Reduced neuromuscular activity and force generation during prolonged cycling

Dehydration: cause of fatigue or sign of pacing in elite soccer?

Numerous studies have suggested that dehydration is a causal factor to fatigue across a range of sports such as soccer; however, empirical evidence is equivocal on this point. It is also possible that exercise-induced moderate dehydration is purely an outcome of significant metabolic activity during a game. The diverse yet sustained physical activities in soccer undoubtedly threaten homeostasis, but research suggests that under most environmental conditions, match-play fluid loss is minimal ( approximately 1-2% loss of body mass), metabolite accumulation remains fairly constant, and core temperatures do not reach levels considered sufficiently critical to require the immediate cessation of exercise. A complex (central) metabolic control system which ensures that no one (peripheral) physiological system is maximally utilized may explain the diversity of research findings concerning the impact of individual factors such as dehydration on elite soccer performance. In consideration of the existing literature, we propose a new interpretative pacing model to explain the self-regulation of elite soccer performance and, in which, players behaviourally modulate efforts according to a subconscious strategy. This strategy is based on both pre-match (intrinsic and extrinsic factors) and dynamic considerations during the game (such as skin temperature, thirst, accumulation of metabolites in the muscles, plasma osmolality and substrate availability), which enables players to avoid total failure of any single peripheral physiological system either prematurely or at the conclusion of a match. In summary, we suggest that dehydration is only an outcome of complex physiological control (operating a pacing plan) and no single metabolic factor is causal of fatigue in elite soccer.

Interpretation of EMG integral or RMS and estimates of "neuromuscular efficiency" can be misleading in fatiguing contraction

In occupational and sports physiology, reduction of neuromuscular efficiency (NME) and elevation of amplitude characteristics, such as root mean square (RMS) or integral of surface electromyographic (EMG) signals detected during fatiguing submaximal contraction are often related to changes in neural drive. However, there is data showing changes in the EMG integral (I(EMG)) and RMS due to peripheral factors. Causes for these changes are not fully understood. On the basis of computer simulation, we demonstrate that lengthening of intracellular action potential (IAP) profile typical for fatiguing contraction could affect EMG amplitude characteristics stronger than alteration in neural drive (central factors) defined by number of active motor units (MUs) and their firing rates. Thus, relation of these EMG amplitude characteristics only to central mechanisms can be misleading. It was also found that to discriminate between changes in RMS or I(EMG) due to alterations in neural drive from changes due to alterations in peripheral factors it is better to normalize RMS of EMG signals to the RMS of M-wave. In massive muscles, such normalization is more appropriate than normalization to either peak-to-peak amplitude or area of M-wave proposed in literature.

The top-down influence of ergogenic placebos on muscle work and fatigue

Placebos have been shown to induce powerful effects in a variety of medical conditions, such as pain and movement disorders, as well as to increase physical performance and endurance in healthy subjects. Here we investigated the effects of an ergogenic placebo on the performance of the quadriceps muscle, which is responsible for the extension of the leg relative to the thigh. In a first experiment, a placebo was administered along with the suggestion that it was caffeine at high dose. This resulted in a significant increase in mean muscle work across subjects, which, however, was not accompanied by a decrease of perceived muscle fatigue. In a second experiment, the placebo caffeine was administered twice in two different sessions. Each time, the weight to be lifted with the quadriceps was reduced surreptitiously so as to make the subjects believe that the 'ergogenic agent' was effective. After this conditioning procedure, the load was restored to the original weight, and both muscle work and perceived fatigue assessed after placebo administration. Compared with the first experiment, the placebo effect was larger, with a significant increase in muscle work and decrease in perceived muscle fatigue. Within the context of the role of peripheral and/or central mechanisms in muscle performance, the present findings suggest a central mechanism of top-down modulation of muscle fatigue. In addition, the difference between the first and second experiment underscores the role of learning in increasing muscle performance with placebos.

Neuromuscular fatigue is greater following highly variable versus constant intensity endurance cycling

The present study compared neuromuscular fatigue of the knee extensor muscles following highly variable versus constant power output cycling. Ten subjects performed two 33-min cycling trials of the same average power output, in a random order. Cycling exercise was performed either at constant (CST) power output, corresponding to 70% of the maximal aerobic power (MAP), or at variable (VAR) power output with alternating high (200, 150 and 100% of MAP during 10, 15 and 20 s, respectively) and moderate (50% of MAP) power output periods. Neuromuscular tests were performed before and immediately after the two trials. Heart rate (HR) was measured during exercise and blood lactate concentration ([La]) at the end of both trials. Reductions in maximal voluntary contraction torque, voluntary activation level and peak doublet were significantly greater after VAR than after CST. HR and [La] were significantly higher during VAR than during CST. Cycling at a varying power output in comparison to constant power resulted in additional muscular fatigue that may be explained by greater anaerobic contribution and muscle solicitation during the highly variable power output protocol.

