Mike Lambert

Autonomic control of heart rate during and after exercise : measurements and implications for monitoring training status.

Endurance training decreases resting and submaximal heart rate, while maximum heart rate may decrease slightly or remain unchanged after training. The effect of endurance training on various indices of heart rate variability remains inconclusive. This may be due to the use of inconsistent analysis methodologies and different training programmes that make it difficult to compare the results of various studies and thus reach a consensus on the specific training effects on heart rate variability. Heart rate recovery after exercise involves a coordinated interaction of parasympathetic re-activation and sympathetic withdrawal. It has been shown that a delayed heart rate recovery is a strong predictor of mortality. Conversely, endurance-trained athletes have an accelerated heart rate recovery after exercise. Since the autonomic nervous system is interlinked with many other physiological systems, the responsiveness of the autonomic nervous system in maintaining homeostasis may provide useful information about the functional adaptations of the body. This review investigates the potential of using heart rate recovery as a measure of training-induced disturbances in autonomic control, which may provide useful information for training prescription.

The Quantification of Training Load, the Training Response and the Effect on Performance

Historically, the ability of coaches to prescribe training to achieve optimal athletic performance can be attributed to many years of personal experience. A more modern approach is to adopt scientific methods in the development of optimal training programmes. However, there is not much research in this area, particularly into the quantification of training programmes and their effects on physiological adaptation and subsequent performance. Several methods have been used to quantify training load, including questionnaires, diaries, physiological monitoring and direct observation. More recently, indices of training stress have been proposed, including the training impulse, which uses heart rate measurements and training load, and session rating of perceived exertion measurements, which utilizes subjective perception of effort scores and duration of exercise. Although physiological adaptations to training are well documented, their influence on performance has not been accurately quantified. To date, no single physiological marker has been identified that can measure the fitness and fatigue responses to exercise or accurately predict performance. Models attempting to quantify the relationship between training and performance have been proposed, many of which consider the athlete as a system in which the training load is the input and performance the system output. Although attractive in concept, the accuracy of these theoretical models has proven poor. A possible reason may be the absence of a measure of individuality in each athlete’s response to training. Thus, in the future more attention should be directed towards measurements that reflect individual capacity to respond or adapt to exercise training rather than an absolute measure of changes in physiological variables that occur with training.

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

AbstractThe 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 exertionHowever, 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 feed forward commands and afferent feedback determine pacing strategies such that an exercise event can be completed without irreversible tissue damage. 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 feed forward commands and afferent feedback determine pacing strategies such that an exercise event can be completed without irreversible tissue damage.

The Conscious Perception of the Sensation of Fatigue

In this review, fatigue is described as a conscious sensation rather than a physiological occurrence. We suggest that the sensation of fatigue is the conscious awareness of changes in subconscious homeostatic control systems, and is derived from a temporal difference between subconscious representations of these homeostatic control systems in neural networks that are induced by changes in the level of activity. These mismatches are perceived by consciousness-producing structures in the brain as the sensation of fatigue. In this model, fatigue is a complex emotion affected by factors such as motivation and drive, other emotions such as anger and fear, and memory of prior activity. It is not clear whether the origin of the conscious sensation of fatigue is associated with particular localised brain structures, or is the result of electrophysiological synchronisation of entire brain activity.

Evidence for neuromuscular fatigue during high-intensity cycling in warm, humid conditions.

