Swimming Stroke Mechanics What we continue to learn from High-speed Videography and Biomechanical Motion Analysis Jan Prins, Ph.D. Aquatic Research Laboratory University of Hawaii Swimming Biomechanics, Inc.
Simplification underlies the success of science. The common notion that scientists are as accurate as possible represents a serious misunderstanding. The trick is to figure out beforehand what level of accuracy is required and then to waste no effort doing substantially better. Steven Vogel in Comparative Biomechanics Princeton Univ. Press, 2003
Note to Reader: These slides have been modified to accommodate the printed format. The majority of the pictures are freeze-frame extracts of the video clips shown during the lecture. Details on viewing the complete video files are provided at the conclusion of this presentation.
Swimming research in Biomechanics conducted in our laboratory: We use use a combination of multiple, synchronized, high-speed video cameras located in the 3 planes of motion. After filming we use motion-analysis software for image capture and data processing.
The pictures will be shown in two formats: First as Reports These Reports are generated by the motion capture software and allow us to combine video with synchronized graphs to observe changes.
The Primary object of the Report To observe how a swimmer s hip velocity, plotted in the longitudinal direction, is influenced by different stroke patterns. Note: The vertical bar in the graph indicates the hip velocity at the instant of the video freeze-frame.
Motion from left to right across the screen.
The second format we have termed Video Enhancements. 1. Video Segments allow us to superimpose a stick-figure directly over the swimmer s body. 2. Video Trails trace a continuous path of selected points during the stroke cycle.
Video Segments
Video Trails
The Motion capture software Using the software we conduct Multi-2D analysis. Used on land for studying gait, the Multi-2D format allows us to film and analyze swimmers from the most practical and well-understood views: Frontal, Lateral, & Vertical.
Four Topics for today s presentation 1. Where do the Maximum Hip Velocities occur within each stroke cycle? 2. Impulse & Swimming 3. Hip velocities during Breakouts 4. Stroke mechanics from a Vertical Perspective looking straight up!
Focus 1: Maximum Hip Velocities In this section we examine at what point during the stroke cycle we see the maximum hip velocities for each stroke. Understanding where these points are, provides insight into maximizing a swimmer s propulsive efficiency.
Note: When observing this data, it is important to separate individual idiosyncrasies from fundamental movements that produce propulsion. This becomes increasingly important as a swimmer gets more efficient in the water.
Focus 1a Freestyle Preliminary Observation: Elite Freestylers instinctively choose to hold their arms at obtuse angles (>90 degree elbow-bend) mid-way through the pull.
James Magnussen: Current World 100m Freestyle Champion
Maximum Hip Velocities in Freestyle High-speed motion analysis shows that peak hip velocity occurs during the middle-third of the pull. It does not take place at the end of the underwater pull, i.e. during the followthrough, as was previously assumed.
Top graph shows changes in hip velocity
Reference Prins, Jan, N.M. Murata, J.S. Allen. Preliminary results of a Multi-2D Kinematic Analysis of Straight-Arm vs. Bent Arm Freestyle Swimming, using High-speed Videography. XIth International Symposium on Biomechanics and Medicine in Swimming. Oslo, Norway, 2010.
Focus 1b: Backstroke Unlike the Freestyle, in the Backstroke, there is a noticeable absence of a single peak velocity during the stroke cycle. Instead, the hip velocity reaches a maximum at two phases in the stroke, indicating a bi-phasic quality to the propulsive forces of the underwater pull.
The first peak occurs mid-stroke, as the hand passes the shoulder line.
The second peak, as expected, occurs during the follow through.
Future Research The swimmer being analyzed is Olympic Silver Medalist, Emily Seebohm. She appears to have little difference between the values of the two peaks. What will be interesting to examine is the differences, if any, in these values, between elite and less experienced swimmers.
Focus 1c: Breaststroke We filmed the Breaststrokers using two different trials. In Trial 1, we tracked the swimmers while they used a pull buoy. In Trial 2, we tracked them doing the complete stroke.
Trial 1: Breaststroke Pull only When using a pull buoy, the maximum velocity took place at the start of in-sweep. The graph in the report shows two peaks, each from successive strokes, with the first pull performed with more effort.
Copyright Jan Prins, 2013
Trial 2: Combining the kick and pull: As expected, two peaks occurred within each stroke cycle. One peak was coincident with the pull and the other with the kick. Similar to when using a pull buoy, maximum hip velocity also appears to occur as the hands progress into the in-sweep pulling phase.
Observation 1: During the combined stroke, the maximum hip velocity generated by the kick was unexpectedly smaller than that produced by the pull. This is surprising, as the kick in breaststroke, when isolated, is always more powerful than the pull.
Observation 1 (continued) At time of writing we have filmed a number of elite breaststrokers, including Sean Mahoney, worldranked in 2012. All these swimmers have shown the same difference in the relative velocities, providing an intriguing question that will require further analysis.
Observation 2: We see a significant drop-off in velocity as the pull transitions into the kick, when the feet are being drawn up! This drop in hip velocity may be primarily attributed to the increased frontal resistance.
Top graph shows reduction in hip velocity
Focus 1d: Butterfly All 3 camera views are included in the synchronized freeze-frame of the butterfly. Peak hip velocity, as expected, occurs during the second half of the underwater pull as the hands approach final elbow extension.