Effect of carbohydrate ingestion and ambient temperature on muscle fatigue development in endurance-trained male cyclists

The aim of the present study was to determine the effect of carbohydrate (CHO; sucrose) ingestion and environmental heat on the development of fatigue and the distribution of power output during a 16.1-km cycling time trial. Ten male cyclists (V̇o2max = 61.7 ± 5.0 ml·kg−1·min−1, mean ± SD) performed four 90-min constant-pace cycling trials at 80% of second ventilatory threshold (220 ± 12 W). Trials were conducted in temperate (18.1 ± 0.4°C) or hot (32.2 ± 0.7°C) conditions during which subjects ingested either CHO (0.96 g·kg−1·h−1) or placebo (PLA) gels. All trials were followed by a 16.1-km time trial. Before and immediately after exercise, percent muscle activation was determined using superimposed electrical stimulation. Power output, integrated electromyography (iEMG) of vastus lateralis, rectal temperature, and skin temperature were recorded throughout the trial. Percent muscle activation significantly declined during the CHO and PLA trials in hot (6.0 and 6.9%, respectively) but not temperate conditions (1.9 and 2.2%, respectively). The decline in power output during the first 6 km was significantly greater during exercise in the heat. iEMG correlated significantly with power output during the CHO trials in hot and temperate conditions ( r = 0.93 and 0.73; P < 0.05) but not during either PLA trial. In conclusion, cyclists tended to self-select an aggressive pacing strategy (initial high intensity) in the heat.

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.

Physiological and electromyographic responses during 40-km cycling time trial: relationship to muscle coordination and performance

The purpose of this study was to compare the oxygen uptake (VO(2)), respiratory exchange ratio (RER), cadence and muscle activity during cycling a 40-km time trial (TT), and to analyse the relationship between muscle activity and power output (PO). Eight triathletes cycled a 40-km TT on their own bicycles, which were mounted on a stationary cycle simulator. The VO(2), RER and muscle activity (electromyography, EMG) from tibialis anterior (TA), gastrocnemius medialis (GA), biceps femoris (BF), rectus femoris (RF) and vastus lateralis (VL) of the lower limb were collected. The PO was recorded from the cycle simulator. The data were collected at the 3rd, 10th, 20th, 30th and 38th km. The root mean square envelope (RMS) of EMG was calculated. The VO(2) and PO presented a significant increase at the 38th km (45.23+/-8.35 ml kg min(-1) and 107+/-7.11% of mean PO of 40-km, respectively) compared to the 3rd km (38.12+/-5.98 ml kg min(-1) and 92+/-8.30% of mean PO of 40-km, respectively). There were no significant changes in cadence and RER throughout the TT. The VL was the only muscle that presented significant increases in the RMS at the 10th km (22.56+/-3.05% max), 20th km (23.64+/-2.52% max), 30th km (25.27+/-3.00% max), and 38th km (26.28+/-3.57%max) when compared to the 3rd km (21.03+/-1.88%max). The RMS of VL and RF presented a strong relationship to PO (r=0.89 and 0.86, respectively, p<0.05). The muscular steady state reported for cycling a 30-min TT seems to occur in the 40-km TT, for almost all assessed muscles, probably in attempt to avoid premature muscle fatigue.

Effect of oral administration of sodium bicarbonate on surface EMG activity during repeated cycling sprints

The purpose of this study was to determine the effect of oral administration of sodium bicarbonate (NaHCO3) on surface electromyogram (SEMG) activity from the vastus lateralis (VL) during repeated cycling sprints (RCS). Subjects performed two RCS tests (ten 10-s sprints) interspersed with both 30-s and 360-s recovery periods 1 h after oral administration of either NaHCO3 (RCSAlk) or CaCO3 (RCSPla) in a random counterbalanced order. Recovery periods of 360 s were set before the 5th and 9th sprints. The rate of decrease in plasma HCO3- concentration during RCS was significantly greater in RCSAlk than in RCSPla, but the rates of decline in blood pH during the two RCS tests were similar. There was no difference between change in plasma lactate concentration in RCSAlk and that in RCSPla. Performance during RCSAlk was similar to that during RCSPla. There were no differences in oxygen uptake immediately before each cycling sprint (preVO2) and in SEMG activity between RCSAlk and RCSPla. In conclusion, oral administration of NaHCO3 did not affect SEMG activity from the VL. This suggests that the muscle recruitment strategy during RCS is not determined by only intramuscular pH.

A 350-S recovery period does not necessarily allow complete recovery of peak power output during repeated cycling sprints

The aim of this study was to determine whether a 350-s recovery period allows recovery of peak power output (PPO) to its initial value under the condition of a blood lactate (La) concentration higher than 10 mmol.L-1 during repeated cycling sprints (RCS). RCS (10x10-s cycling sprints) were performed under two conditions. Under one condition, the recovery period of RCS was fixed at 35 s (RCS35), and under the other condition, a 350-s recovery period was set before the 5th and 9th sets, and a 35-s recovery period was set before the other sets (RCScomb). In RCScomb, PPO in the 5th set recovered to that in the 1st set, but PPO in the 9th set did not. Under both conditions, blood La concentration progressively increased and reached approximately 14 mmol.L-1 at the end of the RCS. In RCScomb, VO2 immediately before the 5th set was not significantly different from that immediately before the 9th set. Mean power frequency (MPF) values estimated by a surface electromyogram from the vastus lateralis in the 5th and 9th sets were significantly higher in RCScomb than in RCS35. In conclusion, a 350-s recovery period does not allow recovery of PPO to its initial value under the condition of a blood La concentration of 14 mmol.L-1 during RCS.