Abstract The purpose of this study was to examine and describe the neuromuscular changes associated with fatigue using a self-paced cycling protocol of 60-min duration, under warm, humid conditions. Eleven subjects [mean (SE) age 21.8 (0.8) years; height 174.9 (3.0) cm; body mass 74.8 (2.7) kg; maximum oxygen consumption 50.3 (1.8) ml · kg · min−1] performed one 60-min self-paced cycling time trial punctuated with six 1-min “all out” sprints at 10-min intervals, while 4 subjects repeated the trial for the purpose of determining reproducibility. Power output, integrated electromyographic signal (IEMG), and mean percentile frequency shifts (MPFS) were recorded at the mid-point of each sprint. There were no differences between trials for EMG variables, distance cycled, mean heart rate, and subjective rating of perceived exertion for the subjects who repeated the trial (n=4). The results from the repeated trials suggest that neuromuscular responses to self-paced cycling are reproducible between trials. The mean heart rate for the 11 subjects was 163.6 (0.71) beats · min−1. Values for power output and IEMG expressed as a percentage of that recorded for the initial sprint decreased during sprints 2–5, with normalised values being 94%, 91%, 87% and 87%, respectively, and 71%, 71%, 73%, and 77%, respectively. However, during the final sprint normalised power output and IEMG increased to 94% and 90% of initial values, respectively. MPFS displayed an increase with time; however, this was not significant (P=0.06). The main finding of this investigation is the ability of subjects to return power output to near initial values during the final of six maximal effort sprints that were included as part of a self-paced cycling protocol. This appears to be due to a combination of changes in neuromuscular recruitment, central or peripheral control systems, or the EMG signal itself. Further investigations in which changes in multiple physiological systems are assessed systematically are required so that the underlying mechanisms related to the development of fatigue during normal dynamic movements such as cycling can be more clearly delineated.

Obesity and overweight in South African primary school children—the Health of the Nation Study

Objectives. To determine the prevalence of overweight and obesity in a sample of South African children aged 6–13 years. Design. Random sampling of schools within each provincial and socio-economic category. Setting. Primary school children from 5 South African provinces. Subjects. 10 195 (5 611 male and 4 584 female) primary school children. Outcome measure. Height and weight were measured and body mass index (BMI) (weight (kg)/height (m)2) was calculated for each grouping (age x gender x ethnic group). Cut-off points for BMI defining obese and overweight for gender and age (6–13 years) were calculated in accordance with international standards. Results. There were significant differences in height and mass between the different ethnic groups and genders. This trend was not evident for the BMI values. The prevalence of obesity within the sample was 3.2% for boys and 4.9% for girls, whereas overweight prevalence was 14.0% for boys and 17.9% for girls. When the contribution of each ethnic group was adjusted to the demographics of South Africa these values were only slightly different. The prevalence of obesity and overweight among boys was 2.4% and 10.9% respectively, while obese and overweight girls comprised 4.8% and 17.5%, respectively. Conclusions. South African children show trends of obesity and overweight, similar to values in developed countries about 10 years ago. Intervention strategies to combat an increasingly sedentary lifestyle may need to be developed for the South African context.

Neural control of force output during maximal and submaximal exercise.

AbstractA common belief in exercise physiology is that fatigue during exercise is caused by changes in skeletal muscle metabolism. This ‘periphera’ fatigue results either from substrate depletion during submaximal exercise or metabolite accumulation during maximal exercise in the exercising muscles. However, if substrate depletion alone caused fatigue, intracellular ATP levels would decrease and lead to rigor and cellular death. Alternatively, metabolite accumulation would prevent any increase in exercise intensity near the end of exercise. At present, neither of these effects has been shown to occur, which suggests that fatigue may be controlled by changes in efferent neural command, generally described as ‘central’ fatigue.In this review, we examine neural efferent command mechanisms involved in fatigue, including the concepts of muscle wisdom during short term maximal activity, and muscle unit rotation and teleoanticipation during submaximal endurance activity. We propose that neural strategies exist to maintain muscle reserve, and inhibit exercise activity before any irreparable damage to muscles and organs occurs. The finding that symptoms of fatigue occur in the nonexercising state in individuals with chronic fatigue syndrome indicates that fatigue is probably not a physiological entity, but rather a sensory manifestation of these neural regulatory mechanisms.

