Biomechanics and Medicine in Swimming XI

Drag Coefficient first glide
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
second glide
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 2. Drag Coefficient versus velocity curve in two distinct gliding positions:
first glide and second glide of the breaststroke underwater stroke.
dIscussIon
The relatively low v values found in this study may be explained by the
level and age of the swimmers tested, all of national level, not international
elite, and mixing junior and senior swimmers. However, the
higher v values were obtained during the first glide, probably due to its
time proximity to the wall impulse, as well as to the higher hydrodynamics
that characterised it.
As expected, in accordance with our previous results (Vilas-Boas, in
press), and with the literature using similar methodology (Clarys, 1979),
swimmers showed a smaller S area in the first glide position than in the
second position. This fact could be explained by the “compressive” effect
over the shoulders and chest width produced by the flexed shoulders,
which could be one of the main determinant factors associated with a reduced
drag in the first glide. It is important to note that the obtained S
values may slightly differ from the actual gliding S values, once it is possible
that the gliding alignment of the body may differ from the standing
position. In fact, we must assume that it is possible that some swimmers
may adopt, at least in some instants during the glide, an inclined body position
with respect to the gliding direction. Our observation of the recorded
video images revealed some inclined gliding directions, with a coaxial and
convenient body alignment, but not any inclined body position regarding
the direction of motion that would change S values. We are convinced that
this approach is also more accurate for S and C D assessment than calculating
active drag C D values using a constant estimation of S based upon body
volume powered 2/3 (Kolmogorov & Duplischeva, 1982).
The D values calculated in this study (Fig. 1) are consistent with previously
passive drag values published for competitive swimmers. Clarys
(1979) reported D values of about 52 N at 1.3m/s for male national-level
swimmers. Vilas-Boas et al. (in press) found values of 34.66 ± 7.869 N and
42.92 ± 5.658 N, respectively for the first and second studied glide positions,
but at lower velocities than the studied in this paper for the first one, and
higher for the second one. In the present study, D ranged between 23.35
N and 58.25 N. For both glides the drag increase with velocity (Fig. 1).
However the first glide is characterised by drag values lower for all velocities
(Fig. 1), probably due to a parallel and concurrent effect of S and C D . The
increased body length and slenderness associated with the flexed shoulders
and extended position of the arms along the longitudinal axis of the body
may reduce C D , as well as may have an effect upon the reduction of S.
For the first glide, the C D changed from 0.52 at 1.3 m/s to 0.44 at 1.6
m/s. After analysis of Table 2, it is possible to state that C D decreases with
v. For the second glide the drag coefficient changed from 0.95 at 0.8 m/s
to 0.47 at 1.5 m/s. The inverse relationship between the C D and v found
in the current study seems to correspond to what was observed in other
experimental situations (e.g. Lyttle et al., 2000). Moreover, the gliding position
with arms extended at the front (first glide) presented lower drag
coefficient values (Fig. 2). This body position is accepted by the swimming
technical and scientific communities as the most hydrodynamic position,
being called streamline position, because it seems to be the one that al-
chaPter2.Biomechanics
lows a higher reduction of the negative hydrodynamic effects of the human
body morphology: a body with various pressure points due to large
changes in its shape. This position seems to smooth the anatomical shape,
especially at the head and shoulders, allowing better penetration of the
body in the water during the underwater phases in swimming.
In Fig. 2. the C D for the first glide position presents lower values for
the different velocities in study, while the second glide position provides
a greater range of values, causing more variability. This finding points out
the presumably higher instability of the second gliding position. Being
so, attention needs to be paid to its training and competitive execution.
conclusIon
The first glide position with arms extended at the front is characterised by
lower values of D and C D , despite it is performed at higher velocities. The
C D decreases with velocity, particularly during the second glide. As a practical
consequence, swimmers and coaches should stress the need for body
position control during its execution, particularly during the more resistive
glide (the second one). These results also pointed out the need of technical
evaluation, control and advice to allow drag reductions during swimming
performance, and not only emphasising propulsion increase possibilities.
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