Arterial oxygenation influences central motor output and exercise performance via effects on peripheral locomotor muscle fatigue in humans

Is it time to retire the A.V. Hill Model?: A rebuttal to the article by Professor Roy Shephard

Recent publications by Emeritus Professor Roy Shephard propose that a "small group of investigators who have argued repeatedly (over the past 13 years) for a 'Central Governor'," should now either "Put up or shut up." Failing this, their 'hypothesis' should be 'consigned to the bottom draw for future reference'; but Professor Shephard's arguments are contradictory. Thus, in different sections of his article, Professor Shephard explains: why there is no need for a brain to regulate exercise performance; why there is no proof that the brain regulates exercise performance; and why the brain's proven role in the regulation of exercise performance is already so well established that additional comment and research is unnecessary. Hence, "The higher centres of an endurance athlete … call forth an initial effort … at a level where a minimal accumulation of lactate in the peripheral muscles is sensed." Furthermore, "a variety of standard texts have illustrated the many mutually redundant feedback loops (to the nervous system) that limit exercise." Yet, the figure from Professor Shephard's 1982 textbook does not contain any links between the nervous system, "many mutually redundant feedback loops" and skeletal muscle. This disproves his contradictory claims that although there is neither any need for, nor any proof of, any role of the brain in the regulation of exercise performance, the physiological mechanisms for this (non-existent) control were already well established in 1982. In contrast, the Central Governor Model (CGM) developed by our "small group … in a single laboratory" after 1998, provides a simple and unique explanation of how 'redundant feedback loops' can assist in the regulation of exercise behaviour. In this rebuttal to his article, I identify (i) the numerous contradictions included in Professor Shephard's argument; (ii) the real meaning of the facts that he presents; (iii) the importance of the evidence that he ignores; and (iv) the different philosophies of how science should be conducted according to either the Kuhnian or the Popperian philosophies of scientific discovery. My conclusion is that the dominance of an authoritarian Kuhnian philosophy, which refuses to admit genuine error or "the need to alter one's course of belief or action," explains why there is little appetite in the exercise sciences for the acceptance of genuinely novel ideas such as the CGM. Furthermore, to advance the case for the CGM, I now include evidence from more than 30 studies, which, in my opinion, can only be interpreted according to a model of exercise regulation where the CNS, acting in an anticipatory manner, regulates the exercise behaviour by altering skeletal muscle recruitment, specifically to ensure that homeostasis is maintained during exercise. Since few, if any, of those studies can be explained by the 'brainless' A.V. Hill Cardiovascular Model on which Professor Shephard bases his arguments, I argue that it is now the appropriate time to retire that model. Perhaps this will bring to an end the charade that holds either (i) that the brain plays no part in the regulation of exercise performance; or, conversely, (ii) that the role of the brain is already so well defined that further research by other scientists is unnecessary. However, this cannot occur in a discipline that is dominated by an authoritarian Kuhnian philosophy.

Interactive processes link the multiple symptoms of fatigue in sport competition

Muscle physiologists often describe fatigue simply as a decline of muscle force and infer this causes an athlete to slow down. In contrast, exercise scientists describe fatigue during sport competition more holistically as an exercise-induced impairment of performance. The aim of this review is to reconcile the different views by evaluating the many performance symptoms/measures and mechanisms of fatigue. We describe how fatigue is assessed with muscle, exercise or competition performance measures. Muscle performance (single muscle test measures) declines due to peripheral fatigue (reduced muscle cell force) and/or central fatigue (reduced motor drive from the CNS). Peak muscle force seldom falls by >30% during sport but is often exacerbated during electrical stimulation and laboratory exercise tasks. Exercise performance (whole-body exercise test measures) reveals impaired physical/technical abilities and subjective fatigue sensations. Exercise intensity is initially sustained by recruitment of new motor units and help from synergistic muscles before it declines. Technique/motor skill execution deviates as exercise proceeds to maintain outcomes before they deteriorate, e.g. reduced accuracy or velocity. The sensation of fatigue incorporates an elevated rating of perceived exertion (RPE) during submaximal tasks, due to a combination of peripheral and higher CNS inputs. Competition performance (sport symptoms) is affected more by decision-making and psychological aspects, since there are opponents and a greater importance on the result. Laboratory based decision making is generally faster or unimpaired. Motivation, self-efficacy and anxiety can change during exercise to modify RPE and, hence, alter physical performance. Symptoms of fatigue during racing, team-game or racquet sports are largely anecdotal, but sometimes assessed with time-motion analysis. Fatigue during brief all-out racing is described biomechanically as a decline of peak velocity, along with altered kinematic components. Longer sport events involve pacing strategies, central and peripheral fatigue contributions and elevated RPE. During match play, the work rate can decline late in a match (or tournament) and/or transiently after intense exercise bursts. Repeated sprint ability, agility and leg strength become slightly impaired. Technique outcomes, such as velocity and accuracy for throwing, passing, hitting and kicking, can deteriorate. Physical and subjective changes are both less severe in real rather than simulated sport activities. Little objective evidence exists to support exercise-induced mental lapses during sport. A model depicting mind-body interactions during sport competition shows that the RPE centre-motor cortex-working muscle sequence drives overall performance levels and, hence, fatigue symptoms. The sporting outputs from this sequence can be modulated by interactions with muscle afferent and circulatory feedback, psychological and decision-making inputs. Importantly, compensatory processes exist at many levels to protect against performance decrements. Small changes of putative fatigue factors can also be protective. We show that individual fatigue factors including diminished carbohydrate availability, elevated serotonin, hypoxia, acidosis, hyperkalaemia, hyperthermia, dehydration and reactive oxygen species, each contribute to several fatigue symptoms. Thus, multiple symptoms of fatigue can occur simultaneously and the underlying mechanisms overlap and interact. Based on this understanding, we reinforce the proposal that fatigue is best described globally as an exercise-induced decline of performance as this is inclusive of all viewpoints.