Metabolic Consequences of Exercise-Induced Muscle Damage

Exercise-induced muscle damage (EIMD) is commonly experienced following either a bout of unaccustomed physical activity or following physical activity of greater than normal duration or intensity. The mechanistic factor responsible for the initiation of EIMD is not known; however, it is hypothesised to be either mechanical or metabolic in nature. The mechanical stress hypothesis states that EIMD is the result of physical stress upon the muscle fibre. In contrast, the metabolic stress model predicts that EIMD is the result of metabolic deficiencies, possibly through the decreased action of Ca2+-adenosine triphosphatase. Irrespective of the cause of the damage, EIMD has a number of profound metabolic effects. The most notable metabolic effects of EIMD are decreased insulin sensitivity, prolonged glycogen depletion and an increase in metabolic rate both at rest and during exercise. Based on current knowledge regarding the effects that various types of damaging exercise have on muscle metabolism, a new model for the initiation of EIMD is proposed. This model states that damage initiation may be either metabolic or mechanical, or a combination of both, depending on the mode, intensity and duration of exercise and the training status of the individual.

Metabolic rate, not percent dehydration, predicts rectal temperature in marathon runners.

This study was designed to determine the factors predicting the post-race rectal temperature in marathon runners. Post-race rectal temperatures of 30 recreational runners (maximum oxygen consumption (VO2max) = 58.3 +/- 5.9 ml O2.kg-1.min-1; mean +/- SD) who completed a 42.2 km marathon at 75.8% (+/- 9.3%) VO2max were measured and related to their levels of dehydration (percent mass loss), their running velocities (km.h-1), and their estimated absolute metabolic rates (1 O2.min-1) for different segments of the 42.2 km race. The influence of certain anthropometric variables was also determined. Percent mass loss during the race (2.5 +/- 1.4%), post-race rectal temperatures (38.9 +/- 0.6 degrees C), and rates of sweat loss (1.0 +/- 0.3 1.h-1) were low. There was no statistical relationship between percent mass loss and post-race rectal temperature. Post-race rectal temperatures were significantly related to the metabolic rates for the full 42.2 km and for the last 21.1 and 6 km of the race, and to the average running velocity for the last 6 km (P less than 0.05 and P less than 0.01). Average sweat rates were related to metabolic rates for 42.2 km and for the last 6 km of the race (P less than 0.05) but were unrelated to running velocity. We conclude that metabolic rate sustained during the latter section of the race, and not the level of dehydration, is the principal determinant of the post-race rectal temperature in marathon runners.

Isokinetic Muscle Strength Predicts Maximum Exercise Tolerance in Renal Patients on Chronic Hemodialysis

Patients with end-stage renal disease receiving chronic hemodialysis have impaired exercise tolerance. To distinguish between a central cardiorespiratory and a peripheral skeletal muscular origin for this fatigue, we measured exercise performance and peak oxygen consumption during a maximum exercise test in 10 patients receiving chronic hemodialysis. Skeletal muscle function was measured with an isokinetic cycle ergometer and a Cybex II isokinetic dynamometer. Peak rates of oxygen consumption (17.7 +/- 3.6 [mean +/- SD] mL O2/kg/min), blood lactate concentrations (3.4 +/- 0.9 mmol/L), peak heart rates (168 +/- 12 beats/min), and rates of ventilation (37.3 +/- 14.6 L/min) were low, but respiratory exchange ratios (1.1 +/- 0.1) were compatible with maximal effort. There was a significant correlation between isokinetic muscle strength and VO2 peak, exercise duration, peak ventilation, and peak blood lactate concentrations (P less than 0.05 to less than 0.001), but not between hemoglobin concentration, total blood hemoglobin content, or hematocrit and these variables. Therefore, in renal dialysis patients, isokinetic muscle strength is a better predictor of exercise capacity than are variables determining blood oxygen carrying capacity. This suggests that altered skeletal muscle function explains the impaired exercise tolerance of anemic patients with end-stage renal disease receiving chronic hemodialysis.