NEGATIVE ANGLE BODY POSITION IS BEST WHEN SWIMMING UNDERWATER

Pease, D., & Vennell, R. (2010). The effect of angle of attack and depth on passive drag. A paper presented at the XIth International Symposium for Biomechanics and Medicine in Swimming, Oslo, June 1619, 2010.

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This study examined the interaction between angle of attack, submergence depth, and drag force. In a flume, an anatomically accurate mannequin, whose orientation could be precisely controlled, was used. Data were obtained with the mannequin oriented with the longitudinal axis at angles of attack of -4, -2, 0, +2 and +4 degrees relative to the water flow. Measurements were taken at depths ranging from 0.2-0.8 m and at velocities ranging from 0.00-2.55 m/sec.

The most significant finding was the difference between a negative and positive angle of attack near the water surface. As an example, the total drag force, at 0.2 m depth for angles of attack of -4 and +4 degrees at a flow velocity of 2.36 m/sec were ~72.90 N and ~104.16 N respectively while at 0.8 m depth the total drag forces were ~60.56 N and ~63.45 N respectively. Despite the approximate equivalence of the frontal area in the respective positive and negative angles of attack, the negative angle demonstrated a lower total drag force which appears to be due largely to a decrease in the wave drag contribution (25% and 55% of total drag respectively) near the surface. As with the earlier study, the depth at which the mannequin was suspended also had a great affect on the measured drag forces with little difference between corresponding angles at greater depths.

Implication. These results highlight the importance of body position near the water surface. It also demonstrates that, due to the decreased drag with a negative angle of attack, if a swimmer chooses a shallow glide depth following a start or turn then it is advantageous to maintain a slightly negative angle of attack for as long as possible before surfacing. Therefore, while it may be beneficial for the athlete to begin the underwater trajectory in a negative angle of attack, once the intention is to surface that should be achieved as quickly as possible in order to limit the effects of the positive angle attack usually associated with surfacing. [This study shows that a flat or even negative angle of body alignment (angle of attack) is better than one of any positive angle.]

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