Temperature and neuromuscular function

This review focuses on the effects of different environmental temperatures on the neuromuscular system. During short duration exercise, performance improves from 2% to 5% with a 1 °C increase in muscle temperature. However, if central temperature increases (i.e., hyperthermia), this positive relation ceases and performance becomes impaired. Performance impairments in both cold and hot environment are related to a modification in neural drive due to protective adaptations, central and peripheral failures. This review highlights, to some extent, the different effects of hot and cold environments on the supraspinal, spinal and peripheral components of the neural drive involved in the up- and down-regulation of neuromuscular function and shows that temperature also affects the neural drive transmission to the muscle and the excitation-contraction coupling.

Spinal opioid receptor-sensitive muscle afferents contribute to the fatigue-induced increase in intracortical inhibition in healthy humans

We investigated the influence of spinal opioid receptor-sensitive muscle afferents on cortical changes following fatiguing unilateral knee-extensor exercise. On separate days, seven subjects performed an identical five sets of intermittent isometric right-quadriceps contractions, each consisting of eight submaximal contractions [63 ± 7% maximal voluntary contraction (MVC)] and one MVC. The exercise was performed following either lumbar interspinous saline injection or lumbar intrathecal fentanyl injection blocking the central projection of spinal opioid receptor-sensitive lower limb muscle afferents. To quantify exercise-induced peripheral fatigue, quadriceps twitch force (Q(tw,pot)) was assessed via supramaximal magnetic femoral nerve stimulation before and after exercise. Motor evoked potentials and cortical silent periods (CSPs) were evaluated via transcranial magnetic stimulation of the motor cortex during a 3% MVC pre-activation period immediately following exercise. End-exercise quadriceps fatigue was significant and similar in both conditions (Q(tw,pot) -35 and -39% for placebo and fentanyl, respectively; P = 0.38). Immediately following exercise on both days, motor evoked potentials were similar to those obtained prior to exercise. Compared with pre-exercise baseline, CSP in the placebo trial was 21 ± 5% longer postexercise (P < 0.01). In contrast, CSP following the fentanyl trial was not significantly prolonged compared with the pre-exercise baseline (6 ± 4%). Our findings suggest that the central effects of spinal opioid receptor-sensitive muscle afferents might facilitate the fatigue-induced increase in CSP. Furthermore, since the CSP is thought to reflect inhibitory intracortical interneuron activity, which may contribute to central fatigue, our findings imply that spinal opioid receptor-sensitive muscle afferents might influence central fatigue by facilitating intracortical inhibition.

Suppression of cerebral hemodynamics is associated with reduced functional capacity in patients with heart failure

This investigation elucidated the underlying mechanisms of functional impairments in patients with heart failure (HF) by simultaneously comparing cardiac-cerebral-muscle hemodynamic and ventilatory responses to exercise among HF patients with various functional capacities. One hundred one patients with HF [New York Heart Association HF functional class II (HF-II, n = 53) and functional class III (HF-III, n = 48) patients] and 71 normal subjects [older control (O-C, n = 39) and younger control (Y-C, n = 32) adults] performed an incremental exercise test using a bicycle ergometer. A recently developed noninvasive bioreactance device was adopted to measure cardiac hemodynamics, and near-infrared spectroscopy was employed to assess perfusions in the frontal cerebral lobe (Δ[THb](FC)) and vastus lateralis muscle (Δ[THb](VL)). The results demonstrated that the Y-C group had higher levels of cardiac output, Δ[THb](FC), and Δ[THb](VL) during exercise than the O-C group. Moreover, these cardiac/peripheral hemodynamic responses to exercise in HF-III group were smaller than those in both HF-II and O-C groups. Although the change of cardiac output caused by exercise was normalized, the amounts of blood distributed to frontal cerebral lobe and vastus lateralis muscle in the HF-III group significantly declined during exercise. The HF-III patients had lower oxygen-uptake efficiency slopes (OUES) and greater Ve-Vo(2) slopes than the HF-II patients and age-matched controls. However, neither hemodynamic nor ventilatory response to exercise differed significantly between the HF-II and O-C groups. Cardiac output, Δ[THb](FC), and Δ[THb](VL) during exercise were directly related to the OUES and Vo(2peak) and inversely related to the Ve-Vco(2) slope. Moreover, cardiac output or Δ[THb](FC) was an effect modifier, which modulated the correlation status between Δ[THb](VL) and Ve-Vco(2) slope. We concluded that the suppression of cerebral/muscle hemodynamics during exercise is associated with ventilatory abnormality, which reduces functional capacity in patients with HF.

