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School of Physiotherapy

The Physiological Effects of Endurance Events

Proposition for Debate - by Lynne Thompson

Contents

Statement of the Topic

The Physiological effects of endurance events

Introduction

Endurance events in sports are now commonly widespread, and seen in sports such as swimming, running and cycling, in races such as the Paris-Dakar road race, marathons, and the Tour de France. With the introduction of triathlons in the 1970s, more of the population is competing in these at a non-elite level.

For all endurance athletes, the critical determinant of success is their ability to sustain a high rate of energy output for extended periods of time. To accomplish this, all systems of the athlete's body must make physiological adaptations. This is accomplished with training exercise-induced changes.

Aerobic capacity (VO2 max), economy of movement (Submaximal VO2), and fractional utilisation of the maximal capacity (%VO2 max) reflect the physiological changes.

Aerobic Capacity

It has been documented in successful endurance athletes have high maximal aerobic capacity. Triathletes, who compete over three different disciplines, also demonstrate this comparing favourably to the single event athlete (O'Toole and Douglas, 1995).

However in the measurements of triathletes VO2 max has been found to fluctuate in accordance to the activity being tested. The VO2 max on a cycling ergometer will be 3-6% less than that seen in treadmill running, while swimming is 13-18% less (O'Toole and Douglas, 1995).

In all endurance events, VO2 max has been studied to relate its importance in the overall performance in the athlete. Generally this does not have a high correlation in the endurance athlete, and it is even more difficult to judge the performance based on these tests in the triathlete. In a study of short course triathletes that compared the VO2 max of the three components of the discipline, swimming was found to have the highest correlation of VO2 max to performance, but the cycling and running components were not necessarily related (Sleivert and Wenger, 1993). This relationship is even less specific in the longer events (O'Toole and Douglas, 1995).

This weak relationship between VO2 max and performance can be explained in the triathlete, when considering the demands of the event. The main factor that may influence this is that all the VO2 max tests are maximal tests, while the triathlete is required to perform all three components with sustained submaximal outputs for a prolonged period of time.

Extrinsic factors that can affect the cardiovascular system, the haemodynamic system, and the thermoregulatory system, all play a role in the endurance event, and have the ability to affect the performance time of the athlete.

VO2 max represents the capacity of an individual for energy transfer; this requires the integration of the respiratory, cardiovascular, and neuromuscular systems. The respiratory system is not seen as a limiting factor in exercise performance. Mahler et al (1982) found that in highly trained marathon runners, there was no difference between their respiratory function and that of the matched sedentary controls. Therefore, the cardiovascular system and the neuromuscular system are the two that do adapt to endurance training. The adaptations that are important to the endurance athlete occur centrally (increased cardiac output), and peripherally (maximal arterio-venous O2 difference).

Improvements of VO2 max seen centrally in the cardiovascular system are based on an increased cardiac output (CO) during exercise. Maximal CO and the subsequent increase in oxygen and substrate carrying capacity increases with endurance exercise training (O'Toole and Douglas, 1995). As CO is dependant on heart rate (HR) and stroke volume (SV) for alteration (McArdle et al 1996), the increase in CO is a result in the increase in SV, as maximal HR does not change, which has been demonstrated by Gledhill et al (1994) in cyclists.

The increase in SV is brought about due to an increase in left ventricular mass and cavity size, but with no sign of hypertrophy (Douglas, 1989; Urhausen and Kindermann, 1999). With an increase in cavity size, the heart has a greater preload and filling of the ventricles, during diastole, and greater systolic emptying (McArdle et al, 1996). Gledhill et al (1994) showed increasing left ventricular emptying rates in cyclists during a maximal exercise test, along with longer left ventricular emptying times. Thereby postulating that the endurance athlete is able to enhance systolic function by decreasing the systolic end function, and also enhancing early diastolic filling.

In the triathlete, special considerations must be taken into account. For the cardiovascular system to make the necessary adaptations, the system must be placed into overload. In the three components of triathlon, different overloads are presented to the cardiovascular system and these must be accounted for. In swimming the body is horizontal, with the added pressure of the water, the HR is slower than on land or in the upright position. In cycling, due to the posture of the athlete, increased arterial pressures in the lower limbs may cause both a pressure and volume overload on the heart during training. In running, the blood volume may decrease due to lack of fluid balance, and cause the heart to function inefficiently (O'Toole and Douglas, 1995).

