Perspectives / Biomechanics


M Edwin DeMont PhD

Comparative Biomechanics Laboratory, Biology Department, St Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada. Email:

Sportscience 3(1),, 1999 (1559 words)

Reviewed by: Todd L Allinger PhD, The Resource Center, The Orthopedic Specialty Hospital, Salt Lake City, Utah, USA; Giannis Giakas PhD, Division of Sport, Health and Exercise, Staffordshire University, Stoke-on-Trent, UK


Two examples demonstrate how research on aquatic animals can benefit research on human swimmers. In the first example, work by comparative biomechanists studying animals that move through the air-water interface has shown that for competitive swimmers the optimal depth during the glide phase is the depth that minimizes surface waves. The second example contributes to the ongoing argument on the nature of thrust generation in freestyle swimming. The dominant view is that lift generates more thrust than drag. These forces arise from steady-state fluid dynamics, but the movement of the thrust-generating surfaces creates unsteady fluid motion. Recent work on aquatic animal locomotion has shown that unsteady mechanisms probably play an important role in generating thrust, so the thrust-generating mechanisms used by competitive swimmers should be reevaluated. Reprint  Help


KEYWORDS: biomechanics, drag, fluid dynamics, force, lift, swim, thrust


Sport scientists studying competitive swimming may benefit fromlearning about Nature's competitive swimmers. In this article, twoexamples will be presented that demonstrate the potential benefits ofthis exercise. The first example will show that research on aquaticanimal locomotion has already examined a question raised by sportscientists. The second example will suggest that an ongoing argumenton the nature of thrust generation in freestyle swimming should bereevaluated using current knowledge generated from studies of aquaticanimal locomotion.

Lyttle (1998) raised the interestingquestion "does a swimmer's depth in the water make a difference inglide speed?" In 1983 I participated in a project that generated ananswer to the question. The research was focussed on the energeticsof leaping in aquatic animals (Blake,1983).

The cost of swimming is a very important component of the energybudget of aquatic animals. A huge research effort has been devoted tounderstanding all aspects of the costs. This research effort ispartially to provide information for fisheries managers, who attemptto sustain a global fisheries industry. Some of this work has shownthat specific morphological and/or behavior traits have evolved toreduce the costs. For example, air-breathing cetaceans (e.g.dolphins), which need to surface to breathe, have developed abehavior that can minimize the costs of long migrations. The problemthese animals have to cope with is that drag near the surface issignificantly higher than for subsurface swimming. The behavior thatsome air-breathing aquatic animals exhibit is to periodically leapout of the water during long migrations (Figure 1). This behaviorminimizes the time at the air-water interface and, in certainconditions, can reduce the overall cost of the migration (Au& Weihs, 1980; Blake, 1983).

Figure 1: Dolphins leap out of the water during long migrations to reduce energy costs caused by surface waves.

The air-water interface creates serious problems for aquaticair-breathing animals; indeed, it is so problematic that few animalsspend time in this region (Vogel, 1994).Movement at the air-water interface generates surface waves, usuallyone in front and one behind the animal. Work is done to generate thewaves, because the water in the waves has been lifted aboveequilibrium (Vogel, 1994). Thus energy iswasted in the generation of these waves, and that lost energy ismanifest as an additional drag force called the wave drag. Thecontribution of wave drag to the total drag of structures movingthrough the air-water interface depends on several factors, such asthe hull speed and wave celerity. An explanation of these factors canbe found in Denny (1993).

The model developed by Blake (1983) toexamine the drag-reduction potential of leaping in air-breathingaquatic animals used a drag-augmentation factor. This factor wasbased on published data collected from an object towed behind a boatat various depths below the surface. The drag-augmentation factordepends on the relative submersion depth (h/d), which is the ratio ofthe distance from the surface to the centerline of the body (h) tothe maximum breadth of the body (d). Thus the drag-augmentationfactor depends on the size of the body. It has a maximum value of 5,for an h/d ratio of 0.5 (near the surface), so a body moving near thesurface would have a drag five times the value when fully submerged.The drag augmentation factor has a value of 1.0 at an h/d ratio of3.0 (or greater), which means that the drag is the same as that for afully submerged body. In practical terms the body is out of theinfluence of the surface, because no waves are generated, there is nowave drag, and total drag is minimized.

For competitive swimmers, glide speed will be reduced as soon assurface waves are generated. Thus during the glide and push-offphase, swimmers should train to swim at a depth that minimizes thegeneration of surface waves. The actual depth at which the minimumoccurs will depend on the size (breadth) of a swimmer. I presentedthis idea to Canadian swim coaches during a course I teach as part ofthe National CoachingCertification Program for SwimmingCanada.

The second example is based on arguments that have been presentedfor years supporting the view that either drag-based or lift-basedmechanisms are used to generate thrust in freestyle swimming.Recently Sanders (1998) argued that sportscientists should be skeptical about the predominant view thatlift-based mechanisms generate thrust in freestyle swimming. Recentwork in comparative animal biomechanics, however, has shown that inNature, thrust generation by appendages of aquatic animals may besomething substantially different from either lift or drag-basedmechanisms (Dickinson, 1996).

The concept of lift and drag forces as applied to competitiveswimming was taken from steady-state fluid dynamic theory. As inanimal locomotion, the movement of thrust-generating limbs in humanswimmers is inherently unsteady: phases of acceleration exist in thestroke. It is now recognized from comprehensive studies of animallocomotion (e.g., Cheng and DeMont, 1996;Cheng et al., 1996; Vogel,1997) that new concepts associated with unsteady fluid dynamicsare required to fully understand animal movement (Lauderand Long, 1996). Quasi-steady state applications, such asSanders (1997), may have limitedapplicability in both animal and human aquatic locomotion, sinceforce coefficients are not constant, but have complex timehistories.

New observations on unsteady effects have shown, for example, thathydrofoils with an impulsive start and high angle of attack canproduce significant transient lift forces (Dickinson,1996). This finding suggests that the application of unsteadyfluid dynamics to competitive swimming may rejuvenate the debate onthe nature of thrust forces. But the debate should not be focussed onwhether a lift-based or drag-based mechanism is used to generatethrust, but rather on how important these steady-state forces are atall. Colwin (1992) already introduced thegeneral idea of applying unsteady fluid dynamics to understand thethrust generating mechanisms used by competitive swimmers, but withrapid advances in the development of theories of unsteady fluiddynamics in animal locomotion, his suggestions need to be updated. Isuspect that significant advances will be made in the performance ofcompetitive swimmers in most of the strokes used, when unsteadymechanisms are included in the analysis of thrust generation. Thetechniques for such an analysis are now available in the field ofcomparative biomechanics.

There is growing interest in applying what Nature does so well tothe design of new technologies. This new field of science is calledBiomimicry (Benyus, 1997). Sport scientistsstudying competitive swimming may benefit from learning aboutNature's highly competitive swimmers.


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Edited by Todd Allinger and Will Hopkins.
Webmastered by Will Hopkins.
Published March 1999.