Biomechanics and Medicine in Swimming XI

ω y (deg/s) ω x (deg/s)
t glide25
Parameter selection for a backstroke trial
t stroke25
t pp
pω x
t turn
pω y
t glide50
t stroke50
time (s)
time (s)
Figure 2: parameter selection for backstroke using its most descriptive
angular velocities, ω x and ω y . The same parameter selection can be adopted
for the further axis and for the other swimming styles. Apart from
the starting phase, it is possible to easily recognise the underwater gliding
and the stroke phase of the first 25m lap, the turning phase and the
final lap with the relevant gliding and stroke phases. Local peaks for
each angular velocity, pω x and pω y , during the turning phase were also
highlighted, together with the t turn and t pp durations.
ω z (deg/s) ω y (deg/s)
t glide25
Parameter selection for a butterfly trial
t stroke25
pω y
pω z
t turn
t glide50
t stroke50
time (s)
time (s)
Figure 3: parameter selection for a butterfly trial using its most descriptive
angular velocities, ω y and ω z . It is possible to outline several differences
from the previous style, particularly in the stroke phase and in the
turning phase, where the negative ω z peak specifically characterizes the
open-turn technique executed by the butterfly swimmer.
Starting from the sensor output, the following further parameters were
defined. According to the literature, the stroke rate is defined for each
lap (sr25 and sr50 ) as number of stroke cycles per minute. Furthermore,
aiming at outlining the strategy undertaken by the swimmer in choosing
the duration of both gliding and stroke phases for the two laps, the
tglide
synthetic indices r25 and r50 were defined as follows: r = . An r index
tstroke
near 1 indicates that the two phases have a similar duration (e.g. for
backstroke), whereas values less than 0.5 are expected for breaststroke
swimmers.
chaPter2.Biomechanics
results
Each swimmer completed the execution without problems and did not
experience any difficulty in using the instrumental apparatus. Results for
the eight swimmers are listed in Table 1. Positive and negative pω values
are according to the convention in Fig. 1.
Biomechanics and Medicine in Swimming XI Chapter 2 Biomechanics b
Table 1. Results obtained for the four swimming styles for each selected
parameter.
Table 1. Results obtained for the four swimming styles for each selected parameter.
pωx
(deg/s)
pωy
(deg/s)
pωz
(deg/s)
tpp
(s)
tturn
(s)
sr25
(cycle/min)
r25
sr50
(cycle/min)
r50
tchr
(s)
Free
M
F
-241
-310
565
324
-111
-69
0.95
0.92
1.35
1.48
60.08
55.17
0.36
0.44
55.65
51.60
0.12
0.11
25.7
27.8
Back
M
F
-352
354
344
321
-18
-47
1.02
1.16
2.15
2.36
67.67
46.81
0.99
0.70
56.07
44.44
0.65
0.56
26.7
32.0
Breast
M
F
-205
-140
-379
-296
-330
-128
0.46
0.42
1.16
1.26
61.71
52.82
0.36
0.29
57.24
44.02
0.16
0.26
29.8
35.8
Fly
M
F
-230
-119
-321
-265
-260
-238
0.54
0.66
1.24
1.14
60.26
61.46
0.41
0.51
59.29
57.04
0.28
0.17
25.6
29.5
DISCUSSION
dIscussIon
The present study verified the feasibility to use wearable inertial sensor devices to
characterise turning, gliding and stroke resumption in swimming. The novelty of the
approach is constituted by the use of gyroscopes to measure angular velocities and by
the consequent easy description of swimming rotational movements. If used in
conjunction with the available measured linear accelerations, several biomechanical
parameters can be extracted that increase the knowledge about each swimming style
and/or about the strategy adopted by each swimmer.
Several quantitative parameters were selected from the sensor device output: the choice
reflected the need to characterise each swimming style and offer possible parameters to
describe the way each athlete executes his/her swimming exercise.