Metabolic changes occur with endurance training in the muscles of the elite athletes, to allow for the efficient usage of oxygen being supplied to the muscles.

The adaptations that occur include: increased number and size of mitrochondria, increased ATP production, decreased amounts of lactic acid, increased triglyceride content, increased energy derived from carbohydrate and fatty acid, lower glycogen depletion in the muscles during exercise, increased enzyme activity, and improved efficiency in utilising oxygen from the blood supply (Taylor et al, 1999). All of these adaptations allow the athlete to increase their VO2 max, and widening of the arterio-venous difference (a-v O2 difference).

The increase in the number and size of the mitochondria, along with enzyme number and activity, enhance the muscles' ability to utilise the oxygen being supplied. The amount of oxygen to the muscle is increased via redirection of the blood supply to the active muscles. The mitochondrial enzymes work rate increases with endurance training to increase the athletes' ability to work aerobically, and conserve muscle glycogen. Enzymes that limit or regulate muscle glycogen use increase under the endurance training condition, and the oxidative capacity is enhanced in both type I and II fibres. There is a close relationship in the time course between the changes in oxidative enzyme activities and the improvement in VO2 max (Taylor and Bachman, 1999).

The oxidative capacity of the active muscles is vital for the endurance athlete to perform to their maximum. However, long distance events are detrimental to this capacity. Hochli et al (1995) found that the oxidative capacity was considerably reduced in participants in the extreme distance run from Paris to Dakar. In these subjects, the mitochondrial volume density was significantly decreased in the fibre volume, the sarcolemma, and the interfibrils.

When testing the endurance athlete for, it is important that the testing procedures are sports specific VO2 max. Under these conditions, cross-country skiers, rowers, and cyclists show the highest VO2 max.

Economy of Movement

The efficiency of motion is the relationship between the energy input and the resultant mechanical output. In the endurance events energy output must be sustained for long periods of time, therefore economy of movement is vital for the endurance athlete.

It has been estimated that in human locomotion (ie walking, running and cycling), the mechanical efficiency ranges between 20 to 30 % (McArdle et al, 1996), while in swimming it is only 5 to 9.5 % (Toussaint and Beck, 1992). There is however a great variation among individuals who have greater natural economy of motion, and can travel faster.

In the triathlete, economy of motion is extremely important especially in the swimming component which a very technically difficult sport. In specialist swimmers, it is suggested that they have better velocities than in triathletes. The elite swimmer has a greater propelling efficiency (the power used to overcome drag over the total power output) than the elite triathlete (Toussaint, 1990).

In cycling, economy of movement has been able to be related to the set up of the bicycle, with the seat position, crank length, body position, aerobars and the shoe/pedal interfaces having the potential to affect the athletes submaximal VO2, and therefore their efficiency of movement (Gregor and Wheeler, 1994).

In distance runners, the highly trained have been found to have better VO2 than the runners with lesser ability. It has also been found that the elite long distance runner have good correlations with economy of movement and performance times, especially in runners with comparable VO2 max levels.

In the triathlon events it must also be considered that the athlete's efficiency of motion is also affected by the previous components, and therefore, the technique of the athlete in each component is not the only factor affecting the performance times (O'Toole and Douglas, 1995).

The majority of the studies in efficiency of movement in swimmers and cyclists have looked at mechanical and biomechanical elements of these sport rather than the physiological factors.

In the cardiovascular system, the direction of blood flow to the active musculature is essential, and most especially in triathlon events. The other adaptations necessary to maintain economy of movement are the maintenance of blood volume, and the peripheral response to thermoregulatory needs. The main adaptation in the cardiovascular system is the ability of the heart to maintain the necessary oxygen supply to the working muscles. The left and right ventricular end diastolic volumes increase, and the submaximal training causes the HR to slow, therefore decreasing the oxygen demands of the heart (O'Toole and Douglas, 1995).