Regulation of pacing strategy during athletic competition

Background

Athletic competition has been a source of interest to the scientific community for many years, as a surrogate of the limits of human ambulatory ability. One of the remarkable things about athletic competition is the observation that some athletes suddenly reduce their pace in the mid-portion of the race and drop back from their competitors. Alternatively, other athletes will perform great accelerations in mid-race (surges) or during the closing stages of the race (the endspurt). This observation fits well with recent evidence that muscular power output is regulated in an anticipatory way, designed to prevent unreasonably large homeostatic disturbances.

Principal Findings

Here we demonstrate that a simple index, the product of the momentary Rating of Perceived Exertion (RPE) and the fraction of race distance remaining, the Hazard Score, defines the likelihood that athletes will change their velocity during simulated competitions; and may effectively represent the language used to allow anticipatory regulation of muscle power output.

Conclusions

These data support the concept that the muscular power output during high intensity exercise performance is actively regulated in an anticipatory manner that accounts for both the momentary sensations the athlete is experiencing as well as the relative amount of a competition to be completed.

Time to move beyond a brainless exercise physiology: the evidence for complex regulation of human exercise performance

In 1923, Nobel Laureate A.V. Hill proposed that maximal exercise performance is limited by the development of anaerobiosis in the exercising skeletal muscles. Variants of this theory have dominated teaching in the exercise sciences ever since, but 90 years later there is little biological evidence to support Hill’s belief, and much that disproves it. The cardinal weakness of the Hill model is that it allows no role for the brain in the regulation of exercise performance. As a result, it is unable to explain at least 6 common phenomena, including (i) differential pacing strategies for different exercise durations; (ii) the end spurt; (iii) the presence of fatigue even though homeostasis is maintained; (iv) fewer than 100% of the muscle fibers have been recruited in the exercising limbs; (v) the evidence that a range of interventions that act exclusively on the brain can modify exercise performance; and (vi) the finding that the rating of perceived exertion is a function of the relative exercise duration rather than the exercise intensity. Here I argue that the central governor model (CGM) is better able to explain these phenomena. In the CGM, exercise is seen as a behaviour that is regulated by complex systems in the central nervous system specifically to ensure that exercise terminates before there is a catastrophic biological failure. The complexity of this regulation cannot be appreciated if the body is studied as a collection of disconnected components, as is the usual approach in the modern exercise sciences.

Cerebral and muscle oxygenation changes during static and dynamic knee extensions to voluntary fatigue in healthy men and women: a near infrared spectroscopy study

The aim of the study was to examine the changes in cerebral and muscle blood volume (Cbv, Mbv) and oxygenation (Cox, Mox) during static and dynamic knee extensions to fatigue in men (N=10; 29±9 years) and women (N=14; 27±8 years). After assessment of 1 repetition maximum (1RM) during unilateral knee extensions with the dominant limb, each subject exercised at 50%, 75% and 100% of 1 RM in random order on separate occasions. Simultaneous changes in Cbv, Cox, Mbv and Mox from the contralateral prefrontal lobe and the dominant limb were measured by near infrared spectroscopy. During all three contractions, Cbv and Cox increased while Mbv and Mox decreased until fatigue in both genders. There were no signs of levelling off or decline in Cbv and Cox during any of these contractions, implying that there was no reduction in cerebral neuronal activation. Conversely, there was a rapid decline in Mbv and Mox during the early stages of the contractions, with a plateau or slight increase towards the end. The respective delta values at 50%, 75% and 100% of 1RM for Cbv (0·088 versus 0·062 versus 0·070), Cox (0·042 versus 0·033 versus 0·038), Mbv (-0·225 versus -0·198 versus -0·196), and Mox (-0·169 versus -0·146 versus -0·158) were not significantly different in the total group (N=24). These findings suggest that fatigue during resistance exercise lasting up to 60 s is mediated peripherally because of reduced blood volume and oxygen availability and is independent of the type and intensity of muscle contraction and gender.

Effect of graded hypoxia on supraspinal contributions to fatigue with unilateral knee-extensor contractions

Supraspinal fatigue, defined as an exercise-induced decline in force caused by suboptimal output from the motor cortex, accounts for over one-quarter of the force loss after fatiguing contractions of the knee extensors in normoxia. We tested the hypothesis that the relative contribution of supraspinal fatigue would be elevated with increasing severities of acute hypoxia. On separate days, 11 healthy men performed sets of intermittent, isometric, quadriceps contractions at 60% maximal voluntary contraction to task failure in normoxia (inspired O(2) fraction/arterial O(2) saturation = 0.21/98%), mild hypoxia (0.16/93%), moderate hypoxia (0.13/85%), and severe hypoxia (0.10/74%). Electrical stimulation of the femoral nerve was performed to assess neuromuscular transmission and contractile properties of muscle fibers. Transcranial magnetic stimulation was delivered to the motor cortex to quantify corticospinal excitability and voluntary activation. After 10 min of breathing the test gas, neuromuscular function and cortical voluntary activation prefatigue were unaffected in any condition. The fatigue protocol resulted in ∼ 30% declines in maximal voluntary contraction force in all conditions, despite differences in time-to-task failure (24.7 min in normoxia vs. 15.9 min in severe hypoxia, P < 0.05). Potentiated quadriceps twitch force declined in all conditions, but the decline in severe hypoxia was less than that in normoxia (P < 0.05). Cortical voluntary activation also declined in all conditions, but the deficit in severe hypoxia exceeded that in normoxia (P < 0.05). The additional central fatigue in severe hypoxia was not due to altered corticospinal excitability, as electromyographic responses to transcranial magnetic stimulation were unchanged. Results indicate that peripheral mechanisms of fatigue contribute relatively more to the reduction in force-generating capacity of the knee extensors following submaximal intermittent isometric contractions in normoxia and mild to moderate hypoxia, whereas supraspinal fatigue plays a greater role in severe hypoxia.

