise pressor reflex (EPR) and the chemoreflex (CR) on the cardiovascular response to exercise. Eleven healthy participants (5 females) completed a total of six bouts of single-leg knee-extension exercise (60% peak work rate, 4 min each) either with or without lumbar intrathecal fentanyl to attenuate group III/IV afferent feedback from lower limbs to modify the EPR, while breathing either ambient air, normocapnic hypoxia (Sa O2 ∼79%, Pa O2 ∼43 mmHg, Pa CO2 ∼33 mmHg, pH ∼7.39), or normoxic hypercapnia (Sa O2 ∼98%, Pa O2 ∼105 mmHg, Pa CO2 ∼50 mmHg, pH ∼7.26) to modify the CR. During co-activation of the EPR and the hypoxia-induced CR (O2 -CR), mean arterial pressure and heart rate were significantly greater, whereas leg blood flow and leg vascular conductance were significantly lower than the summation of the responses evoked by each reflex alone. During co-activation of the EPR and the hypercapnia-induced CR (CO2 -CR), the haemodynamic responses were not different from the summated responses to each reflex response alone (P ≥ 0.1). Therefore, while the interaction resulting from the EPR:O2 -CR co-activation is hyper-additive for blood pressure and heart rate, and hypo-additive for peripheral haemodynamics, the interaction resulting from the EPR:CO2 -CR co-activation is simply additive for all cardiovascular parameters. Thus, EPR:CR co-activation results in significant interactions between cardiovascular reflexes, with the impact differing when the CR activation is achieved by hypoxia or hypercapnia. Since the EPR:CR co-activation with hypoxia potentiates the pressor response and restricts blood flow to contracting muscles, this interaction entails the most functional impact on an exercising human.

Motor point associative stimulation (MPAS) in hand muscles is known to modify motor cortex excitability and improve learning rate, but not plateau of performance, in manual dexterity tasks. Central stimulation of motor cortex, such as transcranial direct current stimulation (tDCS), can have similar effects if accompanied by motor practice, which can be difficult and tiring for patients. Here we asked whether adding tDCS to MPAS could improve manual dexterity in healthy individuals who are already performing at their plateau, with no motor practice during stimulation. We hypothesized that MPAS could provide enough coordinated muscle activity to make motor practice unnecessary, and that this combination of stimulation techniques could yield improvements even in subjects at or near their peak. If so, this approach could have a substantial effect on patients with impaired dexterity, who are far from their peak. MPAS was applied for 30min to two right hand muscles important for manual dexterity. tDCS was simultaneously applied over left sensorimotor cortex. The motor cortex input/output (I/O) curve was assessed with transcranial magnetic stimulation (TMS), and manual dexterity was assessed with the Purdue Pegboard Test. Compared to sham or cathodal tDCS combined with MPAS, anodal tDCS combined with MPAS significantly increased the plateau of manual dexterity. This result suggests that MPAS has the potential to substitute for motor practice in mediating a beneficial effect of tDCS on manual dexterity.

The specific internal environment in the body during exercise can influence the muscle hypertrophy and strength response. High altitude mediated hypobaric hypoxia creates many morphological and physiological changes. One study reported that thigh muscle cross-sectional area (CSA) decreased by 10% after sojourn at high altitude (Himalayas, greater than 5000 m for over 56 days), although whether changes in physical activity and food intake occurred is not clear. In that study, the loss of thigh muscle size was mainly due to a decrease in myofibrillar proteins.1 Mizuno et al2 also reported that muscle fiber CSA decreased at an average of 15% in the vastus lateralis and biceps brachii muscles in both active and less active men after 75 days of altitude (greater than 5250 m) exposure. Tanner and Stager found that energy intake, in part, accounts for changes in body composition variables during a 21-day expedition between 2200 m and 4300 m altitude.3 However, some studies have reported that no significant changes occur in muscle fiber CSA following ~8 weeks of high altitude exposure.4,5 In the study by Lundby et al4, subjects had free access to physical activities such as jogging, hiking, and soccer, which may have counteracted the acute disuse muscle atrophy. In addition, a recent study investigated the effects of chronic hypobaric hypoxia on muscle protein metabolism and suggested that whole body amino acid f lux is increased due to an elevated protein turnover. Furthermore, the myocontractile protein synthesis rate is elevated following chronic hypobaric hypoxia.6 Therefore, the large increase in protein degradation is the underlying mechanism for the loss of skeletal muscle mass under high altitude mediated hypobaric hypoxia;6,7 although total time of exposure, the degree of hypoxia, food intake, and physical activity levels may all be influencing factors. In contrast to chronic hypobaric hypoxia, one study found that muscle fiber size increased after exposure to intermittent normobaric hypoxia. In this study, subjects performed bicycle ergometer exercise (70-80% of VO2max; VO2max was measured in an ambient fractional oxygen concentration [FlO2] of 11.6%, equivalent to an altitude of 4800 m) for 3 weeks under normobaric hypoxia (ambient FlO2 decreasing from 12.7% to a f inal level of 10%) with recovery in normoxia.8 Furthermore, a recent study found that intermittent systemic