Drop in velocity in the Butterfly As shown in the following report, there was a significant drop in hip velocity during the stroke cycle. This occurs during the initial phase of the arm recovery as it is combined with the up-beat of the first kick.
Focus 2: Impulse Definition of Impulse: Force x Time The majority of land-based sporting activities reward impulse that is produced either by the application of large forces for a very brief interval, or relatively large forces exerted for extended periods of time.
Large Force Brief Time Large Force Extended Time
Impulse applied to Starts & Turns The intent should be to produce as high an impulse as possible by applying a large force for a brief time while ensuring that the feet stay in contact with the starting block, or wall. This is coupled with the implied precaution of directing the forces in the desired direction!
Large Force Short Optimum Time
Impulse as a factor in Swimming In swimming, it appears that we need to be able to apply moderate forces over a long a period of time, as determined by the mechanics of each stroke.
Example of Impulse in Freestyle Swimmer tested: James Magnussen. In the report we see frontal and lateral views, with the graph showing hip velocities during the right & left underwater arm pulls, during a complete stroke cycle.
Example 1. (continued) Peak velocities generated by each hand are virtually identical: 1.94 and 1.95 m/sec. However, there is a significant difference between the time each hand is exerting propulsive force. It appears that the impulse generated by the right hand is twice that of the left hand.
Example 1. (continued) There may be two reasons for this difference in impulse. A comparatively shortened stroke length or a reduction in propulsive force exerted by the left hand. Both factors will require further analysis.
0.37 sec s 0.18 sec s
Second Application of Impulse The six-beat vs. the two-beat flutter kick. Swimmer: Jessica Ashford. Event: 800 meter Freestyle Member of Australian Olympic Team to London, 2012.
Example (continued) Peak hip velocity during the two-beat kick was cyclical and brief. Not surprisingly, the six-beat kick provided a more sustained period of peak hip velocity and, therefore, produced greater impulse.
2-Beat Kick 6-Beat Kick
Focus 3: Breakouts We are interested in the changes in hip velocities that occur during the time that the first arm pull is taken, the head break, and the initial progress on the surface.
Example of a Effective Breakout The most visible indicator of an effective breakout is the absence of noticeable dips in hip velocity during the first arm pull followed by the head breaking the surface.
Effective Breakout
Example of an Early Breakout Early breakout occurs when the first arm pull is started and completed, before the head breaks the surface. The reduction of hip velocity is reflected by the dips in the graphs
31% drop off in velocity Early Breakout
Early Breakout (continued) The graph shows the increase in hip velocity as the pull takes place. However, because the head is still underwater, there is a significant slowing down before the head break. In this example, the percentage decrease in velocity between these events was approximately 31%.
Example of a Late Breakout Late breakout takes place when the head breaks the surface, before the first arm pull has begun. As seen in the following example, there are two significant dips in hip velocity before the first pull is taken.
Late Breakout 36% drop off in velocity
Late Breakout (continued) These decreases in velocity could well be caused by wave drag encountered on the surface. The 36% dip in hip velocity was the difference between the highest speed, seen as the body surfaced and the lowest velocity, at the bottom of the first dip.
Focus 4: The Vertical Perspective Our fourth camera allows us to video swimmers as they swim directly over the camera placed on the bottom of the pool. When synchronized with the cameras that are video-taping the frontal and lateral views, the vertical view provides an unique perspective of stroke mechanics.
What a vertical view offers: A view of the paths that the hands and feet make relative to the longitudinal orientation of the body. The ability to view and measure the widths or amplitudes of the pull (particularly important in Breaststroke & Butterfly).
The Vertical view in Breaststroke This view helps us measure wing span. We can observe how hip velocities change, depending on how wide the hands go at the end of the out-sweep. We can also view the effect of rounding out earlier during the insweep (as performed by Rebecca Soni).
Top graph shows changes in wing span.
The Vertical View (continued) The next picture shows the drop in velocity during the period the legs are drawn up in preparation for the kick.
Botton graph shows reduced hip velocity
The Vertical view in Butterfly As the butterfly pull evolves from the key hole pull into the straight or diamond pull, the vertical view documents how impulse changes and hip velocities increase as the pull gets narrower.
Butterfly Vertical View (continued) The first butterfly video clip combines all three camera views. The graph indicates the phase in the stroke cycle where maximum hip velocity occurs. As expected, it is during the last third of the pull, when combined with the second down-beat of the dolphin kick.
Butterfly Vertical View (continued) The lateral view in the next report also explains the reason for the significant drop in hip velocity during the stroke cycle. Stalling occurs, clearly the result of the lag in propulsion that occurs during the arm recovery, inevitably coinciding with the upbeat of the dolphin kick.
Acknowledgements Thanks to Steve Allnutt, Amarens Genee, and Dan Worden who helped prepare this presentation. Thanks also to all our participating swimmers.
Web site in preparation www:swimmingbiomechanics.com This website will include video clips of both the High-speed Video Reports and the Video Enhancement footage as reviewed in this presentation. Projected launch date for Swimming Biomechanics website: April 2015.
Contact Info. jprins@hawaii.edu