Central and peripheral fatigue kinetics during exhaustive constant-load cycling

The kinetics of central and peripheral fatigue development during an intensive constant-load cycling exercise was evaluated to better understand the mechanisms of task failure. Thirteen males cycled to exhaustion at 80% of maximal power output in intermittent bouts of 6 min of exercise with 4-min break between bouts to assess quadriceps fatigue with maximal voluntary contractions and single (1 Hz), paired (10 and 100 Hz) potentiated and interpolated magnetic stimulations of the femoral nerve (TwQ). Surface electromyographic signals (EMG) of the quadriceps muscles were recorded during stimulations and cycling. Total cycling duration (TCD) was 27 min 38 s±7 min 48 s. The mechanical response evoked by magnetic stimulation decreased mostly during the first half of TCD (TwQ1 Hz reduction: -34.4±12.2% at 40% TCD and -44.8±9.2% at exhaustion; P<0.001), while a reduction in maximum voluntary activation was present toward the end of exercise only (-5.4±4.8% and -6.4±5.6% at 80% TCD and exhaustion, respectively; P<0.01). The increase in quadriceps EMG during cycling was significantly correlated to the TwQ reduction for the rectus femoris (r(2) =0.20 at 1 Hz, r(2) =0.47 at 100 Hz, all P≤0.001). We conclude that peripheral fatigue develops early during constant-load intense cycling and is compensated by additional motor drive, while central fatigue appears to be associated with task failure.

Impact of pulmonary system limitations on locomotor muscle fatigue in patients with COPD

We examined the effects of respiratory muscle work [inspiratory (W(r-insp)); expiratory (W(r-exp))] and arterial oxygenation (Sp(O(2))) on exercise-induced locomotor muscle fatigue in patients with chronic obstructive pulmonary disease (COPD). Eight patients (FEV, 48 +/- 4%) performed constant-load cycling to exhaustion (Ctrl; 9.8 +/- 1.2 min). In subsequent trials, the identical exercise was repeated with 1) proportional assist ventilation + heliox (PAV); 2) heliox (He:21% O(2)); 3) 60% O(2) inspirate (hyperoxia); or 4) hyperoxic heliox mixture (He:40% O(2)). Five age-matched healthy control subjects performed Ctrl exercise at the same relative workload but for 14.7 min ( approximately best COPD performance). Exercise-induced quadriceps fatigue was assessed via changes in quadriceps twitch force (Q(tw,pot)) from before to 10 min after exercise in response to supramaximal femoral nerve stimulation. During Ctrl, absolute workload (124 +/- 6 vs. 62 +/- 7 W), W(r-insp) (207 +/- 18 vs. 301 +/- 37 cmH(2)O x s x min(-1)), W(r-exp) (172 +/- 15 vs. 635 +/- 58 cmH(2)O x s x min(-1)), and Sp(O(2)) (96 +/- 1% vs. 87 +/- 3%) differed between control subjects and patients. Various interventions altered W(r-insp), W(r-exp), and Sp(O(2)) from Ctrl (PAV: -55 +/- 5%, -21 +/- 7%, +6 +/- 2%; He:21% O(2): -16 +/- 2%, -25 +/- 5%, +4 +/- 1%; hyperoxia: -11 +/- 2%, -17 +/- 4%, +16 +/- 4%; He:40% O(2): -22 +/- 2%, -27 +/- 6%, +15 +/- 4%). Ten minutes after Ctrl exercise, Q(tw,pot) was reduced by 25 +/- 2% (P < 0.01) in all COPD and 2 +/- 1% (P = 0.07) in healthy control subjects. In COPD, DeltaQ(tw,pot) was attenuated by one-third after each interventional trial; however, most of the exercise-induced reductions in Q(tw,pot) remained. Our findings suggest that the high susceptibility to locomotor muscle fatigue in patients with COPD is in part attributable to insufficient O(2) transport as a consequence of exaggerated arterial hypoxemia and/or excessive respiratory muscle work but also support a critical role for the well-known altered intrinsic muscle characteristics in these patients.

Combination of two oxidant stressors suppresses the oxidative stress and enhances the heat shock protein 27 response in healthy humans

We tested the hypothesis that the combination of 2 oxidant stressors (hyperoxia and fatiguing exercise) might reduce or suppress the oxidative stress. We concomitantly measured the plasma concentration of heat shock proteins (Hsp) that protect the cells against the deleterious effects of reactive oxygen species. Healthy humans breathed pure oxygen under normobaric condition for 50-minute periods during which they stayed at rest or executed maximal static handgrip sustained until exhaustion. They also repeated handgrip bouts in normoxic condition. We performed venous blood measurements of 2 markers of the oxidative stress (thiobarbituric acid reactive substances and reduced ascorbic acid) and Hsp27. Under normoxic condition, the handgrip elicited an oxidative stress and a modest increase in plasma Hsp27 level (+7.1 +/- 5.4 ng/mL). Under hyperoxic condition, (1) at rest, compared with the same time schedule in normoxic condition, we measured an oxidative stress (increased thiobarbituric acid reactive substances and decreased reduced ascorbic acid levels) and the plasma Hsp27 level increased (maximal variation, +12.5 +/- 6.0 ng/mL); and (2) after the handgrip, the oxidative stress rapidly disappeared. The combination of both hyperoxia and handgrip bout doubled the Hsp27 response (maximal variation, +24.8 +/- 9.2 ng/mL). Thus, the combination of 2 hits eliciting an oxidative stress seems to induce an adaptive Hsp27 response that might counterbalance an excessive production of reactive oxygen species.