Protection of muscle membrane excitability during prolonged cycle exercise with glucose supplementation

To determine if exercise-induced depressions in neuromuscular function are altered with oral glucose supplementation, 15 untrained participants (Vo2 peak = 45 +/- 2 ml x kg(-1) x min(-1), mean +/- SE) performed prolonged cycle exercise at approximately 60% Vo2 peak on two occasions: without glucose supplementation (NG) and with oral glucose supplementation (G). The oral G began at 30 min of exercise and was administered every 15 min (total ingested = 1.23 +/- 0.11 g carbohydrate/kg body mass). Quadriceps isometric properties and membrane excitability were assessed prior to exercise, after 90 min of exercise, and at fatigue. Cycle time to fatigue was greater (P < 0.05) in G compared with NG (137 +/- 7 vs. 115 +/- 6 min). Progressive reductions (P < 0.05) in maximal voluntary contraction (MVC, N) were observed for NG at 90 min (441 +/- 29) and at fatigue (344 +/- 33) compared with pre-exercise (666 +/- 30). At fatigue in G, the reduction in MVC was not as pronounced (P < 0.05) as in NG. Motor unit activation assessed with the interpolated twitch technique during an MVC following exercise was not different between conditions. During cycling, the G condition also resulted in a higher (P < 0.05) muscle compound potential (M-wave) amplitude (mV) at both 90 min (+50%) and at fatigue (+87%) compared with NG. Similar effects were also found M-wave area (mV/ms). These results suggest that the ergogenic effect of glucose supplementation occurs not as a result of decreased neural activation but to improved muscle function, possibly as a consequence of protection of muscle membrane excitability.

Physiological function and neuromuscular recruitment in elite South African distance runners

Physiological studies of elite and sub-elite black South African runners show that these athletes are typically about 10–12 kg lighter than white athletes and that they are able to sustain higher exercise intensities for longer than white runners. Such superior performance is not a result of higher V O2max values and hence cannot be due to superior oxygen delivery to the active muscles during maximal exercise, as is predicted by the traditional cardiovascular/anaerobic/catastrophic models of exercise physiology. A marginally superior running economy is also unlikely to be a crucial determinant in explaining this apparent superiority. However, black athletes are able to sustain lower rectal and thigh, but higher mean skin, temperatures during exercise. Furthermore, when exercising in the heat, lighter black athletes are able to maintain higher running speeds than are larger white runners matched for running performance in cool environmental conditions. According to the contrasting theory that the body acts as a complex system during exercise, the superiority of black African athletes should be sought in an enhanced capacity to maintain homeostasis in all their inter-dependent biological systems despite running at higher relative exercise intensities and metabolic rates. In this case, any explanation for the success of East African runners will be found in the way in which their innate physiology, training, environment, expectations and genes influence the function of those parts of their subconscious (and conscious) brains that appear to regulate the protection of homeostasis during exercise as part of an integrative, complex biological system.

Constant versus variable-intensity during cycling: effects on subsequent running performance

The aim of this study was to investigate the metabolic responses to variable versus constant-intensity (CI) during 20-km cycling on subsequent 5-km running performance. Ten triathletes, not only completed one incremental cycling test to determine maximal oxygen uptake and maximal aerobic power (MAP), but also three various cycle-run (C-R) combinations conducted in outdoor conditions. During the C-R sessions, subjects performed first a 20-km cycle-time trial with a freely chosen intensity (FCI, approximately 80% MAP) followed by a 5-km run performance. Subsequently, triathletes were required to perform in a random order, two C-R sessions including either a CI, corresponding to the mean power of FCI ride, or a variable-intensity (VI) during cycling with power changes ranging from 68 to 92% MAP, followed immediately by a 5-km run. Metabolic responses and performances were measured during the C-R sessions. Running performance was significantly improved after CI ride (1118 +/- 72 s) compared to those after FCI ride (1134 +/- 64 s) or VI ride (1168 +/- 73 s) despite similar metabolic responses and performances reported during the three cycling bouts. Moreover, metabolic variables were not significantly different between the run sessions in our triathletes. Given the lack of significant differences in metabolic responses between the C-R sessions, the improvement in running time after FCI and CI rides compared to VI ride suggests that other mechanisms, such as changes in neuromuscular activity of peripheral skeletal muscle or muscle fatigue, probably contribute to the influence of power output variation on subsequent running performance.

Non-random fluctuations in power output during self-paced exercise

OBJECTIVES

To analyse the power output measured during a self-paced 20-km cycling time trial, during which power output was free to vary, in order to assess the level and characteristics of the variability in power output that occurred during the exercise bout.

METHODS

Eleven well-trained cyclists performed a 20-km cycling time trial, during which power output was sampled every 200 m. Power spectrum analysis was performed on the power output data, and a fractal dimension was calculated for each trial using the Higuchi method.

RESULTS

In all subjects, power output was maintained throughout the trial until the final kilometre, when it increased significantly, indicating the presence of a global pacing strategy. The power spectrum revealed the presence of 1/f-like scaling of power output and multiple frequency peaks during each trial, with the values of the frequency peaks changing over the course of the trial. The fractal dimension (D-score) was similar for all subjects over the 20-km trial and ranged between 1.5 and 1.9.

CONCLUSIONS

The presence of an end spurt in all subjects, 1/f-like scaling and multiple frequency peaks in the power output data indicate that the measured oscillations in power output during cycling exercise activity may not be system noise, but may rather be associated with system control mechanisms that are similar in different individuals.

Is fatigue all in your head? A critical review of the central governor model

The central governor model has recently been proposed as a general model to explain the phenomenon of fatigue. It proposes that the subconscious brain regulates power output (pacing strategy) by modulating motor unit recruitment to preserve whole body homoeostasis and prevent catastrophic physiological failure such as rigor. In this model, the word fatigue is redefined from a term that describes an exercise decline in the ability to produce force and power to one of sensation or emotion. The underpinnings of the central governor model are the refutation of what is described variously as peripheral fatigue, limitations models, and the cardiovascular/anaerobic/catastrophe model. This argument centres on the inability of lactic acid models of fatigue to adequately explain fatigue. In this review, it is argued that a variety of peripheral factors other than lactic acid are known to compromise muscle force and power and that these effects may protect against "catastrophe". Further, it is shown that a variety of studies indicate that fatigue induced decreases in performance cannot be adequately explained by the central governor model. Instead, it is suggested that the concept of task dependency, in which the mechanisms of fatigue vary depending on the specific exercise stressor, is a more comprehensive and defensible model of fatigue. This model includes aspects of both central and peripheral contributions to fatigue, and the relative importance of each probably varies with the type of exercise.