Highest pωy were found for freestyle in both genders. This circumstance is directly
related to the well-known highest velocities usually obtained in this swimming style at
the end of the 25m lap. This parameter was higher for males than for females, as it
happens usually for mean velocity, even if related differences are slighter than for linear
velocity. In fact, the two parameters are linked by the equation - v = ω*R - being R the
distance between the most external point and the rotational axis and, thus, related to the
individual flexibility. Considering that females are usually more flexible than males,
thus more able in reducing R, it is possible to argue that a higher R for female
compensates the higher mean velocity usually obtained by male swimmers. The sign of
the rotation depends upon the style turning technique: freestyle and backstroke use a
ecutes flip-turn his/her technique, swimming thus involving exercise. a positive rotation about the y axis, vice-versa
breaststroke and butterfly use an open-turn that involves a negative rotation about the
same axis corresponding to the recall of the leg before the push-off.
Further peak angular velocities, pωx and pωz, characterised differently the other
swimming styles. Highest pωx were found for the backstroke flip-turn that is first
characterised by a sudden rotation about the cranio-caudal axis, consistently with its
theoretical execution. For all styles, the negative peak indicated that every athlete
carried out the rotation leading the right arm, thus executing a clockwise rotation about
the cranio-caudal axis. This is, in fact, a choice dependent upon the single swimmer.
Highest pωz, conversely, were found for both breaststroke and butterfly. This
circumstance is consistent with the open-turn technique used for these two styles: after
the feet contact on the pool wall, the swimmer suddenly turns the upper body leading
the rotation using the right arm, thus generating a clockwise rotation about the z axis
(negative pωz peak).
The present study verified the feasibility to use wearable inertial sensor
devices to characterise turning, gliding and stroke resumption in
swimming. The novelty of the approach is constituted by the use of
gyroscopes to measure angular velocities and by the consequent easy
description of swimming rotational movements. If used in conjunction
with the available measured linear accelerations, several biomechanical
parameters can be extracted that increase the knowledge about each
swimming style and/or about the strategy adopted by each swimmer.
Several quantitative parameters were selected from the sensor device
output: the choice reflected the need to characterise each swimming
style and offer possible parameters to describe the way each athlete ex-
Highest pωy were found for freestyle in both genders. This circumstance
is directly related to the well-known highest velocities usually obtained
in this swimming style at the end of the 25m lap. This parameter
was higher for males than for females, as it happens usually for mean velocity,
even if related differences are slighter than for linear velocity. In fact, the
two parameters are linked by the equation - v = ω*R - being R the distance
between the most external point and the rotational axis and, thus, related
to the individual flexibility. Considering that females are usually more flexible
than males, thus more able in reducing R, it is possible to argue that
a higher R for female compensates the higher mean velocity usually obtained
by male swimmers. The sign of the rotation depends upon the style
turning technique: freestyle and backstroke use a flip-turn technique, thus
involving a positive rotation about the y axis, vice-versa breaststroke and
butterfly use an open-turn that involves a negative rotation about the same
axis corresponding to the recall of the leg before the push-off.
Further peak angular velocities, pωx and pωz , characterised differently
the other swimming styles. Highest pωx were found for the backstroke
flip-turn that is first characterised by a sudden rotation about
the cranio-caudal axis, consistently with its theoretical execution. For all
styles, the negative peak indicated that every athlete carried out the rotation
leading the right arm, thus executing a clockwise rotation about the
cranio-caudal axis. This is, in fact, a choice dependent upon the single
swimmer. Highest pωz, conversely, were found for both breaststroke and
butterfly. This circumstance is consistent with the open-turn technique
used for these two styles: after the feet contact on the pool wall, the
swimmer suddenly turns the upper body leading the rotation using the
right arm, thus generating a clockwise rotation about the z axis (negative
pωz peak).
Still focusing on the turning phase, tpp and tturn were higher for backstroke,
consistently with the backstroke turning technique that includes
a rotation (positive if clockwise, negative if counter-clockwise) about
the X-axis before the execution of the rotation about the Y-axis. Also, tpp value indicates that flip-turns are characterised by two distinct rotations
(about y and x axes), while open-turns are complex movements in which
rotations follow each other in a faster way.
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