Lift or Drag? Let's Get Skeptical About Freestyle Propulsion
Sanders, Edith Cowan
University, Perth, Australia
It is better to debate a question without settling it, than settle a question without debating it.
Joseph Joubert, 1754-1824, French essayist and moralist (Antony, 1996).
Having a small minority in our midst who get it wrong is bad enough, but how devastating would it be if nearly everybody gets it wrong? What if sport scientists do their homework by reading authoritative texts, but it's later found that the information presented in those sources is incorrect or misleading? In such a case, who's to blame? Would you blame the scientist who wrote the text? Or the scientists who read the views of the author and accepted them as facts?
Is it possible that some of the information we present as fact is really not fact? I'm going to use an example from my keynote address at the International Symposium of Biomechanics in Sports in Konstanz, Germany, and look at a biomechanics swimming situation in which it's possible that most of us have had it wrong. Let's get skeptical!
Propulsion in Freestyle
The issue of whether propulsion in freestyle swimming is due primarily to lift or drag appeared to have been settled in the early 1970s. Prior to that time coaches believed that the best way to propel the body forward was to pull the hand directly backwards; that is, to use drag forces. The drag force produced is opposite the direction of hand motion. It was thought that the hand plane should be almost square to the direction of motion. Coaches applied this idea by teaching swimmers to pull directly backwards with the hand at right angles to the pulling direction.
Following observations that the hands of champion freestyle swimmers scribed curved paths during the pull phase of the stroke, Brown and Counsilman (1971) and Counsilman (1971) promoted the idea that good swimmers use sculling actions with their hands pitched to utilize lift forces as the dominant means of propulsion. By definition, lift forces are perpendicular to the flow relative to the hand. Assuming that the hand moves into "still" water, this means that the lift forces are also perpendicular to the line of motion of the hand. Initially, the lift in freestyle swimming was thought to be generated in accordance with Bernoulli's Principle: when "foil-like" objects move through a fluid at high speed and small angles to the flow large lift forces are generated and the drag forces are comparatively small. The lift forces arise from a difference in pressure as the fluid travels further and faster around the more curved side of the foil than the less curved side. Thus, a swimmer's hand could act as a foil because the back of the hand is more curved than the front. To generate lift by the Bernoulli Principle the hand should be sculled so that the angle between the hand plane and line of motion of the hand is small. This generates forces which are mostly lift rather than drag. A revolution in coaching practices followed Counsilman's work. Coaches taught swimmers to "sweep" with the hands. Lift as the main source of propulsive force in freestyle swimming became almost universally accepted. Some swimming texts depicted the hand as a "foil" or as a "propeller".
Bernoulli's Principle is only one explanation of the kinetics of the lift force (Sprigings and Koehler, 1990). Lift force may also be generated by pushing water backwards using intermediate angles of pitch (Costill, Maglischo, and Richardson, 1992). In addition, drag and lift both contribute to the net force produced by the hand. Some texts depict the hand at instants through the pull phase of the stroke and indicate the relative magnitudes of lift and drag vectors. Ideally, the combination of lift and drag is such that the resultant force is in the desired direction of travel. It's common for the depictions to indicate lift as the predominant source of propulsion. Thus, the perception that most of the propulsive force in freestyle swimming is due to lift rather than drag has persisted.
A number of research papers have supported this view (Barthels and Adrian, 1974; Schleihauf, 1974; Schleihauf, 1979; Schleihauf, Gray, & DeRose, 1983; Schleihauf, Higgins, & Hinrichs, 1988; Reischle, 1979). Additionally, there are some compelling reasons why we might accept the idea that sound freestyle technique is characterized by the use of lift forces in preference to drag forces. The first is related to the curved nature of the hand path. We find it natural and logical to reason that if a hand path is curved then forces must be generated by lift. Otherwise, why would good swimmers use a curved hand path? Also, we find an advantage to a curved hand path is that the hand moves through a greater distance and/or speed thereby allowing forces to be applied longer and/or be greater in each stroke. We reason for forces to be in the desired direction when a curved hand path is used, then lift must make an important contribution. Thus, to achieve the advantages of a longer hand path a swimmer learns to use sculling motions to produce forces from lift rather than drag. Further, sculling actions may allow the large muscle groups to be used more effectively than when the hand is pulled straight back. Much of the sculling may be produced by the trunk rotators and incorporated into the natural rolling actions which accompany breathing and hand exit. Such a technique may be mechanically and physiologically efficient. Perhaps the most convincing argument is that less energy may be transferred to the water and "wasted" when forces are generated from lift than from drag (Toussaint and Beek, 1992).
