The errors in the use of heart rate and time motion analysis as an estimation of energy

expenditure

The null-hypothesis that "For each player, during match play there is no significant difference between the estimation of energy

expenditure from heart rate and time-motion analysis" was rejected, this raised the question of which of the two techniques was the more valid measure of energy expenditure in "multiple-sprint" activity. The study in Chapter 7 was used to test the null-hypothesis, which stated that "During intermittent activity on the treadmill, there is no

significant difference between the use of heart rate to estimate energy expenditure and the measurement of energy expenditure from the collection of expired air". Although this hypothesis was rejected (p<0.01), the errors in the use of heart rate were in the region of 3.7 ± 5.1%, which shows that this methodology can be used to give a good measure of energy expenditure during multiple-sprint activity. These differences are much smaller than the errors suggested by previous studies in "multiple-sprint" activity (Bailor et al., 1989; Ogushi et al.,

1993), the larger errors found in these studies may have been caused by the methodologies adopted (see 2.2.3) rather than actual differences. Heart rate analysis has been found to be a non-obtrusive method of measuring the energy expenditure of players during "multiple-sprint" activity.

The use of heart rate will result in a slight over-estimation of total energy expenditure for the majority of subjects, but it can now be assumed that the rate of energy expenditure estimated for women’s hockey in Chapter 4 from heart rate analysis is a good indication of the actual energy expenditure in women's hockey. As the individual

errors for each subject will be slightly different, the comparison of the rate of energy expenditure between subjects should be used with caution.

In order to determine whether time-motion analysis could be used to estimate energy expenditure the following null-hypothesis was tested "During intermittent activity on the treadmill, there is no significant difference between the use of time-motion analysis to estimate energy expenditure and the measurement of energy expenditure from the

collection of expired air". This hypothesis was rejected. There were large errors associated with this method 16.6 ± 4.84 % in the treadmill study in Chapter 7. It is probable that these are even greater in the field situation where it is more difficult to determine the true energy cost of specific activities. This study has shown that due to the

additional energy requirements of changing activity, acceleration and deceleration, it cannot be used to accurately estimate energy

expenditure. This is in agreement with Reilly & Borrie (1992) who

suggested that the use of time-motion analysis to estimate energy expenditure during hockey match-play will underestimate the true value.

Using heart rate and time-motion analysis simultaneously will provide a greater understanding of the physiological stress placed on players during "multiple-sprint" sports. Heart rate analysis to measure a players energy expenditure and the physiological stress she is under, movement analysis to measure the work rate in terms of contribution to the game and the work: rest ratios. It must be remembered, though, that the measurement of both energy expenditure and work rate only tell us a player's physical contribution to the game. Neither of these analysis determines the player's effectiveness.

8.4.3 The relationship between energy expenditure

and movement intensity

There was a good correlation between the energy expenditure

estimated from both heart rate analysis and time motion analysis when expressed in relation to body weight and high intensity activity (r=0.80 and r=0.71 respectively). However this correlation was reduced when energy expenditure was expressed as kj.match'^ (r=0.55 and r=0.44 respectively). This demonstrates the effect of a player's weight on total energy expenditure, consequently the use of energy expenditure adjusted for body weight should be used to compare players and the rate of energy expenditure between sports. A second factor affecting total energy expenditure will be efficiency, for example in the study in Chapter 6 subject A had an energy cost of running at 12 km.h‘^ of 0.97 kj.kg'^.min’^, whereas subject F had an energy cost of 0.90 kj.kg"^.min' ^ for the same workload. This would suggest that total time spent in high intensity activity is a better indication of a players contribution

to the game rather than work rate expressed in kj.kg'^.min"^ as although there is a good correlation between the two indices, a less efficient player may appear to have a higher work rate than a more efficient player.

8.4.4 Fatigue during the second half

The results of the study in Chapter 4 accepted the null-hypothesis that "There is no significant difference between the energy expenditure (estimated from heart rate analysis) for the first and second halves." However, the null-hypothesis which stated that "There is no

significant difference between the energy expenditure (estimated from time-motion analysis) for the first and second halves" was

rejected. From the time-motion analysis (Chapter 5) it can be seen that players spend less time in high intensity activity during the second half. In contrast, the heart rate analysis would suggest that players have a similar work rate in the first and second halves. The similar heart rates during the second half may have been caused by a change in the relationship between heart rate and oxygen uptake at given workloads. If this is the case then heart rate would not be a good estimation of energy expenditure during the second half. This changing relationship has been observed in endurance activity (Astrand & Rodahl, 1986) and is referred to as heart rate drift. An alternative explanation might be that as the player becomes fatigued during the second half and her movements become less efficient, then the oxygen cost of specific activities will increase. In this case the estimation of energy expenditure during the second half can be obtained from heart rate as the heart rate would reflect the increased oxygen cost of each activity. Hence a drop in the quantity of high intensity work may not necessarily equate to a drop in energy expenditure if the energy cost of each activity increases.

8.5 The metabolic demands of women’s hockey

The metabolic demands of women’s hockey have been estimated in this study to range from 40.1 kj.min"^ to 64.4 kj..min"^ or a total energy

expenditure of 2806-4508 kj.match These values firstly identify

the high rates of energy expenditure during top level women's hockey and secondly the range of demands on players. These

studies were conducted during National League matches so it is likely that at international level the energy requirements and the intensity of the game will be even greater. The presence of ball boys/girls means that the ball is replaced immediately it leaves the pitch, so reducing stoppage time. The greater aerobic fitness found in studies on National Squads (Canadian national squad 59.3 ml.kg’^.min'^ Ready & van der Merwe (1986/87); Welsh national squad, 54.5 ml.kg'^.min"^, Reilly & Bretherton, (1986)) would suggest that players at National level are capable of sustaining play at a high intensity for longer than that observed in this study.

8.5.1 Positional requirements

The null-hypothesis that "In women's hockey, there is no significant difference in the energy expenditure (estimated from heart rate) in relation to position played" was accepted. No positional differences have been found in either the time-motion analysis or the estimation of energy expenditure from heart rate. This is in contrast to the findings in soccer where midfielders have been found to have higher energy expenditures (Ali & Farrally, 1990) and to cover greater distances (Reilly & Thomas, 1976) than both strikers and defenders. This would suggest that differences in both energy expenditure and work rate during women's hockey match play are determined to a large extent by individual factors for, example, body weight, efficiency and aerobic capacity rather than by positional requirements. This questions the recommendations of Cooke (1985) and Aitken & Thompson (1988) who both suggested that hockey players should follow individualised training programmes, both stating that midfielders should concentrate on the development of the aerobic system. Aitken & Thompson (1988) suggested that this should also be the emphasis for defenders, whereas Cooke (1985) suggested that defenders should emphasise the training of the anaerobic systems. The results in Chapter 5 of the time motion analysis suggest that all players need to develop the ability to reproduce sprmting speed which has been related to aerobic capacity (Tumilty et al., 1993) .

8.5.2 Range of metabolic demands for individual