Impact of pulmonary system limitations on locomotor muscle fatigue in patients with COPD

We examined the effects of respiratory muscle work [inspiratory (W(r-insp)); expiratory (W(r-exp))] and arterial oxygenation (Sp(O(2))) on exercise-induced locomotor muscle fatigue in patients with chronic obstructive pulmonary disease (COPD). Eight patients (FEV, 48 +/- 4%) performed constant-load cycling to exhaustion (Ctrl; 9.8 +/- 1.2 min). In subsequent trials, the identical exercise was repeated with 1) proportional assist ventilation + heliox (PAV); 2) heliox (He:21% O(2)); 3) 60% O(2) inspirate (hyperoxia); or 4) hyperoxic heliox mixture (He:40% O(2)). Five age-matched healthy control subjects performed Ctrl exercise at the same relative workload but for 14.7 min ( approximately best COPD performance). Exercise-induced quadriceps fatigue was assessed via changes in quadriceps twitch force (Q(tw,pot)) from before to 10 min after exercise in response to supramaximal femoral nerve stimulation. During Ctrl, absolute workload (124 +/- 6 vs. 62 +/- 7 W), W(r-insp) (207 +/- 18 vs. 301 +/- 37 cmH(2)O x s x min(-1)), W(r-exp) (172 +/- 15 vs. 635 +/- 58 cmH(2)O x s x min(-1)), and Sp(O(2)) (96 +/- 1% vs. 87 +/- 3%) differed between control subjects and patients. Various interventions altered W(r-insp), W(r-exp), and Sp(O(2)) from Ctrl (PAV: -55 +/- 5%, -21 +/- 7%, +6 +/- 2%; He:21% O(2): -16 +/- 2%, -25 +/- 5%, +4 +/- 1%; hyperoxia: -11 +/- 2%, -17 +/- 4%, +16 +/- 4%; He:40% O(2): -22 +/- 2%, -27 +/- 6%, +15 +/- 4%). Ten minutes after Ctrl exercise, Q(tw,pot) was reduced by 25 +/- 2% (P < 0.01) in all COPD and 2 +/- 1% (P = 0.07) in healthy control subjects. In COPD, DeltaQ(tw,pot) was attenuated by one-third after each interventional trial; however, most of the exercise-induced reductions in Q(tw,pot) remained. Our findings suggest that the high susceptibility to locomotor muscle fatigue in patients with COPD is in part attributable to insufficient O(2) transport as a consequence of exaggerated arterial hypoxemia and/or excessive respiratory muscle work but also support a critical role for the well-known altered intrinsic muscle characteristics in these patients.

Cerebral oxygenation is reduced during hyperthermic exercise in humans

AIM

Cerebral mitochondrial oxygen tension (P(mito)O(2)) is elevated during moderate exercise, while it is reduced when exercise becomes strenuous, reflecting an elevated cerebral metabolic rate for oxygen (CMRO(2)) combined with hyperventilation-induced attenuation of cerebral blood flow (CBF). Heat stress challenges exercise capacity as expressed by increased rating of perceived exertion (RPE).

METHODS

This study evaluated the effect of heat stress during exercise on P(mito)O(2) calculated based on a Kety-Schmidt-determined CBF and the arterial-to-jugular venous oxygen differences in eight males [27 +/- 6 years (mean +/- SD) and maximal oxygen uptake (VO(2max)) 63 +/- 6 mL kg(-1) min(-1)].

RESULTS

The CBF, CMRO(2) and P(mito)O(2) remained stable during 1 h of moderate cycling (170 +/- 11 W, approximately 50% of VO(2max), RPE 9-12) in normothermia (core temperature of 37.8 +/- 0.4 degrees C). In contrast, when hyperthermia was provoked by dressing the subjects in watertight clothing during exercise (core temperature 39.5 +/- 0.2 degrees C), P(mito)O(2) declined by 4.8 +/- 3.8 mmHg (P < 0.05 compared to normothermia) because CMRO(2) increased by 8 +/- 7% at the same time as CBF was reduced by 15 +/- 13% (P < 0.05). During exercise with heat stress, RPE increased to 19 (19-20; P < 0.05); the RPE correlated inversely with P(mito)O(2) (r(2) = 0.42, P < 0.05).

CONCLUSION

These data indicate that strenuous exercise in the heat lowers cerebral P(mito)O(2), and that exercise capacity in this condition may be dependent on maintained cerebral oxygenation.

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.

Cerebral oxygenation decreases during exercise in humans with beta-adrenergic blockade

AIM

Beta-blockers reduce exercise capacity by attenuated increase in cardiac output, but it remains unknown whether performance also relates to attenuated cerebral oxygenation.

