Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
Hydrodynamic Characterization of the First and
Second Glide Positions of the Underwater Stroke
Technique in Breaststroke
costa, l. 1 ; ribeiro, J. 1 ; Figueiredo, P. 1 ; Fernandes, r.J. 1 ; Marinho,
d. 2,3 ; silva, A.J. 3,4 ; rouboa, A. 3,4 ; Vilas-Boas, J.P. 1 ; Machado,
l. 1
1 University of Porto, Faculty of Sport, Cifi2d, Porto, Portugal
2 University of Beira Interior, Covilhã, Portugal
3 Research Center In Sports, Health and Human Development, Vila Real,
Portugal
4 University of Trás-os-Montes and Alto Douro, Vila Real, Portugal
The aim of the present study was to characterize two different ventral
glide positions, both used after start and turns: the arms extended at the
front, and the arms extended along the trunk, respectively the first and
the second glide positions of the breaststroke underwater stroke. Inverse
Dynamics was used for obtaining the passive drag and the drag coefficient,
based upon the velocity to time curve of each glide, monitored
through a swim-meter. The first glide position presented lower values
of drag and drag coefficient, managing to reach higher velocities. The
cross sectional area (planimetry) values were lower for first glide position
in comparison with the second one (759.95 ±124.12 cm2 vs. 814.46 ±
111.23 cm2). These results point out the need of technical evaluation
and control in order to reduce drag during swimming performance.
KeY Words: swimming, breaststroke, passive drag, drag coefficient,
gliding position
IntroductIon
In swimming the total event time is determined by the start, swim, turn
and finish partial times (Haljand and Saagpakk, 1994). Starts and turns
appear to be a significant part of the total swimming event time (Vilas-
Boas and Fernandes, 2003). Chatard et al. (1990) stated that the gliding
phase after starts and turns corresponds to 10 to 25% of the total
event time (depending on events and the length of the swimming pool).
D’Acquisto et al. (1988) showed that, during the breaststroke events, the
gliding phases represent 44% of the total swimming time. These authors
showed that the gliding phase distinguishes elite from good level breaststroke
swimmers, with higher gliding time for the elite swimmers. Race
analysis suggested that rather than the start technique used by swimmers;
it is the position under the water that mostly determines the success of the
start (Cossor and Mason, 2001). Therefore, it is essential to analyse and
understand this phase. The passive drag of swimmers moving underwater
in a streamlined position has been measured experimentally (Clarys, 1979;
Kolmogorov et al., 1997; Lyttle et al., 2000; Toussaint et al., 2004; Vilas-
Boas et al., in press). However, there are other gliding phases in which the
swimmers assume a prone position but with the arms extended at the side
of the trunk, as in the second glide position after the breaststroke underwater
arm stroke. To our knowledge, these two glide positions were compared
experimentally only by Vilas-Boas et al. (in press), and numerically
by Marinho et al. (2009). In both cases, the glides were compared in common
velocities, not considering the total range of velocities at which they
are usually performed. The aim of the present study was to experimentally
characterize the first and second gliding positions of the breaststroke underwater
stroke used after starts and turns at the total range of velocities
commonly performed, considering: (i) gliding velocity (v); (ii) body cross
sectional area (S); (iii) drag coefficient (C D ), and (iv) passive drag (D).
Methods
Six Portuguese national level male swimmers (18.2 ± 4.0 years old, 178.2
± 9.0 cm of height and 64.4 ± 11.4 kg of body mass) participated in this
62
study. Testing sessions were conducted in a 25 m pool, 2 m deep, with water
temperature at 27.5 ºC. It was used a similar methodology to the described in
Vilas-Boas et al. (in press), namely the S determination using planimetry, and
the passive drag assessed through inverse dynamics based upon the velocity to
time (v (t) ) curve of each glide, monitored through a swim-meter (Lima et al.
2006). Acceleration (a) was obtained through the numerical derivative of the
velocity curve. The drag force was computed using the expression:
D = m . a . (1)
To quantify the drag coefficient, the following equation was used, assuming
water density (ρ) as 1000 kg/m3 :
C D = 2 D / ρ S v 2 (2)
Each swimmer performed 3 repetitions of the breaststroke underwater stroke,
at maximal intensity (2 min of interval). During the rest interval, swimmers
received proper feedback in order to improve their performance. The video
images and data processing (v (t) curves and acceleration to time curves representations)
of the first and second glide of the underwater breaststroke were
obtained according to Vilas-Boas et al. (in press). Not all swimmers attained
all velocity values, being retained the v values (independently in each glide)
for which there were values from at least three swimmers. The processes of D
and C D calculations were described also in Vilas-Boas et al. (in press).
results
The main results of this study are presented in Table 1 and in Figures
1 and 2. Table 1 shows the S values for each swimmer in the first and
second glides while Fig. 1 and 2 show D and C D , respectively, for each
glide and for all v values attained.
Table 1. Individual and mean ± SD values of the cross-sectional area of
the body (S) assessed for the two studied breaststroke gliding positions.
Swimmer
S (cm2)
for first glide position
S (cm2)
for second glide position
1 971.13 942.1
2 719.73 886.91
3 641.7 668.47
4 689.17 769.24
5 692.6 718.05
6 845.34 901.99
Mean ± SD 759.95 ±124.12 814.46 ± 111.23
The mean ± SD v values for the first and second glide positions were
1.50 ± 0.22m/s and 1.15 ± 0.24m/s, respectively. The first glide presented
higher values of mean v, while the second gliding was characterized
by higher values of acceleration (0.61 ± 0.24m/s 2 to 0.66 ± 0.06m/
s 2 versus 0.40 ± 0.06 m/s 2 to 0.63 ± 0.16m/s 2 for common values of v,
second and first glide respectively). The common velocities for the two
gliding phases were 1.20, 1.30, 1.40 and 1.50m/s (Fig. 1 and 2)
Drag (N) first glide
80
second glide
70
60
50
40
30
20
10
0
0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Velocity (m/s)
Figure 1. Drag versus velocity curve in two distinct gliding positions:
first glide and second glide of the breaststroke underwater stroke