Effect of altered pre-exercise carbohydrate availability on selection and perception of effort during prolonged cycling

This study assessed the effect of altered carbohydrate (CHO) availability on self-selected work rate during prolonged time-trial cycling. Eight endurance-trained men undertook two experimental cycling time-trials after glycogen-depleting exercise and 2 days of: (a) high (9.3 +/- 0 g CHO kg(-1) day(-1)) (HC) and (b) low CHO intakes (0.6 +/- 0.1 g CHO kg(-1) day(-1)) (LC), via a double-blinded crossover design. All feedback regarding performance was removed during both exercise trials. Self-selected external power output was not different during the first 2 h of exercise between experimental conditions (P > 0.05), despite reported sensations of increased tiredness before and during exercise, significantly reduced whole body CHO oxidation (P < 0.05), plasma lactate concentrations (P < 0.05) and earlier onset of fatigue during exercise in LC versus HC. Perceived exertion was not different throughout exercise between conditions (P > 0.05). Mean power output declined significantly in LC versus HC (P < 0.05) after approximately 2 h of exercise, and was associated with significant reductions in cadence, heart rate and plasma glucose concentration (P < 0.05). These results demonstrate that when compared with time-trial cycling performed after a HC diet, reduced CHO availability does not initially alter self-selected work rate in endurance athletes who are deceived of their CHO status prior to exercise. This finding suggests that reduced work rate during exercise following lowered CHO intake may, in part, be a consequence of the subject's awareness of dietary CHO restriction rather than solely a physiologically mediated action. Further research is required to distinguish the influence of circulating glucose and peripheral glycogen availability on pacing strategy during prolonged exercise.

Models to explain fatigue during prolonged endurance cycling

Much of the previous research into understanding fatigue during prolonged cycling has found that cycling performance may be limited by numerous physiological, biomechanical, environmental, mechanical and psychological factors. From over 2000 manuscripts addressing the topic of fatigue, a number of diverse cause-and-effect models have been developed. These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue. This review outlines the more conventional linear models of fatigue and addresses specifically how these may influence the development of fatigue during cycling. The review concludes by showing how these linear models of fatigue might be integrated into a more recently proposed nonlinear complex systems model of exercise-induced fatigue.

The rate of heat storage mediates an anticipatory reduction in exercise intensity during cycling at a fixed rating of perceived exertion

The aim of the present study was to examine the regulation of exercise intensity in hot environments when exercise is performed at a predetermined, fixed subjective rating of perceived exertion (RPE). Eight cyclists performed cycling trials at 15 degrees C (COOL), 25 degrees C (NORM) and 35 degrees C (HOT) (65% humidity throughout), during which they were instructed to cycle at a Borg rating of perceived exertion (RPE) of 16, increasing or decreasing their power output in order to maintain this RPE. Power output declined linearly in all three trials and the rate of decline was significantly higher in HOT than in NORM and COOL (2.35 +/- 0.73 W min(-1), 1.63 +/- 0.70 and 1.61 +/- 0.80 W min(-1), respectively, P < 0.05). The rate of heat storage was significantly higher in HOT for the first 4 min of the trials only, as a result of increasing skin temperatures. Thereafter, no differences in heat storage were found between conditions. We conclude that the regulation of exercise intensity is controlled by an initial afferent feedback regarding the rate of heat storage, which is used to regulate exercise intensity and hence the rate of heat storage for the remainder of the anticipated exercise bout. This regulation maintains thermal homeostasis by reducing the exercise work rate and utilizing the subjective RPE specifically to ensure that excessive heat accumulation does not occur and cellular catastrophe is avoided.

The Role of Information Processing Between the Brain and Peripheral Physiological Systems in Pacing and Perception of Effort

This article examines how pacing strategies during exercise are controlled by information processing between the brain and peripheral physiological systems. It is suggested that, although several different pacing strategies can be used by athletes for events of different distance or duration, the underlying principle of how these different overall pacing strategies are controlled is similar. Perhaps the most important factor allowing the establishment of a pacing strategy is knowledge of the endpoint of a particular event. The brain centre controlling pace incorporates knowledge of the endpoint into an algorithm, together with memory of prior events of similar distance or duration, and knowledge of external (environmental) and internal (metabolic) conditions to set a particular optimal pacing strategy for a particular exercise bout. It is proposed that an internal clock, which appears to use scalar rather than absolute time scales, is used by the brain to generate knowledge of the duration or distance still to be covered, so that power output and metabolic rate can be altered appropriately throughout an event of a particular duration or distance. Although the initial pace is set at the beginning of an event in a feedforward manner, no event or internal physiological state will be identical to what has occurred previously. Therefore, continuous adjustments to the power output in the context of the overall pacing strategy occur throughout the exercise bout using feedback information from internal and external receptors. These continuous adjustments in power output require a specific length of time for afferent information to be assessed by the brain's pace control algorithm, and for efferent neural commands to be generated, and we suggest that it is this time lag that crates the fluctuations in power output that occur during an exercise bout. These non-monotonic changes in power output during exercise, associated with information processing between the brain and peripheral physiological systems, are crucial to maintain the overall pacing strategy chosen by the brain algorithm of each athlete at the start of the exercise bout.

Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance

The aim of this study was to investigate the effect of a high-fat diet (HFD) followed by 1 day of carbohydrate (CHO) loading on substrate utilization, heart rate variability (HRV), effort perception [rating or perceived exertion (RPE)], muscle recruitment [electromyograph (EMG)], and performance during a 100-km cycling time trial. In this randomized single-blind crossover study, eight well-trained cyclists completed two trials, ingesting either a high-CHO diet (HCD) (68% CHO energy) or an isoenergetic HFD (68% fat energy) for 6 days, followed by 1 day of CHO loading (8-10 g CHO/kg). Subjects completed a 100-km time trial on day 1 and a 1-h cycle at 70% of peak oxygen consumption on days 3, 5, and 7, during which resting HRV and resting and exercising respiratory exchange ratio (RER) were measured. On day 8, subjects completed a 100-km performance time trial, during which blood samples were drawn and EMG was recorded. Ingestion of the HFD reduced RER at rest (P < 0.005) and during exercise (P < 0.01) and increased plasma free fatty acid levels (P < 0.01), indicating increased fat utilization. There was a tendency for the low-frequency power component of HRV to be greater for HFD-CHO (P = 0.056), suggestive of increased sympathetic activation. Overall 100-km time-trial performance was not different between diets; however, 1-km sprint power output after HFD-CHO was lower (P < 0.05) compared with HCD-CHO. Despite a reduced power output with HFD-CHO, RPE, heart rate, and EMG were not different between trials. In conclusion, the HFD-CHO dietary strategy increased fat oxidation, but compromised high intensity sprint performance, possibly by increased sympathetic activation or altered contractile function.

Regulation of energy expenditure during prolonged athletic competition

BACKGROUND

Athletic competitions, such as the Tour de France, demand both momentary bursts of very high power output and the ability to provide high levels of energy expenditure for several weeks. As such, they provide a model of the ability for sustained muscular activity, which is important in terms of how humans are understood, not only as athletes, but also within an evolutionary context.

METHODS

Laboratory correlated HR responses were made of elite professional cyclists (N=7) during successive competitions in one of the three grand tours in cycling in successive years, with the intent of evaluating the magnitude and pattern of energy expenditure. HR recordings were normalized into a training impulse (TRIMP) score, summating the intensity and duration of each race, and tracked over the duration of successive tours.

RESULTS

Although the day-by-day pattern of HR responses in exercise intensity zones associated with exercise intensities below the first ventilatory threshold, between the first and second ventilatory thresholds, and above the second ventilatory threshold varied in response to the course and competitive situation, the net accumulation of both time in each of the HR zones and TRIMP was remarkably constant from one tour to the next, both within the group at large as well as within individual athletes. The magnitude of accumulation of TRIMP was similar to that of previous reports on elite tour cyclists.

CONCLUSIONS

We interpret these results as evidence that humans adopt a pacing strategy designed to optimally distribute energy reserves over the duration of each tour.

Effect of fluid ingestion on neuromuscular function during prolonged cycling exercise

OBJECTIVES

To investigate the effects of fluid ingestion on neuromuscular function during prolonged cycling exercise.

METHODS

Eight well trained subjects exercised for 180 minutes in a moderate environment at a workload requiring approximately 60% maximal oxygen uptake. Two conditions, fluid (F) and no fluid (NF) ingestion, were investigated.

RESULTS

During maximal voluntary isometric contraction (MVC), prolonged cycling exercise reduced (p<0.05) the maximal force generating capacity of quadriceps muscles (after three hours of cycling) and root mean square (RMS) values (after two hours of cycling) with no difference between the two conditions despite greater body weight loss (p<0.05) in NF. The mean power frequency (MPF) for vastus lateralis muscle was reduced (p<0.05) and the rate of force development (RFD) was increased (p<0.05) only during NF. During cycling exercise, integrated electromyographic activity and perceived exertion were increased in both conditions (p<0.05) with no significant effect of fluid ingestion.

CONCLUSIONS

The results suggest that fluid ingestion did not prevent the previously reported decrease in maximal force with exercise duration, but seems to have a positive effect on some indicators of neuromuscular fatigue such as mean power frequency and rate of force development during maximal voluntary contraction. Further investigations are needed to assess the effect of change in hydration on neural mechanisms linked to the development of muscular fatigue during prolonged exercise.

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans: summary and conclusions

It is hypothesised that physical activity is controlled by a central governor in the brain and that the human body functions as a complex system during exercise. Using feed forward control in response to afferent feedback from different physiological systems, the extent of skeletal muscle recruitment is controlled as part of a continuously altering pacing strategy, with the sensation of fatigue being the conscious interpretation of these homoeostatic, central governor control mechanisms.

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans

It is a popular belief that exercise performance is limited by metabolic changes in the exercising muscles, so called peripheral fatigue. Exercise terminates when there is a catastrophic failure of homoeostasis in the exercising muscles. A revolutionary theory is presented that proposes that exercise performance is regulated by the central nervous system specifically to ensure that catastrophic physiological failure does not occur during normal exercise in humans.

A signalling role for muscle glycogen in the regulation of pace during prolonged exercise

INTRODUCTION

In this study we examined the pacing strategy and the end muscle glycogen contents in eight cyclists, once when they were carbohydrate loaded and once when they were non-loaded.

METHODS

Cyclists completed 2 hours of cycling at approximately 73% of maximum oxygen consumption, which included five sprints at 100% of peak sustained power output every 20 minutes, followed immediately by a 1 hour time trial. Muscle biopsies were performed before and immediately after exercise, while blood samples were taken during the 2 hour steady state rides and immediately after exercise.