METHODS

Acting as their own controls, eight healthy subjects performed a continuous incremental cycle test to exhaustion with or without administration of the non-selective beta-blocker propranolol. Changes in cerebral blood flow velocity were measured with transcranial Doppler ultrasound and those in cerebral oxygenation were evaluated using near-infrared spectroscopy and the calculated cerebral mitochondrial oxygen tension derived from arterial to internal jugular venous concentration differences.

RESULTS

Arterial lactate and cardiac output increased to 15.3 +/- 4.2 mM and 20.8 +/- 1.5 L min(-1) respectively (mean +/- SD). Frontal lobe oxygenation remained unaffected but the calculated cerebral mitochondrial oxygen tension decreased by 29 +/- 7 mmHg (P < 0.05). Propranolol reduced resting heart rate (58 +/- 6 vs. 69 +/- 8 beats min(-1)) and at exercise exhaustion, cardiac output (16.6 +/- 3.6 L min(-1)) and arterial lactate (9.4 +/- 3.7 mM) were attenuated with a reduction in exercise capacity from 239 +/- 42 to 209 +/- 31 W (all P < 0.05). Propranolol also attenuated the increase in cerebral blood flow velocity and frontal lobe oxygenation (P < 0.05) whereas the cerebral mitochondrial oxygen tension decreased to a similar degree as during control exercise (delta 28 +/- 10 mmHg; P < 0.05).

CONCLUSION

Propranolol attenuated the increase in cardiac output of consequence for cerebral perfusion and oxygenation. We suggest that a decrease in cerebral oxygenation limits exercise capacity.

Reaction time to peripheral visual stimuli during exercise under normoxia and hyperoxia

The purpose of this study was to test if increased oxygen availability affected simple reaction time (RT) to peripheral visual stimuli during exercise. Twelve male participants performed RT tasks at rest, during cycling with three different workloads (100, 150, and 200 W), and after exercise. We fractionated RT into Premotor time and Motor time. Under normoxia, Premotor time significantly increased during exercise at 200 W (mean +/- SD, 224.7 +/- 34.8 ms) relative to that at rest (213.3 +/- 34.1 ms) (P < 0.05). In contrast, we found no difference in Premotor time between at rest (214.0 +/- 27.0 ms) and at 200 W (213.0 +/- 21.6 ms) under hyperoxia. Furthermore, Premotor time significantly decreased at 150 W (201.3 +/- 22.4 ms) relative to that at rest under hyperoxia (P < 0.05). These results suggest that increased oxygen availability during exercise has beneficial effects on perceptual performance.

Locomotor exercise induces long-lasting impairments in the capacity of the human motor cortex to voluntarily activate knee extensor muscles

Muscle fatigue is a reduction in the capacity to exert force and may involve a "central" component originating in the brain and/or spinal cord. Here we examined whether supraspinal factors contribute to impaired central drive after locomotor endurance exercise. On 2 separate days, 10 moderately active individuals completed a locomotor cycling exercise session or a control session. Brief (2 s) and sustained (30 s) isometric knee extension contractions were completed before and after locomotor exercise consisting of eight, 5-min bouts of cycling at 80% of maximum workload. In the control session, subjects completed the isometric contractions in a rested state. Twitch responses to supramaximal motor nerve stimulation and transcranial magnetic stimulation were obtained to assess peripheral force-generating capacity and voluntary activation. Maximum voluntary contraction (MVC) force during brief contractions decreased by 23 +/- 6.3% after cycling exercise and remained 12 +/- 2.8% below baseline 45 min later (F(1,9) > 15.5; P < 0.01). Resting twitch amplitudes declined by approximately 45% (F(1,9) = 28.3; P < 0.001). Cortical voluntary activation declined from 90.6 +/- 1.6% at baseline to 80.6 +/- 2.1% after exercise (F(1,9) = 28.0; P < 0.001) and remained significantly reduced relative to control 30-45 min later (80.6 +/- 3.4%; F(1,9) = 10.7; P < 0.01). Thus locomotor exercise caused a long-lasting impairment in the capacity of the motor cortex to drive the knee extensors. Force was reduced more during sustained MVC after locomotor exercise than in the control session. Peripheral mechanisms contributed relatively more to this force reduction in the control session, whereas supraspinal fatigue played a greater role in sustained MVC reduction after locomotor exercise.

On the mechanisms that limit oxygen uptake during exercise in acute and chronic hypoxia: role of muscle mass

