Biomechanics and Medicine in Swimming XI

These trials allowed verifying that the force signal produced by the wave
was reasonably symmetric (Fig. 3). In normal turning situations, swimmers
touch the platform, allowing it to record the swimmer’s pushing
action against the wall. However, as already stated, before the swimmer
touches the wall, a water wave hits the platform being also recorded. To
remove this extraneous force, two assumptions were made: (i) the wave
force to time curve being symmetrical, and (ii) that the peak value of the
water wave force occurs at the time instant of swimmer’s wall contact.
From the force platform data was extracted the time of water wave force
start, and from the image-based kinematics the time of swimmer’s first
contact was obtained. We assume this to be coincident with half water
wave curve. This half force to time curve of the water wave was then mirrored
and its values were subtracted from the total force values registered
by the platform (Fig. 2) to allow obtaining: (i) the swimmer’s contact
force to time curve, and (ii) the water wave force to time curve.
Solid line: force registered; dotted
line: wave mirroring; vertical
line: time of contact
Solid line: force registered; dotted
line: force after wave removal;
vertical line: time of contact
Figure 2. Graphical representation of the removal of the water wave of
the total force to time curve obtained during a crawl flip turn.
With a sample of 17 swimmers, 9 males and 8 females, aged 17.88 ±
3.19 years, height 1.73 ± 0.09, body mass 64.48 ± 11.90 kg, all participants
at the Absolute Portuguese National Championships, 154 valid
turns were analysed. The force to time curve produced by the water wave
was eliminated graphically from the force to time curve produced by
the swimmer’s contact. Moreover, the wave force to time curve was also
treated to characterize their main characteristics. Descriptive statistics
were used for data analysis.
RESULTS
The force curves generated by the water wave obtained without the
swimmer touching the wall resulted in a mean curve (Fig. 3).
Figure 3. Mean force curve of the water wave generated by the displacement
of the swimmer during the turn without touching the force
platform.
chaPter2.Biomechanics
It is possible to observe that the behavior of the curve is not linear but
follows a pattern tending to a symmetrical condition, as observed before
by Fujishima (1999). Mean results obtained for the quantitative parameters
of the water wave obtained with the turns performed without
touching the wall are presented in Table 1, as well as the data regarding
eight turns from the same swimmer touching the wall and the 154 turns
performed by the total sample of 17 swimmers.
Table 1. Mean values (Mean) ± standard deviations (SD) obtained for
the variables: maximum and mean force value produced by the water
wave, and wave impulse, grouped in normalized and absolute values in 3
situations of wave measurement during the flip turn.
WAVE DATA
Turning trials
8 turns without
touching the wall
8 turns touching
the wall
154 turns touching
the wall
NORMALIZADED VALUES ABSOLUT VALUES
Maximal
(N/bw)
Mean ±
SD
Mean (N/
bw)
Mean ±
SD
Impulse
(N.s/bw)
Mean ±
SD
0.38 ± 0.77 0.16 ± 0.33 0.01 ± 0.002
0.52 ± 0.12 0.24 ± 0.06 0.01 ± 0.008
0.49 ± 0.24 0.19 ± 0.12 0.04 ± 0.02
Maximal
(N)
Mean ±
SD
320.17 ±
65.59
448.88 ±
106.32
323.17 ±
191.94
Mean
(N)
Mean ±
SD
140.62 ±
28.05
206.97 ±
65.76
128.68 ±
90.44
Impulse
(N.s)
Mean ±
SD
10.5 ± 0.17
15.44 ± 4.07
27.16 ± 19.29
Figures 4 and 5 represent an example of the swimmer’s contact/impulse
force to time curve, before (Fig. 4) and after removal (Fig. 5) of the water
wave specific curve. In this case, the wave was almost fully incorporated
into the swimmers’ contact curve, and the interference of the wave phenomenon
into the swimmers’ kinetics is presumably important, making
it more difficult to assess the effect of the wave over the impulse curve.
Figure 4. Example of a force to time curve produced during a flip turn
with the effect of the water wave impact.
Figure 5. Example of a force to time curve produced during a flip turn
without the effect of the water wave impact.
149