Challenges to the view that lift plays the dominant role in freestyle propulsion have been few and, in general, have been ignored or dismissed. Wood and Holt (1979), Holt and Holt (1989), and Valiant et al (1982) presented evidence in favour of drag being the dominant force. More recently, Cappaert (1993) and Cappaert and Rushall (1994) quantified the direction of hand motion and the orientation of the hand using three-dimensional analysis techniques. Cappaert used hand orientation and path data in conjunction with Schleihauf's lift and drag coefficients (Schleihauf, 1979) to estimate life and drag forces. All of these studies, which were of champion swimmers, indicated that drag forces are more important than lift forces in all strokes other than breaststroke.
Rushall et al (1994) proffered convincing arguments in favour of drag as the dominant propulsive force in freestyle swimming. They contended that the arguments in favor of lift as the dominating force were ill-conceived and that much of the total propulsive force comes from the forearm. Because of its bluff shape nearly all the force generated by the forearm must be due to drag. Further, the forearm has a substantially straighter path than the hand. Thus, freestyle technique may be directed toward generating propulsion from the forearm using drag rather than deliberately using a sculling action to optimize lift forces by the hand. Unfortunately, although research such as Berger et al. (1995) has quantified the drag and lift coefficients of the forearm and combined forearm and hand, no research has effectively quantified the relative contributions of the forearm and hand in actual swimming. One of the major methodological problems to be overcome is that the forearm moves at very different velocities along its length during the swimming stroke. Schleihauf (1984) estimated that the contribution of the forearm in swimming is very small compared to that of the hand. This is because the hand moves at a greater velocity than the forearm. If this is the case, then a focus on the forces produced by the hand remains warranted and the question of whether lift or drag is the more important remains open for consideration.
Recent studies quantifying whole body motion indicate that the hand paths of successful swimmers are not as curved as initially thought (Cappaert, 1993). Thus, swimmers are tending to use a straight pull rather than to maximize pulling distance and speed by using a curved path. If the path is not very curved then the major contributor to force in the desired direction must be drag regardless of whether the hand is angled to the flow.
Through a combination of experiment and simulation, Liu et al. (1993) and Hay et al. (1993) showed that the curved path of a swimmer's hand is due to body roll. In fact, when the arm is simulated to move directly backwards with respect to the swimmer's reference frame, the path of the hand in the external reference frame is more curved than in actual swimming. This means that swimmers actually straighten the curve somewhat. This has important implications. It means that the curved hand path is not deliberate. Rather than swimmers adducting the arm to produce the "insweep" and then abducting to produce the "outsweep", as is commonly demonstrated by coaches when instructing on poolside, the swimmers are actually reducing the curve by abducting in the early part of the pull and adducting during the latter part of the pull. The fact that swimmers attempt to straighten the path rather than to use sculling actions is strong indirect evidence that swimmers rely on drag forces rather than lift forces.
Recently, I attempted to shed more light on the lift versus drag issue using hand lift and drag coefficient data obtained from a testing tank at the Iowa Institute for Hydraulic Research (Sanders, 1997a; Sanders, 1997b). The lift and drag coefficients obtained from the hands tested in the Iowa facility indicated that the greatest forces are obtained when the hand plane is close to 90 degrees to the flow. At this orientation the force is due almost entirely to drag. Lift makes its greatest contribution to resultant force at angles near 45 degrees. However, even at these angles, the contribution due to drag is as great as the contribution due to lift. When these coefficient data were used in conjunction with three-dimensional kinematic data to estimate forces in actual swimming, it was found that drag made a larger contribution than lift throughout the propulsive part of the pull. During the most propulsive phase of the stroke the pitch angle was between 50 and 60 degrees, which means that the hand was pitched to take advantage of drag forces with a smaller contribution due to lift. During the most propulsive phase of the stroke the direction of fluid flow was from the wrist towards the fingers. This is contrary to the situation commonly envisaged and depicted in swimming texts, in which the hand is represented as a foil generating lift forces from lateral movements which produce a flow across the hand.
In this example, many of us were quick to accept theory as fact before sufficient evidence was available. It may still be too early to state that freestyle propulsion is dominated by drag, but the commonly held belief that it is dominated by lift may be ill-founded and incorrect.
Antony, J. (1996). Oxford Dictionary of Political Quotations (p.200). Oxford, England: Oxford University Press.
Barthels, K., & Adrian, M.J. (1974). Three-dimensional spatial hand patterns of skilled butterfly swimmers. In J. Clarys and L. Lewille (Eds.), Swimming II (pp. 154-160). Baltimore, Maryland: University Park Press.
Berger, M.A.M., de Groot, G., & Hollander, A.P. (1995). Hydrodynamic drag and lift forces on human hand arm models. Journal of Biomechanics, 28, 125-133.
