Interactions between endogenous nitric oxide and hypoxemia in activation of group IV muscle afferents

Modulation of exercise-induced spinal loop properties in response to oxygen availability

This study investigated the effects of acute hypoxia on spinal reflexes and soleus muscle function after a sustained contraction of the plantar flexors at 40% of maximal voluntary isometric contraction (MVC). Fifteen males (age 25.3 ± 0.9 year) performed the fatigue task at two different inspired O₂ fractions (FiO₂ = 0.21/0.11) in a randomized and single-blind fashion. Before, at task failure and after 6, 12 and 18 min of passive recovery, the Hoffman-reflex (H max) and M-wave (M max) were recorded at rest and voluntary activation (VA), surface electromyogram (RMSmax), M-wave (M sup) and V-wave (V sup) were recorded during MVC. Normalized H-reflex (H max/M max) was significantly depressed pre-exercise in hypoxia compared with normoxia (0.31 ± 0.08 and 0.36 ± 0.08, respectively, P < 0.05). Hypoxia did not affect time to task failure (mean time of 453.9 ± 32.0 s) and MVC decrease at task failure (-18% in normoxia vs. -16% in hypoxia). At task failure, VA (-8%), RMSmax/M sup (-11%), H max/M max (-27%) and V sup/M sup (-37%) decreased (P < 0.05), but with no FiO2 effect. H max/M max restored significantly throughout recovery in hypoxia but not in normoxia, while V sup/M sup restored significantly during recovery in normoxia but not in hypoxia (P < 0.05). Collectively, these findings indicate that central adaptations resulting from sustained submaximal fatiguing contraction were not different in hypoxia and normoxia at task failure. However, the FiO₂-induced differences in spinal loop properties pre-exercise and throughout recovery suggest possible specific mediation by the hypoxic-sensitive group III and IV muscle afferents, supraspinal regulation mechanisms being mainly involved in hypoxia while spinal ones may be predominant in normoxia.

Nervous system function during exercise in hypoxia

Aerobic exercise capacity decreases with exposure to hypoxia. This article focuses on the effects of hypoxia on nervous system function and the potential consequences for the exercising human. Emphasis is put on somatosensory muscle afferents due to their crucial role in the reflex inhibition of muscle activation and in cardiorespiratory reflex control during exercise. We review the evidence of hypoxia influences on muscle afferents and discuss important consequences for exercise performance. Efferent (motor) nerves are less affected at altitude and are thought to stay fairly functional even in severe levels of arterial hypoxemia. Altitude also alters autonomic nervous system functions, which are thought to play an important role in the regulation of cardiac output and ventilation. Finally, the consequences of hypoxia-induced cortical adaptations and dysfunctions are evaluated in terms of neurotransmitter turnover, brain electrical activity, and cortical excitability. Even though the cessation of exercise or the reduction of exercise intensity, when reaching maximum performance, implies reduced motor recruitment by the nervous system, the mechanisms that lead to the de-recruitment of active muscle are still not well understood. In moderate hypoxia, muscle afferents appear to play an important role, whereas in severe hypoxia brain oxygenation may play a more important role.

Muscle-derived ROS and thiol regulation in muscle fatigue

Muscles produce oxidants, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), from a variety of intracellular sources. Oxidants are detectable in muscle at low levels during rest and at higher levels during contractions. RNS depress force production but do not appear to cause fatigue of healthy muscle. In contrast, muscle-derived ROS contribute to fatigue because loss of function can be delayed by ROS-specific antioxidants. Thiol regulation appears to be important in this biology. Fatigue causes oxidation of glutathione, a thiol antioxidant in muscle fibers, and is reversed by thiol-specific reducing agents. N-acetylcysteine (NAC), a drug that supports glutathione synthesis, has been shown to lessen oxidation of cellular constituents and delay muscle fatigue. In humans, NAC pretreatment improves performance of limb and respiratory muscles during fatigue protocols and extends time to task failure during volitional exercise. These findings highlight the importance of ROS and thiol chemistry in fatigue, show the feasibility of thiol-based countermeasures, and identify new directions for mechanistic and translational research.

Effects of hypoxia on muscle response to tendon vibration in humans

Previous animal studies have shown that hypoxia markedly reduces the activation of muscle spindles. The present study was undertaken to determine whether a reduced oxygen supply to muscle affects the tonic vibration reflex (TVR) in humans. In resting healthy volunteers, the effects of inhalation of hypoxic gas, apnea, and total forearm ischemia produced by cuff inflation were studied on separate days. The TVR was recorded in flexor digitorum superficialis and the neuromuscular conduction time (CT) was measured from the compound muscle action potential; the latency and amplitude of the H reflex were also determined. TVR depression began during inhalation of the hypoxic gas, at the end of apnea, and during cuff inflation, and persisted during the recovery period. The H-reflex amplitude concomitantly increased or remained unchanged. Thus, hypoxia seems to directly alter muscle spindle reactivity. Such alterations of sensorimotor control may occur in patients suffering from respiratory or circulatory insufficiency and may contribute to their exercise limitation.

Electromyographic signs of neuromuscular fatigue are concomitant with further increase in ventilation during static handgrip

We questioned if a non-linear increase in ventilation defining a ventilatory threshold (V(Th)) accompanied the electromyographic (EMG) signs of neuromuscular fatigue. Indeed, the intramuscular accumulation of metabolites may activate the afferent nervous pathways responsible for both the 'muscle wisdom' phenomenon and the respiratory centre activation. During inframaximal (50%) handgrip sustained until exhaustion, minute ventilation (V(E)), V(E)/V(O2) and V(E)/V(CO2) ratios were measured simultaneously with surface EMG of the 'flexor digitorum' muscle. V(Th) was defined as a non-linear V(E) increase and/or an abrupt V(E)/V(O2) increase without any concomitant increase in the V(E)/V(CO2) ratio. Handgrip was repeated during complete arterial blood flow interruption in order to suppress any venous return from the exercising forearm. In both control and blood flow interruption conditions, an abrupt increase in the V(E)/V(O2) ratio was measured in the majority of trials (13 of 15 and 14 of 15, respectively) and the EMG signs of neuromuscular fatigue (a decline in median frequency and/or a non-linear increase in low-frequency EMG energies, E(L)) were concomitant with the V(Th) determination. Thus, V(Th) occurs during sustained static contraction and is concomitant with EMG signs of neuromuscular fatigue. Neurogenic factors seem to be responsible for the two responses which persist despite the absence of any release of metabolites in the circulation.