Economy of movement also requires improved in metabolism to gain efficiency in substrate utilisation. Aerobic energy transfer enzymes cause enhanced metabolism of fat and carbohydrates in distance events. With training, the contribution of fat decreases compared to carbohydrate and substrate usage, this allows the activity to continue for prolonged periods of time as the muscle glycogen depletion is delayed (Taylor and Bachman, 1999).

In terms of efficiency, burning fat as a fuel occurs at a far higher oxygen cost, than when compared to the use of carbohydrates as the fuel (McArdle et al 1996).

Muscle fibre type has also been said to play an important part in the economy of movement in the distance athletes. It has been suggested that individuals who perform well in the endurance events have a greater number of Type I and IIa fibres, ranging from 56-100 % in the locomotion muscles of the leg (Taylor and Bachman, 1999). Studies have found that the Type I fibres hypertrophy in adults involved in endurance training, but this does not occur in prepubescent children (Taylor and Bachman, 1999). It is generally accepted that the increased number of slow twitch fibres in endurance athletes is due to mainly genetic factors rather than the training affect occurring in these events.

The neural adaptations involved in producing the appropriate firing sequence of the active muscles can be considered to play a vital role in the efficiency of movement in all endurance events. These adaptations are bought about through training schedules where the tempo of training is performed at the same or slightly higher pace of the actual competition. This ensures that the neural system is able to activate the correct muscles in the most efficient manner automatically, even in the stressed atmosphere of a race.

Fractional Utilisation of Maximal Capacity

The fractional utilisation of maximal capacity (%VO2 max) represents the effects of economy of movement and VO2 max. The stress placed on the body's physiological systems by exercise is based on the relative rather than the absolute intensity of the exercise. The intensity of the exercise may be quantified on as a percentage of maximal capacity (%VO2 max) (O'Toole and Bachman, 1995).

The %VO2 max in endurance athletes can be assessed in two ways: the measurement of %VO2 max at a specific submaximal exercise speed, and the highest %VO2 max sustained during a competition.

Running performance has been associated with a specific submaximal speed %VO2 max. In this situation a lower %VO2 max is thought to be associated with improved running performance. The lower %VO2 max at specific submaximal speeds (15 km/hr) produces less physiological stresses on the athlete's body (Sjodin and Svendenberg, 1985).

In an endurance event the athlete aims to run at a fast pace, while keeping the lactic acid production in the muscle and blood at a level where acidosis does not occur. An athlete's ability to perform close to VO2 max during a competition can be represented by %VO2 max. In distance running, the athlete's ability to use a large %VO2 max while keeping the production of lactate acid at a minimum is crucial to performance (O'Toole and Bachman, 1995).

In an individual athlete, the ability to sustain enough energy and have a high enough %VO2 max to participate competitively in an endurance event is dependent on the ventilatory and lactate thresholds.

The lactate threshold of athletes has been measured by two main theories: the fixed lactate threshold of 4.0 mmol/l, and the individualised lactate threshold.

From the 1970s, a fixed lactate level was described for the evaluation of an individual. However this is now being questioned and the move is to test for an individualised lactate level to deliver a more accurate prediction of the athlete's performance (Loat and Rhodes, 1993).

The individualised lactate level is measured during both exercise and recovery, and assuming that the blood lactate kinetics in recovery reflect that of exercise, the blood lactate levels can be accurately evaluated. Although the individualised lactate level relies on some assumptions, it is now being used instead of the fixed 4.0 mmol/l blood lactate levels (Loat and Rhodes, 1993).

The ventilatory threshold is based on inspection of one or two parameters (eg excretion of CO2, VO2 max, ventilation, and the respiratory exchange ratio), over time and/or velocity to identify the breakaway threshold, where the linear relationship of these parameters deviates. Originally this has been achieved by visual inspection, but there are now computerised models being used (Loat and Rhodes, 1993; McArdle et al, 1996).

In the triathlete, the energy output and the %VO2 max representing the ventilatory or lactate threshold may vary in the three parts of the event, whereas in the single event competitor this is not an issue. The lactate thresholds based on 4.0mmol/l are comparable between triathletes and competitive runners and swimmers (Loat and Rhodes, 1993). In endurance training, the aim is to shift the lactate curve to the right, so that the threshold occurs at a higher %VO2 max (Loat and Rhodes, 1993; O'Toole and Douglas, 1995).