Brown, R.M., & Counsilman, J.E. (1971). The role of lift in propelling swimmers. In J.M. Cooper, (Ed.), Biomechanics (pp.179-188). Chicago, Illinois: Athletic Institute.
Counsilman, J.E. (1971). The application of Bernoulliís Principle to Human Propulsion in Water. In L. Lewillie and J. Clarys (Eds.), First International Symposium on Biomechanics of Swimming (pp.59-71).Universite Libre de Bruxelles, Brussels, Belgium,
Cappaert, J. (1993). 1992 Olympic Report. Limited circulation communication to all FINA Federations. United States Swimming, Colorado Springs, CO.
Cappaert, J., & Rushall, B.S. (1994). Biomechanical Analyses of Champion Swimmers. Spring Valley, California: Sports Science Associates.
Costill, D.L., Maglischo, E.W., & Richardson, A.B. (1992). Swimming. London, England: Blackwell Scientific Publications.
Hay, J.G., Liu, Q. & Andrews, J.G. (1993). The influence of body roll on handpath in freestyle swimming: A computer simulation study. Journal of Applied Biomechanics, 9, 227-237.
Holt, L.E., & Holt, J.B. (1989). Swimmming
velocity with and without lift forces. Unpublished paper, Sports
Science Laboratory, Dalhousie University, Canada.
Liu, Q., Hay, J.G., & Andrews, J.G. (1993). The influence of body roll on handpath in freestyle swimming: An experimental study. Journal of Applied Biomechanics, 9, 238-253.
Reischle, K. (1979). A kinematic investigation of movement patterns in swimming with photo-optical methods. In J. Terauds and E.W. Bedingfield (Eds). Swimming III (pp.127-136). Baltimore, Maryland: University Park Press.
Rushall, B.S., Sprigings, E.J., Holt, L.E.,
& Cappaert, J.M. (1994). A re-evaluation of forces in swimming.
Journal of Swimming Research.10, 6-30.
Sanders, R.H. (1997a) Extending the "Schleihauf" model for estimating forces produced by a swimmers hand. In B.O. Eriksson and L. Gullstrand Proceedings of the XII FINA World Congress on Sports Medicine. Goteborg, Sweden 12-15 April 1997, pp.421-428.
Sanders, R.H. (1997b). Hydrodynamic characteristics of a swimmers hand with adducted thumb: Implications for technique. In B.O. Eriksson and L. Gullstrand Proceedings of the XII FINA World Congress on Sports Medicine (pp.429-434). Goteborg, Sweden 12-15 April 1997.
Sanders, R.H. "Lifting Performance in
Aquatic Sports". Keynote address at the XVI International Symposium
of Biomechanics in Sports, Konstanz, Germany, July 21-25,
Schleihauf, R.E. (1974). A biomechanical analysis of freestyle. Swimming Technique, 11, 89-96.
Schleihauf, R.E. (1979). A hydrodynamic analysis of swimming propulsion. In J. Terauds and E.W. Bedingfield (Eds). Swimming III (pp.70-109). Baltimore, Maryland: University Park Press.
Schleihauf, R.E. (1984). The biomechanical analysis of swimming propulsion in the sprint front crawl stroke. Doctoral Thesis. Teachers College, Columbia University.
Schleihauf, R.E.,Gray, L., & DeRose, J. (1983). Three-dimensional analysis of swimming propulsion in the sprint front crawl stroke. In A.P. Hollander et al. (Eds). Biomechanics and Medicine in Swimming (pp.173-184). Champaign, Illinois: Human Kinetics. University Park Press.
Schleihauf, R.E., Higgins, J.R., & Hinrichs, R. (1988). Propulsive techniques: front crawl stroke, butterfly, backstroke, and breaststroke. In B. Ungerechts et. al. (Eds). Swimming Science V (pp.53-59). Champaign, Illinois: Human Kinetics.
Sprigings, E.J., & Koehler, J.A. (1994). The choice between Bernoulliís or Newtonís model in predicting dynamic lift. International Journal of Sport Biomechanics,6, 235-245.
Toussaint, H.M., & Beek, P.J. (1992). Biomechanics of competitive front crawl swimming. Sports Medicine, 13, 8-24.
Valiant, G.A., Holt, L.E., & Alexander,
A.B. (1982). The contributions of lift and drag components of the
arm/forearm to a swimmerís propulsion. In J. Terauds
(Ed.) Biomechanics in Sports: Proceedings of the International
Symposium of Biomechanics in Sports. Research Center for Sports, Del
Wood, T.C., & Holt, L.E. (1979). A fluid dynamic analysis of the propulsive potential of the hand and forearm in swimming. In J. Terauds and E.W. Bedingfield (Eds.), Swimming III. Baltimore, University Park Press.