Barbosa T, Keskinen K, Fernandes R, Colaço P, Lima A e Vilas-Boas JP (2004). Speed flutuation as a determinant factor of energy cost in butterfly stroke. In E van Praagh, J Coudert, N Fellmann, P Duché (eds.), Book of Abstracts of the 9th Annual Congress of the European Colledge of Sport Science. Clermond-Ferrand, France: 337.

Introduction
Although numerous papers around biomechanical or bioenergetic characteristics of different swimming techniques have been published, only a few approaches combine these two domains. Alves et al. (1996) explored the links between speed fluctuation and energy cost (EC) in front crawl and backstroke. Vilas-Boas (1996) made a similar study in breaststroke. In backstroke at low velocities and in breaststroke it was suggested that an increase in speed fluctuation would induce an increase of the EC (Alves et al., 1996; Vilas-Boas, 1996).
While there are no scientific approaches between the speed fluctuation and the EC concerning the butterfly stroke, the purpose of this study was to examine the relationships between the speed fluctuation of the centre of mass and the EC, in butterfly stroke.

Methods
Five Portuguese swimmers of national level performed one maximal and two sub-maximal (85% and 75%) 200-m butterfly swims in a 25-m swimming pool. Cardio-pulmonary and gas exchange parameters were measured breath by breath for each swim to analyze oxygen consumption (VO2) and other energetic parameters by portable metabolic cart (K4b2, Cosmed, Rome, Italy). A respiratory snorkel and valve system with low hydrodynamic resistance was used to measure pulmonary ventilation and to collect breathing air samples. Blood samples from the ear lobe were collected before and after each swim to analyze blood lactate concentration (YSI 1500L, Yellow Springs, US). Total energy expenditure (Ėtot) and EC were calculated for each swim. The swims were videotaped (50 Hz) in sagital plane with a set of two cameras providing dual projection from both underwater and above the water surface. The two cameras were placed stationary on the opposite wall of the pool perpendicular to the line of motion. The cameras were real time synchronised and the images were edited on a mixing table to create one single image of dual projection. APAS system (Ariel Dynamics Inc, USA) was used to analyse speed fluctuation for the centre of mass. Coefficients of variation for the horizontal velocity of the centre of mass along with the stroke cycle were calculated. Linear regressions between the bioenergetic and biomechanical variables were computed, as well as, its coefficients of determination and correlation (p≤0.05).

Results
There was a significant relationship between Ėtot and velocity (r=0.827, p=0.0005) so that the energy expenditure increased linearly with the velocity of displacement. Statistically significant correlation coefficient between the EC and the speed fluctuation (r=0.807, p=0.0009) was found, the coefficient of determination being r2=0.651. This means that the associate variance of both variables was 65.1%, the increase in the EC being strongly associated with the increase in the speed fluctuation. The individual coefficients of correlation and determination between the EC and the speed fluctuation were very high (mean r2 = 0.986±0.018, ranging from 0.973 to 1.000). The mean of individual correlation coefficients was higher than the overall correlation coefficient (r=0.993±0.009 vs r=0.807) of the pooled data.

Discussion/Conclusion
The increase in EC was significantly associated with the increase in the speed fluctuation. The mean individual correlation coefficients were higher than the overall correlation coefficient. It is concluded that the speed fluctuation of the centre of mass was related to less efficient swimming and vice versa in butterfly. We suggest that the swimmers should strive to improve their technique performances by avoiding large variations in the speed fluctuation.

References
Alves F et al. (1996). Determinants of energy cost of front crawl and backstroke swimming and competitive performance. Biomechanics and Medicine in swimming VII: 185-192.
Vilas-Boas JP (1996). Speed fluctuation and energy cost of different breaststroke technique. Biomechanics and Medicine in swimming VII: 167-171.