RESULTS

Carbohydrate loading improved mean power output during the 1 hour time trial (mean (SEM) 219 (17) v 233 (15) W; p<0.05) and enabled subjects to use significantly more muscle glycogen than during the trial following their normal diet. Significantly, the subjects, kept blind to all feedback except for time, started both time trials at similar workloads ( approximately 30 W), but after 1 minute of cycling, the workload average 14 W higher throughout the loaded compared with the non-loaded time trial. There were no differences in subjects' plasma glucose and lactate concentrations and heart rates in the carbohydrate loaded versus the non-loaded trial. Of the eight subjects, seven improved their time trial performance after carbohydrate loading. Finishing muscle glycogen concentrations in these seven subjects were remarkably similar in both trials (18 (3) v 20 (3) mmol/kg w/w), despite significantly different starting values and time trial performances (36.55 (1.47) v 38.14 (1.27) km/h; p<0.05). The intra-subject coefficient of variation (CV) for end glycogen content in these seven subjects was 10%, compared with an inter-subject CV of 43%.

CONCLUSIONS

As seven subjects completed the time trials with the same end exercise muscle glycogen concentrations, diet induced changes in pacing strategies during the time trials in these subjects may have resulted from integrated feedback from the periphery, perhaps from glycogen content in exercising muscles.

Evidence for complex system integration and dynamic neural regulation of skeletal muscle recruitment during exercise in humans

A model is proposed in which the development of physical exhaustion is a relative rather than an absolute event and the sensation of fatigue is the sensory representation of the underlying neural integrative processes. Furthermore, activity is controlled as part of a pacing strategy involving active neural calculations in a "governor" region of the brain, which integrates internal sensory signals and information from the environment to produce a homoeostatically acceptable exercise intensity. The end point of the exercise bout is the controlling variable. This is an example of a complex, non-linear, dynamic system in which physiological systems interact to regulate activity before, during, and after the exercise bout.

Skeletal muscle pathology in endurance athletes with acquired training intolerance

BACKGROUND

It is well established that prolonged, exhaustive endurance exercise is capable of inducing skeletal muscle damage and temporary impairment of muscle function. Although skeletal muscle has a remarkable capacity for repair and adaptation, this may be limited, ultimately resulting in an accumulation of chronic skeletal muscle pathology. Case studies have alluded to an association between long term, high volume endurance training and racing, acquired training intolerance, and chronic skeletal muscle pathology.

OBJECTIVE

To systematically compare the skeletal muscle structural and ultrastructural status of endurance athletes with acquired training intolerance (ATI group) with asymptomatic endurance athletes matched for age and years of endurance training (CON group).

METHODS

Histological and electron microscopic analyses were carried out on a biopsy sample of the vastus lateralis from 18 ATI and 17 CON endurance athletes. The presence of structural and ultrastructural disruptions was compared between the two groups of athletes.

RESULTS

Significantly more athletes in the ATI group than in the CON group presented with fibre size variation (15 v 6; p = 0.006), internal nuclei (9 v 2; p = 0.03), and z disc streaming (6 v 0; p = 0.02).

CONCLUSIONS

There is an association between increased skeletal muscle disruptions and acquired training intolerance in endurance athletes. Further studies are required to determine the nature of this association and the possible mechanisms involved.

Muscle coordination changes during intermittent cycling sprints

Maximal muscle power is reported to decrease during explosive cyclical exercises owing to metabolic disturbances, muscle damage, and adjustments in the efferent neural command. The aim of the present study was to analyze the influence of inter-muscle coordination in fatigue occurrence during 10 intermittent 6-s cycling sprints, with 30-s recovery through electromyographic activity (EMG). Results showed a decrease in peak power output with sprint repetitions (sprint 1 versus sprint 10: -11%, P<0.01) without any significant modifications in the integrated EMG. The timing between the knee extensor and the flexor EMG activation onsets was reduced in sprint 10 (sprint 1 versus sprint 10: -90.2 ms, P<0.05), owing to an earlier antagonist activation with fatigue occurrence. In conclusion, the maximal power output, developed during intermittent cycling sprints of short duration, decreased possibly due to the inability of muscles to maintain maximal force. This reduction in maximal power output occurred in parallel to changes in the muscle coordination pattern after fatigue.

