Biomechanics and Medicine in Swimming XI

Evaluation of the Gliding Capacity of a Swimmer
roig, A.
GIRSANE Research Group from Olympic Training Centre, Barcelona,
Spain
Reducing resistance is probably the fastest way to improve performance
and the most efficient to reduce energetic cost. Therefore, the aim of this
study was twofold: (i) to give advice to coaches to re-orient their training
sessions towards reduction of resistance forces and suggest exercises
for its improvement and, (ii) to develop and apply a new test for the
evaluation of gliding. The proposed gliding test evaluated the maximum
speed reached by the swimmer after push-off and the passive hydrodynamic
resistance when gliding through the water. A well-balanced
solution between accuracy, validity and applicability for the evaluation
of underwater glide was found.
Key words: Gliding coefficient, hydrodynamic position, resistance
force, starts, turns
IntroductIon
The capacity to move its own body through the water with the lowest
resistance should be included among the most important qualities of a
swimmer. Advancing through a liquid, the swimmer evacuates water and
occupies its place, causing the hydrodynamic resistance that acts in the
same swimming direction but in the opposite sense.
The underwater phase represents an important part of the entire race
time (Chow, 1984). This is even more true in a 25m-pool race, where the
number of turns is doubled.
During the phases of pure gliding, in starts or turns, the swimmer tries
to maintain the highest velocity of all race obtained after push-off
(highest peak of speed in figure 1).
Figure 1. Velocity graph of a breaststroker during start.
The higher the speed of the swimmer, the higher will be the resistance
to overcome. Therefore, in those phases of the race, where velocity is
increased (start and turns), the swimmer will have to pay much more attention
on reducing resistance forces. An accurate underwater technique
combines the capacity to self-propel in the water and to reduce braking
forces to the minimum for a beneficial use of the generated propulsion.
Both qualities coexist when a swimmer travels in the water and should
be optimised to obtain the highest velocity.
Many factors may determine the gliding capacity of a body inside
the water. One of them, the fluid density (water is about 1000 times
more dense than air) is not controllable by the swimmer and is equal
for all participants. Others, as frontal area, skin and swimsuit friction, or
body shape, can be modified and have to be improved in training sessions.
A slender and aligned body in the moving direction will produce
a lower resistance. New swimsuits seem to slightly alter the parameters
chaPter2.Biomechanics
that increase the swimmers gliding, modifying buoyancy and alignment
(slender the body shape and avoid feet falling) and reducing friction by
means of removing seams and using new textile that minimizes turbulence
generation.
Many authors like Clarys (1979), Maiello et al. (1998) and Lyttle et
al. (2000) have studied the effect of depth of glide onto resistance forces,
concluding with different opinions, some of them even opposed.
Diminishing resistance is probably the fastest way to improve performance
and the most efficient way to reduce energetic cost. It is for
this reason that we pursued a double goal: To give advise to coaches
to re-orient their training sessions towards the reduction of resistance
forces by suggesting exercises for its improvement and to develop and
apply a new test for the evaluation of gliding.
Methods
The proposed gliding test is an adaptation of the one developed by
Klauck et al. (1976) and evaluates simultaneously, the maximum speed
reached by the swimmer after push-off from the wall and the passive
hydrodynamic resistance when gliding through the water. Gliding test
is being applied to swimmers training at the Olympic Training Centre
in Barcelona, belonging to a wide variety of performance levels, including
Olympic, World and European Championships and Paralympics’
swimmers.
Three underwater cameras are placed at the pool’s sidewall at a distance
of 2m between them (Figure 2). The first camera distance from the
wall varies and is equivalent to the distance reached by the swimmer’s
head with his feet extended. For example, if first camera was placed at
a distance of 1.9 m, the following ones were placed at 3.9 m and 5.9 m.
A 4th travelling “dolly” camera followed the swimmer along the pool’s
sidewall.
Figure 2. Camera and buoy placement for the evaluation of resistance
factor.
The swimmer pushed off from the wall and adopted the most hydrodynamic
position until his body stopped. Two trials were recorded for each
chosen body orientation: ventral (facing down), dorsal for backstrokers
(facing up) and lateral position. In addition, the swimmer performed
a turn, starting 10 meters before the wall and glide, as done before, to
maximum distance. This variant detects if push-off or glide were affected
by previous swim or flip.
Image review of the exercise permitted the detection of possible
mistakes on technique. A list of indicators were pre-defined for the
analysis of power application against the wall and resistance reduction
while gliding: feet gap, support surface of feet against the wall, knee
flexion, head within arms, shoulder rising to lengthen the body, overlapping
of hands, extension of elbows, non-arched trunk and alignment of
hands, elbows, shoulders, hips, knees, feet and toes in the trajectory line
of glide. The video images were also used for the calculation of time of
head passing in front of each camera and the time spent to cover the
distance of 2 meters between each pair.
Signals from a velocity meter were recorded, also. This device, attached
to the swimmer’s hip by a thin belt, registered the instantaneous speed
during the exercise. The following parameters were obtained from values
of the velocity graph:
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