Biomechanics and Medicine in Swimming XI

cm, 73.1 ± 6.4 kg and 20 ± 1.2 years, respectively. Before the experiment
began, each swimmer received instruction on the no-step start and two
types of step starts over a two-weeks; this was achieved by conducting
daily training sessions. In each trial of a relay start, swimmers were permitted
to swing their arms and to perform counter-movement.
In the trial, we select three relay starts: a no-step start (NS), singlestep
start (SS) and double-step start (DS) (Figure 1). For each start
technique, each swimmer performed six trials with maximum effort; the
trials were performed in random order. Each swimmer executed the relay
start trial by timing it with the goal touch of the previous front crawl
swimmer and then sprinted 15 m by front crawl swimming.
No-step start
Single-step start start
Double-step start start
Figure 1. Three types of relay starts performed in this study.
Two high-speed cameras (FAST-CAM PCI, Photron Inc., JAPAN)
filmed the trials at 250 frames per second. Camera 1 was positioned
such that its optical axes were perpendicular to the plane of motion.
Camera 2 was positioned 2.5 m above the floor of the side pool deck and
filmed the scene at the instant at which the incoming swimmer touched
his finishing point and the outgoing swimmer take-off from the starting
block. The relay time was defined as the time elapsed between the touch
and take-off and was calculated by counting the frames in the video image
captured by Camera 2.
Ground reaction forces during the relay starts were sampled at 1000
Hz by a waterproof force plate (5253B11, Kistler JAPAN Inc.) that was
modified into the starting block. The force data were smoothed using a
low-pass Butterworth digital filter with a low pass cutoff frequency of 40
Hz. The velocity at take-off (take-off velocity) was calculated from the
data on the ground reaction force by performing time integration until the
time of take-off. The take-off angle was defined as the angle between the
resultant take-off-velocity vector and horizontal line (upward direction:
positive; downward direction: negative). The horizontal ground reaction
forces were used to distinguish the forces generated by steps or the body
leaning forward and the legs driving out. The force generated by legs driving
forward was defined as the force after the time (t 1 ) when the horizontal
ground reaction force reached over 30% of body weight. The force before
t 1 was obtained was defined as the force by step or body leaning forward
(Figure 2). The horizontal reaction force generated by the legs driving was
integrated with respect to time, and this value was used to calculate the velocity
generated by the legs driving. A one-way repeated measure ANOVA
was performed for the start type; this was followed by Bonferroni multiple
comparison in each variable. The statistical significance was set at P < 0.05.
Ground reaction force [N]
Vertical
+
chaPter2.Biomechanics
horizontal
+
1400
1200
1000
800
Horizontal component
600
Force by
400
200
0
30% of subject’s body weght
legs driving
-200
t1 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1
Time [sec]
0
Figure 2. The definition of the horizontal ground reaction force generated
by taking steps and leaning forward and the force generated by the
legs driving.
results
Table 1 lists the mean horizontal take-off velocity, take-off angle and relay
time in each trial. The relay times were increased significantly in order of
no-step start, single-step start and double-step start. The horizontal velocity
generated by legs driving in the no-step start was significantly greater
than that in the double-step start. There was no significant difference in
the horizontal take-off velocity and take-off angle. Table 2 lists the mean
values of the standard deviation in the six trials for each condition.
Table 1. Mean values of horizontal take-off velocity, horizontal velocity
by legs driving, take-off angle and relay time in each trial.
Horizontal
take-off velocity
Horizontal
velocity by legs driving
Take-off angle Relay time
(m/s) (m/s)
(°)
(s)
No-Step 4.09 ± 0.34 3.50 ± 0.34 -11.92 ± 6.15 0.018 ± 0.029
Single-Step 4.09 ± 0.31 3.28 ± 0.24 -13.18 ± 5.27 0.052 ± 0.044 *
Double-Step 4.13 ± 0.32 3.22 ± 0.25 * -13.70 ± 4.80 0.087 ± 0.060 * †
*: P < 0.05 in comparison with no-step start trials.
†: P < 0.05 in comparison with single-step start trials.
Table 2. Mean standard deviations of horizontal take-off velocity, horizontal
velocity generated by legs driving, take-off angle and relay time
in each trial.
Horizontal
take-off velocity
Horizontal
velocity by legs driving
Take-off angle Relay time
(m/s) (m/s)
(°)
(s)
No-Step 0.15 ± 0.08 0.10 ± 0.05 2.17 ± 1.58 0.054 ± 0.023
Single-Step 0.25 ± 0.09 0.15 ± 0.05 2.11 ± 0.59 0.075 ± 0.031
Double-Step 0.23 ± 0.09 0.13 ± 0.03 2.25 ± 1.08 0.077 ± 0.035
Figure 3 shows the changes in the ground reaction force until take-off,
as a typical example for Sub. A. Similar patterns were observed when the
three trials involving the other subjects were compared. For all subjects,
there were a total of eight trials for each step start.
dIscussIon
McLean et al. (2000) reported that the horizontal take-off velocity after
the double-step start was significantly greater than that after the no-step
start. Our hypothesis was similar to the results of McLean’s study. However,
there was no significant difference among the horizontal take-off
velocities achieved by the three relay start techniques (see Table 1). The
advantage of step starts is that horizontal forces are generated until the
legs driving (See Figure 3). No-step starts were superior to step starts
in terms of the horizontal force generated by the legs driving. The horizontal
velocity generated by legs driving in the no-step start was significantly
greater than that in the double-step start. In the step start, the
swimmers had greater horizontal velocities before the legs started driving.
It was difficult for the swimmer to generate a large horizontal force
during the leg drive in the step starts. In addition, the time required for
the legs to drive decreased because the swimmer was already moving
forward with a greater velocity before the legs began driving.
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