The Science of Cycling

The aim of this review is to provide greater insight and understanding regarding the scientific nature of cycling. Research findings are presented in a practical manner for their direct application to cycling. The two parts of this review provide information that is useful to athletes, coaches and exercise scientists in the prescription of training regimens, adoption of exercise protocols and creation of research designs. Here for the first time, we present rationale to dispute prevailing myths linked to erroneous concepts and terminology surrounding the sport of cycling. In some studies, a review of the cycling literature revealed incomplete characterisation of athletic performance, lack of appropriate controls and small subject numbers, thereby complicating the understanding of the cycling research. Moreover, a mixture of cycling testing equipment coupled with a multitude of exercise protocols stresses the reliability and validity of the findings. Our scrutiny of the literature revealed key cycling performance-determining variables and their training-induced metabolic responses. The review of training strategies provides guidelines that will assist in the design of aerobic and anaerobic training protocols. Paradoxically, while maximal oxygen uptake (V-O(2max)) is generally not considered a valid indicator of cycling performance when it is coupled with other markers of exercise performance (e.g. blood lactate, power output, metabolic thresholds and efficiency/economy), it is found to gain predictive credibility. The positive facets of lactate metabolism dispel the 'lactic acid myth'. Lactate is shown to lower hydrogen ion concentrations rather than raise them, thereby retarding acidosis. Every aspect of lactate production is shown to be advantageous to cycling performance. To minimise the effects of muscle fatigue, the efficacy of employing a combination of different high cycling cadences is evident. The subconscious fatigue avoidance mechanism 'teleoanticipation' system serves to set the tolerable upper limits of competitive effort in order to assure the athlete completion of the physical challenge. Physiological markers found to be predictive of cycling performance include: (i) power output at the lactate threshold (LT2); (ii) peak power output (W(peak)) indicating a power/weight ratio of > or =5.5 W/kg; (iii) the percentage of type I fibres in the vastus lateralis; (iv) maximal lactate steady-state, representing the highest exercise intensity at which blood lactate concentration remains stable; (v) W(peak) at LT2; and (vi) W(peak) during a maximal cycling test. Furthermore, the unique breathing pattern, characterised by a lack of tachypnoeic shift, found in professional cyclists may enhance the efficiency and metabolic cost of breathing. The training impulse is useful to characterise exercise intensity and load during training and competition. It serves to enable the cyclist or coach to evaluate the effects of training strategies and may well serve to predict the cyclist's performance. Findings indicate that peripheral adaptations in working muscles play a more important role for enhanced submaximal cycling capacity than central adaptations. Clearly, relatively brief but intense sprint training can enhance both glycolytic and oxidative enzyme activity, maximum short-term power output and V-O(2max). To that end, it is suggested to replace approximately 15% of normal training with one of the interval exercise protocols. Tapering, through reduction in duration of training sessions or the frequency of sessions per week while maintaining intensity, is extremely effective for improvement of cycling time-trial performance. Overuse and over-training disabilities common to the competitive cyclist, if untreated, can lead to delayed recovery.

Deception and perceived exertion during high-intensity running bouts

This investigation examined the overall and localized perceived exertion responses to repeated bouts of submaximal, high-intensity running when subjects were deceived. Well-trained male and female n = 40) runners were randomly assigned to four groups who completed three 1680-m bouts of running at 80-86% peak treadmill running speed. The two experimental groups, Expected Similar and Expected Increase, were deceived of the actual run intensities while the two control groups, Control Increase and Control Similar, were informed of the actual protocol. After each run, ratings of perceived exertion (RPE) were taken for the whole body, chest, legs, head, and other areas. No significant differences were found in overall RPE between deceived and control groups. However, there was a tendency for the Expected Increase group, deceived into believing the intensity would be higher than they were subsequently made to run, to experience an attenuated increase in RPE between runs compared to the control group (Control Increase) who were honestly informed. For all groups, legs and chest were given consistently higher localized exertion scores than the head and other areas. It appears that a precise system of afferent feedback mediates the overall perceived exertion response during high-intensity running, and psychological intervention that alters pre-exercise expectations has minimal feedforward effect on exertion ratings taken postexercise.

Superior performance of African runners in warm humid but not in cool environmental conditions

The purpose of this study was to examine the running performances and associated thermoregulatory responses of African and Caucasian runners in cool and warm conditions. On two separate occasions, 12 (n = 6 African, n = 6 Caucasian) well-trained men ran on a motorized treadmill at 70% of peak treadmill running velocity for 30 min followed by an 8-km self-paced performance run (PR) in cool (15 degrees C) or warm (35 degrees C) humid (60% relative humidity) conditions. Time to complete the PR in the cool condition was not different between groups ( approximately 27 min) but was significantly longer in warm conditions for Caucasian (33.0 +/- 1.6 min) vs. African (29.7 +/- 2.3 min, P < 0.01) runners. Rectal temperatures were not different between groups but were higher during warm compared with cool conditions. During the 8-km PR, sweat rates for Africans (25.3 +/- 2.3 ml/min) were lower compared with Caucasians (32.2 +/- 4.1 ml/min; P < 0.01). Relative rates of heat production were less for Africans than Caucasians in the heat. The finding that African runners ran faster only in the heat despite similar thermoregulatory responses as Caucasian runners suggests that the larger Caucasians reduce their running speed to ensure an optimal rate of heat storage without developing dangerous hyperthermia. According to this model, the superior running performance in the heat of these African runners can be partly attributed to their smaller size and hence their capacity to run faster in the heat while storing heat at the same rate as heavier Caucasian runners.

Effects of supramaximal exercise on the electromyographic signal

AIM

To determine the neuromuscular recruitment characteristics during supramaximal exercise.

METHODS

Ten healthy subjects completed the Wingate anaerobic test (WAT) cycling protocol. Electromyographic (EMG) data and rate of fatigue were recorded throughout the cycling.

RESULTS

The mean (SD) rate of fatigue (decrease in power output) was 44.5 (8.6)%. No significant change was found in EMG amplitude. A significant decrease (p<0.01) in mean power frequency spectrum was found over the 30 second period.

CONCLUSIONS

During WAT, mean power frequency spectrum was attenuated with no decline in EMG amplitude, which may be caused by an accumulation of metabolites in the periphery. However, it is also possible that the feedback loop from intramuscular metabolism to the central nervous system is unable, within the 30 second period of the WAT, to affect neural recruitment strategy.

Nutritional aspects in ultra-endurance exercise

PURPOSE OF REVIEW

Despite much current debate regarding central and peripheral neural mechanisms which may be responsible for the onset of fatigue during prolonged exercise, maintenance of nutritional and hydration status remains critical for successful participation in ultra-endurance exercise. This review focuses on substrate and fluid homeostasis during ultra-endurance exercise and the use of nutritional supplementation both as ergogenic aid and to attenuate exercise-induced immunosuppression.