Peak aerobic power in humans (VO2,peak) is markedly affected by inspired O2 tension (FIO2). The question to be answered in this study is what factor plays a major role in the limitation of muscle peak VO2 in hypoxia: arterial O2 partial pressure (Pa,O2) or O2 content (Ca,O2)? Thus, cardiac output (dye dilution with Cardio-green), leg blood flow (thermodilution), intra-arterial blood pressure and femoral arterial-to-venous differences in blood gases were determined in nine lowlanders studied during incremental exercise using a large (two-legged cycle ergometer exercise: Bike) and a small (one-legged knee extension exercise: Knee)muscle mass in normoxia, acute hypoxia (AH) (FIO2 = 0.105) and after 9 weeks of residence at 5260 m (CH). Reducing the size of the active muscle mass blunted by 62% the effect of hypoxia on VO2,peak in AH and abolished completely the effect of hypoxia on VO2,peak after altitude acclimatization. Acclimatization improved Bike peak exercise Pa,O2 from 34 +/- 1 in AH to 45 +/- 1 mmHg in CH(P <0.05) and Knee Pa,O2 from 38 +/- 1 to 55 +/- 2 mmHg(P <0.05). Peak cardiac output and leg blood flow were reduced in hypoxia only during Bike. Acute hypoxia resulted in reduction of systemic O2 delivery (46 and 21%) and leg O2 delivery (47 and 26%) during Bike and Knee, respectively, almost matching the corresponding reduction in VO2,peak. Altitude acclimatization restored fully peak systemic and leg O(2) delivery in CH (2.69 +/- 0.27 and 1.28 +/- 0.11 l min(-1), respectively) to sea level values (2.65 +/- 0.15 and 1.16 +/- 0.11 l min(-1), respectively) during Knee, but not during Bike. During Knee in CH, leg oxygen delivery was similar to normoxia and, therefore, also VO2,peak in spite of a Pa,O2 of 55 mmHg. Reducing the size of the active mass improves pulmonary gas exchange during hypoxic exercise, attenuates the Bohr effect on oxygen uploading at the lungs and preserves sea level convective O2 transport to the active muscles. Thus, the altitude-acclimatized human has potentially a similar exercising capacity as at sea level when the exercise model allows for an adequate oxygen delivery (blood flow x Ca,O2), with only a minor role of Pa,O2 per se, when Pa,O2 is more than 55 mmHg.

Opioid-mediated muscle afferents inhibit central motor drive and limit peripheral muscle fatigue development in humans

We investigated the role of somatosensory feedback from locomotor muscles on central motor drive (CMD) and the development of peripheral fatigue during high-intensity endurance exercise. In a double-blind, placebo-controlled design, eight cyclists randomly performed three 5 km time trials: control, interspinous ligament injection of saline (5K(Plac), L3-L4) or intrathecal fentanyl (5K(Fent), L3-L4) to impair cortical projection of opioid-mediated muscle afferents. Peripheral quadriceps fatigue was assessed via changes in force output pre- versus postexercise in response to supramaximal magnetic femoral nerve stimulation (DeltaQ(tw)). The CMD during the time trials was estimated via quadriceps electromyogram (iEMG). Fentanyl had no effect on quadriceps strength. Impairment of neural feedback from the locomotor muscles increased iEMG during the first 2.5 km of 5K(Fent) versus 5K(Plac) by 12 +/- 3% (P < 0.05); during the second 2.5 km, iEMG was similar between trials. Power output was also 6 +/- 2% higher during the first and 11 +/- 2% lower during the second 2.5 km of 5K(Fent) versus 5K(Plac) (both P < 0.05). Capillary blood lactate was higher (16.3 +/- 0.5 versus 12.6 +/- 1.0%) and arterial haemoglobin O(2) saturation was lower (89 +/- 1 versus 94 +/- 1%) during 5K(Fent) versus 5K(Plac). Exercise-induced DeltaQ(tw) was greater following 5K(Fent) versus 5K(Plac) (-46 +/- 2 versus -33 +/- 2%, P < 0.001). Our results emphasize the critical role of somatosensory feedback from working muscles on the centrally mediated determination of CMD. Attenuated afferent feedback from exercising locomotor muscles results in an overshoot in CMD and power output normally chosen by the athlete, thereby causing a greater rate of accumulation of muscle metabolites and excessive development of peripheral muscle fatigue.

Somatosensory feedback from the limbs exerts inhibitory influences on central neural drive during whole body endurance exercise

We investigated whether somatosensory feedback from contracting limb muscles exerts an inhibitory influence on the determination of central command during closed-loop cycling exercise in which the subject voluntarily determines his second-by-second central motor drive. Eight trained cyclists performed two 5-km time trials either without (5K(Ctrl)) or with lumbar epidural anesthesia (5K(Epi); 24 ml of 0.5% lidocaine, vertebral interspace L(3)-L(4)). Percent voluntary quadriceps muscle activation was determined at rest using a superimposed twitch technique. Epidural lidocaine reduced pretime trial maximal voluntary quadriceps strength (553 +/- 45 N) by 22 +/- 3%. Percent voluntary quadriceps activation was also reduced from 97 +/- 1% to 81 +/- 3% via epidural lidocaine, and this was unchanged following the 5K(Epi), indicating the presence of a sustained level of neural impairment throughout the trial. Power output was reduced by 9 +/- 2% throughout the race (P < 0.05). We found three types of significant effects of epidural lidocaine that supported a substantial role for somatosensory feedback from the exercising limbs as a determinant of central command throughout high-intensity closed-loop cycling exercise: 1) significantly increased relative integrated EMG of the vastus lateralis; 2) similar pedal forces despite the reduced number of fast-twitch muscle fibers available for activation; 3) and increased ventilation out of proportion to a reduced carbon dioxide production and heart rate and increased blood pressure out of proportion to power output and oxygen consumption. These findings demonstrate the inhibitory influence of somatosensory feedback from contracting locomotor muscles on the conscious and/or subconscious determination of the magnitude of central motor drive during high intensity closed-loop endurance exercise.

