The dancer : physical effort, muscle fibre types, and energy intake and expenditure

Abstract

The aims of this thesis were:

- to estimate aerobic fitness in dancers and analyse possible changes during a three-year dance course and after a detraining period; to compare different dance styles with regard to the duration of single exercises and the following rest periods;

- to estimate the physical effort in dancers during training sessions in different styles of dance; - to compare the physical effort during training sessions in dance teachers with that of dance students;

- to illuminate whether a deviating muscle fibre type distribution, if any, were training-induced or the result of selection of individuals with suitable fibre characteristics, muscle fibre type distribution was compared between children, at the very beginning of their professional dance training and dancers, who had performed professional dance training for 10 years. Moreover, muscle fibre type composition was analysed in dancers after detraining.

- To investigate the dietary habits of female dancers and to compare the estimated daily energy intake (Ein) with the estimated daily energy expenditure (Eexp) and to explore whether a discrepancy found in female dance students between estimated daily Ein and estimated daily Eexp could be explained by a depressed basal energy expenditure (BMR).

Results and conclusions: The estimated maximal oxygen uptake (VO2max) in the dance teachers and the dance students was 20-30% higher than in a randomly selected population with average physical activity. This and the 20% increase of estimated VO2max in the students over the three-year dance course indicate that dance training implies rather high demands on the cardiovascular system. Dance training is a predominantly intermittent type of exercise of brief active and rest periods. This concerns classical ballet as well as other dances styles as modern dance, jazz dance and character dance. The physical effort expressed as heart rate (HR) in % of maximal HR was approximately 70% for both teachers and students with no difference between the dance styles. This would correspond to a perceived exertion of "somewhat hard" to "hard" The blood lactate concentration indicates a moderate activation of the anaerobic energy production during dance training. Muscle fibre type composition in young dancers, even at the very beginning of their professional dance training, differs from that in the average individual and is characterised by a high % of type I fibres, similar to that found in 20 years old dancers. This, together with the fact that detraining did not change the muscle fibre type composition, supports the idea that the high percentage of type I fibres in dancers is due to a selection of individuals with suitable muscle fibre composition to the dance profession rather than being an effect of training. The 20% higher estimated daily Eexp than daily Ein as found in the dance students and no change in body weight over a six month period suggest that the Eexp was overestimated or the Ein was underestimated. In our estimation of daily Eexp a normal BMR was assumed. If the BMR had been lower than expected this would have led to an overestimation of the daily Eexp. In a subsequent study a normal BMR was demonstrated in the dance students. Thus, no support was found for a depressed BMR in the dancers, which consequently cannot contribute to explain the apparent discrepancy between daily Eexp and daily Ein. This may underline the uncertainty of dietary surveys, when applied to groups engaged in extreme physical activity as also pointed out by others.