RECENT FINDINGS

Current evidence continues to support mandatory high carbohydrate intakes (1). before the event to maximize muscle glycogen stores, (2). during the event to prevent hypoglycaemia and (3). after the event to optimize post-event repletion of endogenous carbohydrate stores. No consistent performance benefit has yet been shown following a high-fat diet. Greater utilization of intrafascicular triglyceride stores appears to account for additional fat utilization in females. Recent trends towards excessive fluid intake have resulted in frequent reports of hyponatraemic hyperhydration in ultra-distance athletes, with greater incidence in women than in men. Carbohydrate supplementation during the event attenuates immunosuppressive hormonal and cytokine responses to ultra-endurance exercise, but may impair vitamin C absorption, while the ergogenic value of caffeine supplementation in ultra-endurance performance is currently being questioned.

SUMMARY

Meeting macronutrient and fluid intake demands remains an important priority for ultra-endurance athletes. Yet these athletes are reported to present with a high incidence of disordered eating patterns during periods of training, and excessive fluid replacement strategies have resulted in an increased incidence of water intoxication with resultant central nervous system dysfunction.

Endurance training and performance in runners: research limitations and unanswered questions

The purpose of this review is to discuss several limitations common to research concerning running and, secondly, to identify selected areas where additional research appears needed. Hopefully, this review will provide guidance for future research in terms of topics, as well as design and methodology. Limitations in the research include: lack of longitudinal studies, inadequate description of training status of individuals, lack of confirmation of state of rest, nourishment and hydration, infrequent use of allometric scaling to express oxygen uptake, relative neglect of anaerobic power and physical structure as determinants of performance, neglect of the central nervous system, and reliance on laboratory data. Further research in a number of areas is needed to enhance our knowledge of running performance. This includes: body mass as a performance determinant, evaluation of methods used to measure economy of running, assessing the link between strength and running performance, and further examination of training methods. While the amount of research on distance running is voluminous, the present state of knowledge is somewhat restricted by the limitations in research design and methodology identified here.

Mechanisms contributing to knee extensor strength loss after prolonged running exercise

The aim of this study was to identify the mechanisms that contribute to the decline in knee extensor (KE) muscles strength after a prolonged running exercise. During the 2 days preceding a 30-km running race [duration 188.7 +/- 27.0 (SD) min] and immediately after the race, maximal percutaneous electrical stimulations (single twitch, 0.5-s tetanus at 20 and 80 Hz) were applied to the femoral nerve of 12 trained runners. Superimposed twitches were also delivered during isometric maximal voluntary contraction (MVC) to determine the level of voluntary activation (%VA). The vastus lateralis electromyogram was recorded. KE MVC decreased from pre- to postexercise (from 188.1 +/- 25.2 to 142.7 +/- 29.7 N x m; P < 0.001) as did %VA (from 98.8 +/- 1.8 to 91.3 +/- 10.7%; P < 0.05). The changes from pre- to postexercise in these two variables were highly correlated (R = 0.88; P < 0.001). The modifications in the mechanical response after the 80-Hz stimulation and M-wave peak-to-peak amplitude were also significant (P < 0.001 and P < 0.05, respectively). It can be concluded that 1) central fatigue, neuromuscular propagation, and muscular factors are involved in the 23.5 +/- 14.9% reduction in MVC after a prolonged running bout at racing pace and 2) runners with the greatest KE strength loss experience large activation deficit.

Neuromuscular fatigue during a long-duration cycling exercise

The effects of prolonged cycling on neuromuscular parameters were studied in nine endurance-trained subjects during a 5-h exercise sustained at 55% of the maximal aerobic power. Torque during maximal voluntary contraction (MVC) of the quadriceps muscle decreased progressively throughout the exercise (P < 0.01) and was 18% less at the end of exercise compared with the preexercise value. Peak twitch torque, contraction time, and total area of mechanical response decreased significantly (P < 0.05) after the first hour of exercise. In contrast, changes in M-wave characteristics were significant only after the fourth hour of the exercise. Significant reductions (P < 0.05) in electromyographic activity normalized to the M wave occurred after the first hour for the vastus lateralis muscle but only at the end of the exercise for the vastus medialis muscle. Muscle activation level, assessed by the twitch interpolation technique, decreased by 8% (P < 0.05) at the end of the exercise. The results suggest that the time course is such that the contractile properties are significantly altered after the first hour, whereas excitability and central drive are more impaired toward the latter stages of the 5-h cycling exercise.

The influence of sensory cues on the perception of exertion during exercise and central regulation of exercise performance

The perception of effort during exercise and its relationship to fatigue is still not well understood. Although several scales have been developed to quantify exertion Borg's 15-point ratings of perceived exertion (RPE) scale has been adopted as a valid and reliable instrument for evaluating whole body exertion during exercise. However, Borg's category-ratio scale is useful in quantifying sensations of exertion related to those variables that rise exponentially with increases in exercise intensity. Previous research has examined the extent to which afferent feedback arising from cardiopulmonary and peripheral variables mediates the perception of exertion. However, the literature has not identified a single variable that consistently explains exertion ratings. It is concluded that effort perception involves the integration of multiple afferent signals from a variety of perceptual cues. In a process defined as teleoanticipation, the changes in perceived exertion that result from these afferent signals may allow exercise performance to be precisely regulated such that a task can be completed within the biomechanical and metabolic limits of the body. The accuracy with which individuals can regulate exercise intensity based upon RPE values, the decrease in muscle recruitment (central drive) that occurs before fatigue, and the extent to which perceived exertion and heart rate can be altered with hypnosis and biofeedback training all provide evidence for the existence of such a regulatory system. Future research is needed to precisely quantify the extent to which efferent feedforward commands and afferent feedback determine pacing strategies such that an exercise event can be completed without irreversible tissue damage.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.