EVEN WHEN LIFT IS PROPOSED AS BEING THE MAJOR PROPULSIVE FORCES IN THEORETICAL MODELS ITS CONTRIBUTION IS NOT VALIDATED
Berger, M. A., Hollander, A. P., & de Groot, G. (1997). Technique and energy losses in front crawl swimming. Medicine and Science in Sports and Exercise, 29, 1491-1498.
Although lift and drag forces occur during propulsion in swimming, the authors propose that lift is beneficial because of the relative small energy loss to the water. Consequent assumptions in this article contend that lift is the primary component of propulsive forces and decisions and the investigation design are based on that tenet.
Elite swimmers and triathletes (M = 7; F = 4) swam crawl stroke with legs fixed (no kick) and supported by a small buoy. Ss performed at least two 400-m swims paced by pool lights to maintain constant aerobic intensity velocities. Oxygen consumption was determined. One trial was "free" while the other used the MAD system, a system that measures certain forces. Forces, which only considered lift components relative to total forces, were calculated only for the hands. Propelling efficiency was determined from a force production perspective (Ep) and a physiological perspective (Epp).
It was found that the contributions of lift and drag components to total force were different between Ss. In figures of force creation for two Ss it was shown that lift forces were not of the same magnitude as drag forces in the phase of the crawl stroke where propulsive forces accelerate the swimmer. The authors did not mention this.
It was reported that on the average lift forces contributed more than 50% of the forces of the total stroke. However, the total stroke includes non-propulsive phases (entry repositioning and finish extraction) so this figure is misleading. What should have been reported was the contribution of the force components during acceleration, that is, when the stroke has a positive effect upon forward progress.
"Surprisingly, the Fl-r [lift force to drag force ratio] showed no significant correlation with Epp [physiological propelling efficiency]. Therefore, differences observed in Epp cannot be explained by a different contribution of the lift force of the basis of the present study." (p. 1495)
Several other findings emerged that are the basis for questioning the theoretical assumptions of the investigation. Variations in hand and arm orientation had little influence on the calculation of propelling efficiency from a three-dimensional kinematic analysis. Many correlations between kinematic parameters and specific force parameters were significant, most probably because of the high degree of dependency between the factors considered.
Essentially, what this investigation showed was that a very theoretical treatment of forces in swimming [containing many assumptions] did not correlate with very practical measures of physiological costs. When varied, the questionable force parameters were not related to changes in energy costs of the movements. This could be due to a difference in the energy costs of free (but with legs fixed and supported) swimming and swimming in the MAD system (same leg restrictions but also alterations in hand and arm function). Significant differences would produce no significant correlations whereas the study assumed there were no differences between the two.
The two different ways of calculating/measuring propelling efficiency produced different and unrelated results and implications.
Implications. This study sheds little light on propelling efficiency in swimming. Given all the assumptions involving force calculations and the "unnatural" way they were measured, no inferences should be drawn with regard to force production in "free" crawl stroke swimming. However, the measurement of propelling efficiency might be considered to be more valid.
However, since both forms of swimming were evaluated with supported but restricted leg actions, whether implications have any relevance to true "free" swimming, that is swimming with no restrictions to leg or hand movements, must be seriously considered.
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