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THE PHYSIOLOGICAL BASIS OF FATIGUE

INTRODUCTION

In one sense, competitive swimming is a battle against fatigue on two fronts. Firstly, during competition, it is normally a race between getting to the wall before the onset of debilitating fatigue – the experience of one US swimmer being overtaken in the last 10m is well known to all coaches. All swimmers can swim 25m faster than 100m race pace – unfortunately only the best swimmers can maintain that pace or something near to it over the full race distance. Secondly, the substantial training loads required to support high levels of competitive performance, are themselves are a battle against fatigue. Endurance can be defined as the ability to resist or delay the onset of fatigue – with fatigue itself defined as the loss of force production or power output.

What are the physiological causes of fatigue? Many of the causes have been identified and have received considerable publicity in both the scientific and sporting communities. This is particularly evident in relation to the so-called metabolic causes of fatigue such as depletion of fuels (e.g. muscle glycogen) and the accumulation of metabolic end products (e.g. lactate). These concepts are familiar to most coaches. Whilst experience, scientific research and routine monitoring have shown repeatedly that these metabolic factors can be limiting factors of performance, it is apparent that other factors influence the ability to sustain a given power output (swimming speed). These factors include: the need to maintain homeostasis of fluid and electrolyte levels in the general circulation and within skeletal muscles, regulatory mechanisms in skeletal muscle, neuromuscular fatigue and the often neglected concept of Neuro psychophysiology (will power).

Metabolic Causes of Fatigue

The carbohydrate and muscle glycogen story is well known to most coaches. The body’s reserves of carbohydrates (blood glucose, liver glycogen and muscle glycogen) are relatively limited and will be depleted rapidly during long and/or intensive training sessions. The training and competitive performance of a glycogen depleted swimmer will be significantly limited. Dr David Costill, a leading US Sports Scientist, has conducted a number of studies, which have highlighted the deleterious impact of successive days of swimming training on muscle glycogen levels. Fortunately, the depletion of glycogen reserves can be reversed rapidly with an adequate dietary intake of carbohydrate and the use of high-energy carbohydrate sports drinks during and after training. These issues have been dealt with in detail by Dr Louise Burke in previous issues of Australian Swim Coach.

Metabolic End Products

One of the main end products of the anaerobic metabolism of glycogen is lactic acid (the so-called lactic acid energy system). This system is an important contributor to the energy requirements of most competitive swimming races. Increased dependence on this energy pathway will lead to an accumulation of lactic acid within the muscle and eventually the general circulation (bloodstream) – the accumulation occurs when the production rate of lactic acid exceeds the removal rate. The lactic acid produced dissociates (splits) into the lactate anion and the hydrogen ion. Excessive accumulation of the hydrogen ion – beyond a level which can be adequately handled by buffering systems within the muscle and circulation – will lead to a metabolic acidosis indicated by a reduction in muscle and blood pH. A significant drop in the intramuscular pH will severely limit muscular contraction, as a number of rate limiting enzymes are pH sensitive. This sequence of events has been studied for many years and is well understood by scientists. This work has also led to the examination of the potential benefits of supplements such as bicarbonate and creatine phosphate, which increase the body’s natural buffering capacity, limit the deleterious effects of lactic acid accumulation and thus sustain (or improve) swimming performance.

Non-Metabolic Factors

The view that the physiological causes of fatigue are simply metabolic in origin may have limited our understanding of the training process of elite swimmers. Attributing fatigue exclusively to depletion of carbohydrates and/or the accumulation of lactate is an oversimplification. Whilst it is known that muscular fatigue is largely influenced by these metabolic processes, there is evidence showing dissociation between a reduction in power output and changes in metabolic variables such as ATP turnover and lactic acid accumulation. Counsilman and Counsilman (1993) point out that the ability to resist fatigue, delay its onset or even increase work output as the swimmer fatigues, involves a complex sequence of physiological events of which energy depletion and waste product accumulation constitute only one link. Most coaches will have observed the phenomena where a swimmer at the end of long hard quality set is able to produce his or her fastest repeat, when presumably, they are becoming quite fatigued. Although some swimmers may save themselves for a big final effort, it is apparent that other swimmers who have been right at the limit are able to find something extra for the finish.

Neuro psychophysiology – The Importance of Will Power

The answer to this paradox is thought to lie in the relationship between neural, physiological and psychological factors. Study of these relationships has led to the emergence of a new scientific field of study – Neuro psychophysiology and various cousins such as Neuro psycho immunology. Scientists contend that the ability to combine and optimise the various parts of this pathway – from the central nervous system, to the peripheral nerves, and finally the control of skeletal muscle contraction – may offer an explanation for these phenomena. This concept might also explain the observation that it is always difficult to swim a really fast time trial in training, and it requires a high-level competition for swimmers to perform at or near their best.

It appears that the extra sense of urgency, motivation and/or psychological pressure of high-level competition is the catalyst for the full activation of this pathway. To win an Olympic Gold Medal the coach and swimmer must achieve something special. The willpower to sustain intense training over many months and the willpower to beat all opponents to the wall during competition may be one difference between good and great swimmers. The question is how can the psychophysiological pathway be improved or controlled so that it is optimised at the right time? The answer or indeed the parameters to this question are unknown at present. What is clear is that coaches need to be aware of both the physiological and psychological development of their swimmers. If a coach can develop the skills to master and overcome physiological and psychophysiological factors of fatigue in their swimmers in training, the probability of success in competition should rise accordingly. The ability to harness this potential and have a swimmer swim above themselves is the key to international success. Is this one of the secrets of the great Australian swimming coaches … the motivators such as Don Talbot, Laurie Lawrence and Bill Sweetenham, and the cajolers such as Forbes Carlile, John Carew, Terry Gathercole and Joe King?

 

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