The alterations of lactate levels govern the %VO2 max, and these are specific to the type of exercise. Most of the adaptations that govern the blood lactate levels are due to the changes in the periphery to enhance metabolism, with training induced increases in capillary density, size and number of mitochondria. In the athlete the changes must be done within the specific sports requirements. Therefore, the triathlete must train specifically for all three components of the events and these may occur at different energy outputs in each of the three events (O'Toole and Douglas, 1995; Davis et al, 1979).

Other Considerations for the Athletes

Muscle Damage

For the endurance athlete, intensive training and long competitions are accompanied by muscle damage.

Evidence has been found in the muscles, with increases in intramuscular enzymes and myoglobin found in the blood after events. The athlete complains of post event muscular soreness, and this can be validated by histological findings. These findings include degenerative changes within the muscles, and necrosis occurs with macrophages and neutrophils being present in the damaged muscle cells, and surrounding interstial tissue. Following the damage to the muscle, the fibres are regenerated, therefore not allowing a net loss of fibres (Armstrong, 1986).

The cause of damage to the muscle is not fully known, with many theories suggested. These have included such things as metabolic overload, where the buffering system is worked to the point of not being able to protect the body against the harmful substances, such as lactic acid, which build up in the muscles during prolonged exercise, especially in the untrained individual.

Eccentric contractions have been found to cause greater amounts of damage within the muscle, as they produce high tension locally.

Training reduces the amount of muscle damage in a specific task, however even in highly trained individuals, the endurance events and will cause an increase in the muscle damage due to the high and prolonged intensities of the exercise. Training is the only factor that has been found to prevent damage in the active muscle during endurance events (Armstrong, 1986).

Cardiac Considerations

The risk of cardiac muscle damage has been suggested in the endurance athletes, with the possibility of small areas of myocardial necrosis occurring during pronged intense exercise. This hypothesis was not supported in a study looking into the levels of enzymes that indicate myocardial damage during a professional cycling race that lasted 22 days. The authors of this study suggest that there is no permanent damage of the myocardium of top level athletes, although there was an increase of these enzymes over the course of the 22 days (Bonetti et al, 1995).

A depression of the left ventricular systolic function was found in a large number of competitors in an ultradistance, ironman triathletes, an occurrence thought to be produced by fatigue of the myocardium (Douglas, 1989).

These changes appear to occur regularly in many endurance athletes, but the cause of these findings is unclear, but may be due to sustaining an increased HR over long periods of time (O'Toole and Douglas, 1995).

The above are just two of the many considerations that an endurance athlete must be aware of. Others that are also import to the athlete's performance are nutrition and energy requirements, recovery following an event, fluid and nutrition intake during the event to maintain the body's physiological needs, and prevention of injury.

Conclusion

The endurance event place demands on most of the physiological systems of an athlete's body. Training is vital to help meet these demands, and adaptations must be made through training to allow the athlete to perform at their optimal level.

VO2 max (aerobic capacity), submaximal VO2 max (economy of movement) and %VO2 max (fractional utilisation of maximal capacity) are all measurable responses that indicate the athlete's ability to make these physiological adaptations.

The exercise training induced physiological adaptations made via training are reflected by the above measurements, and these can be used to predict the athlete's ability to perform well at these events.

Training plays a large role in the ability of an athlete to compete at a high level, and as mentioned is vital to provide the necessary physiological adaptations. It must be remembered that the training must be sports specific, and the measurements taken to determine levels of metabolites and the performance of the cardiovascular system must also be performed in a sports specific test.

The triathlete demonstrates a need for specificity of testing in each of the three components of the event, and the appropriate test must be applied to measure each of the values in swimming, cycling and running. This is necessary as the measurement of VO2 max, submaximal VO2 max, %VO2 max, and lactate and ventilatory thresholds do not have high correlation when comparing the three components.

Nutrition, energy needs, fluid intake, and muscular damage occurring in events must also be seen as vital considerations by the athlete, and the support network in trying to achieve the best physiological state for the athlete to perform under.

The endurance athlete must be aware of all considerations of taking part in endurance events, and be able to train according to their particular event in order to gain their optimal performance times when competing.

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