Effect of acute hypoxia on central fatigue during repeated isometric leg contractions

To determine whether hypoxia has a direct influence on the central command independently of the working muscles, 16 subjects performed intermittent isometric unilateral knee extensions until exhaustion either in normobaric hypoxia (inspired O(2) fraction=0.11, arterial oxygen saturation approximately 84%) or in normoxia while the knee extensor muscles were exposed to circulatory occlusion with a 250 mmHg cuff. Among the subjects, 11 also performed the tests in hypoxia and normoxia without occlusion. Single electrical stimulations were regularly delivered to the femoral nerve to measure the changes in the knee extensor peak twitch force. With the cuff, the average slope of decrease in peak twitch did not depend on the inspired oxygen fraction. Performance was slightly but significantly lower during hypoxia than in normoxia (8.2+/-2.6 vs 9.4+/-3.1 repetitions, P<0.05) with the cuff on. The number of repetitions was much higher during hypoxia with maintaining leg blood flow (15.6+/-4.5 repetitions) than with circulatory occlusion in normoxia. In conclusion, this study showed that a direct effect of hypoxia in reducing the motor drive to the working muscles exists but this effect is moderate.

Respiratory system determinants of peripheral fatigue and endurance performance

We briefly summarize recent evidence pertaining to how mechanisms primarily under the control of the respiratory system-namely, arterial oxyhemoglobin desaturation, respiratory muscle work and fatigue, and cyclical fluctuations in intrathoracic pressure-may contribute to exercise limitation. Respiratory influences on cardiac output and on sympathetic vasoconstrictor activity and blood flow distribution are shown to be important determinants of performance. We also address how a compromised O2 transport exacerbates the rate of development of peripheral muscle fatigue and, in turn, precipitates central fatigue and exercise limitation.

Locomotor muscle fatigue increases cardiorespiratory responses and reduces performance during intense cycling exercise independently from metabolic stress

Locomotor muscle fatigue, defined as an exercise-induced reduction in maximal voluntary force, occurs during prolonged exercise, but its effects on cardiorespiratory responses and exercise performance are unknown. In this investigation, a significant reduction in locomotor muscle force (−18%, P < 0.05) was isolated from the metabolic stress usually associated with fatiguing exercise using a 100-drop-jumps protocol consisting of one jump every 20 s from a 40-cm-high platform. The effect of this treatment on time to exhaustion during high-intensity constant-power cycling was measured in study 1 ( n = 10). In study 2 ( n = 14), test duration (871 ± 280 s) was matched between fatigue and control condition (rest). In study 1, locomotor muscle fatigue caused a significant curtailment in time to exhaustion (636 ± 278 s) compared with control (750 ± 281 s) ( P = 0.003) and increased cardiac output. Breathing frequency was significantly higher in the fatigue condition in both studies despite similar oxygen consumption and blood lactate accumulation. In study 2, high-intensity cycling did not induce further fatigue to eccentrically-fatigued locomotor muscles. In both studies, there was a significant increase in heart rate in the fatigue condition, and perceived exertion was significantly increased in study 2 compared with control. These results suggest that locomotor muscle fatigue has a significant influence on cardiorespiratory responses and exercise performance during high-intensity cycling independently from metabolic stress. These effects seem to be mediated by the increased central motor command and perception of effort required to exercise with weaker locomotor muscles.

Convective oxygen transport and fatigue

During exercise, fatigue is defined as a reversible reduction in force- or power-generating capacity and can be elicited by “central” and/or “peripheral” mechanisms. During skeletal muscle contractions, both aspects of fatigue may develop independent of alterations in convective O2delivery; however, reductions in O2supply exacerbate and increases attenuate the rate of accumulation. In this regard, peripheral fatigue development is mediated via the O2-dependent rate of accumulation of metabolic by-products (e.g., inorganic phosphate) and their interference with excitation-contraction coupling within the myocyte. In contrast, the development of O2-dependent central fatigue is elicited 1) by interference with the development of central command and/or 2) via inhibitory feedback on central motor drive secondary to the peripheral effects of low convective O2transport. Changes in convective O2delivery in the healthy human can result from modifications in arterial O2content, blood flow, or a combination of both, and they can be induced via heavy exercise even at sea level; these changes are exacerbated during acute and chronic exposure to altitude. This review focuses on the effects of changes in convective O2delivery on the development of central and peripheral fatigue.

Hyperoxia improves 20 km cycling time trial performance by increasing muscle activation levels while perceived exertion stays the same

Increasing inspiratory oxygen tension improves exercise performance. We tested the hypothesis that this is partly due to changes in muscle activation levels while perception of exertion remains unaltered. Eleven male subjects performed two 20-km cycling time-trials, one in hyperoxia (HI, FiO2 40%) and one in normoxia (NORM, FiO2 21%). Every 2 km we measured power output, heart rate, blood lactate, integrated vastus lateralis EMG activity (iEMG) and ratings of perceived exertion (RPE). Performance was improved on average by 5% in HI compared to NORM (P < 0.01). Changes in heart rate, plasma lactate concentration and RPE during the trials were similar. For the majority of the time-trials, power output was maintained in HI, but decreased progressively in NORM (P < 0.01) while it increased in both trials for the last kilometre (P < 0.0001). iEMG was proportional to power output and was significantly greater in HI than in NORM. iEMG activity increased significantly in the final kilometer of both trials (P < 0.001). This suggests that improved exercise performance in hyperoxia may be the result of increased muscle activation leading to greater power outputs. The finding of identical RPE, lactate and heart rate in both trials suggests that pacing strategies are altered to keep the actual and perceived exercise stress at a similar level between conditions. We suggest that a complex, intelligent system regulates exercise performance through the control of muscle activation levels in an integrative manner under conditions of normoxia and hyperoxia.