Biomechanics and Medicine in Swimming XI

Influences of the Back Plate on Competitive
Swimming Starting Motion in Particular Projection
Skill
nomura, t. 1 , takeda, t. 2 , takagi, h. 2
1 Kyoto Institute of Technology, Kyoto, Japan
2 University of Tsukuba, Tsukuba, Japan
The purpose of this study was to identify the influences of the back
plate to competitive swimming starting motion in particular projection
skill. Ten male college elite competitive swimmers performed the track
start from the conventional platform and from the platform with the
back plate. The influences of the back plate to competitive swimming
starting motion in particular projection skill were identified as follows:
(1) At the set position, the centre of mass displaced to a anterior position
(-0.205±0.054 m); (2) The rear leg knee joint angle of the set
position was about 90 degree (84.3±11.3 degree); (3) Just before take
off, the body was accelerated into the horizontal direction (horizontal
8.76±0.87 m/s 2 , vertical 0.16±1.13 m/s 2 ); (4) At take off, the projection
angle could be kept near horizontal (-8.2±5.2 degree); (5) There were no
significant differences in flight phase.
Key words: Back plate block, competitive swimming start, Projection
skill
IntroductIon
A faster start has characteristics that move the centre of mass fast in the
forward direction while the feet are in contact with the starting block,
maximize the force exerted through the feet in the backward direction,
and maximize the force exerted through the hands against the starting
block in the forward and upward direction (Guimarães & Hay, 1985).
Various investigations have been conducted on styles of set position as
grab, track, handle, and so on (Bkanksby et al., 2002; Issurin & Verbitsky,
2003; Takeda & Nomura, 2006; Galbraith et al., 2008; Welcher
et al., 2008). They found that there was no significant difference of start
time among the grab start, the track start, and the handle start. Furthermore,
the 94% of variance in the start time was explained by the
water time that was the duration of the period between from first water
contact to 9 m (Guimarães & Hay, 1985). It was considered that the
swimming start consisted of plural skills, as projection, immersion and
underwater actions. The underwater speed depends on the horizontal
velocity at the entry. Furthermore, the horizontal velocity at entry results
from the horizontal velocity at takeoff. Therefore, an important
factor is the starting motion on the block to start fast. On the other
hand, few studies have been considering the conditions of the starting
block (Pereira et al., 2003). The starting platform with an adjustable back
plate (posterior foot support) setting is approved by the international
swimming governing body (FINA) in the facility rule 2.7. But there is
no scientific evidence of the influences of the platform with the back
plate on the performance of competitive swimming start. Therefore, the
purpose of this study was to identify the influences of the back plate
on competitive swimming starting motion in particular projection skill.
Methods
Ten male college elite competitive swimmers were used as subjects in
this study (Height = 177.5 ± 5.3 cm, Weight = 74.6 ± 8.4 kg, Age = 21.1
± 1.3 yrs). The characteristics of subjects are showed in Table 1. Subjects
performed the track start in two conditions. One trial started from the
conventional platform (CON). The other trial started from the platform
with the back plate (BKP).
Table 1. Characteristics of subjects.
No. Height Mass Age Specialty Distance
chaPter2.Biomechanics
Personal
Best
(cm) (kg) (yrs) (m) (min’sec)
0 188.0 86 23 Free style 50 23.33
1 176.5 69 21 Butterfly 100 52.40
2 175.0 73 22 Breast stroke 100 1’01.51
3 175.8 81 21 Individual Medley 400 4’22.30
4 179.0 81 21 Free style 100 50.09
5 174.0 67 22 Free style 100 50.78
6 171.0 65 21 Butterfly 100 53.77
7 181.0 81 19 Breast stroke 100 1’01.78
8 172.0 62 19 Free style 200 3’59.38
9 183.0 81 22 Breaststroke 100 1’01.71
The starting block specifications were as follows; Height from water
surface was 0.75m. The conventional platform was 0.5m width × 0.7m
length, with a 10º slope. As a back plate a pedal for a track and field’s
starting equipment was used. It was set 0.44m from the front edge of the
platform, and with a 35º slope from the platform.
A digital video camera equipment (Fps = 60Hz, Shutter speed = 250Hz)
was connected to a starting and timing system and used to record the
performance of each trial from a sagittal view. Seventeen calibration
points were set over a 2.1m width × 2.5m height frame placed at the
start area on the swimmer’s plane of movement. The digital video was
analyzed by digitizing 13 points of the body: left finger tip, left wrist,
left elbow, left shoulder, left hip, left ear, vertex, left knee, left ankle, left
toe, right knee, right ankle, right toe. It was assumed that arm and torso
were symmetrical. Kinematic variables were calculated using 2D-DLT
method with a self-made motion analysis software (Note Player). The
accuracy of coordinates was evaluated by the difference of the actual
value and the estimated value (horizontal = 0.034m, vertical = 0.013m).
Butterworth low-pass filter with a cut-off frequency of 6 Hz was used to
remove noise from all raw data.
The objective in performing a swimming start while the feet are in
contact with the starting block is to move the centre of mass (CM) as
fast as possible in the forward direction. For the purpose of this study,
the projection motion was divided into set position, acceleration phase,
take-off and flight phase. The set position was a still position previous to
the starting signal. The acceleration phase was from the starting signal
until take-off. The take-off is the instant on time when the forward leg
leaved from the block. The flight phase was defined from the take-off
until the first contact with the water surface. Kinematic variables for the
projection motion consisted of 15 items, as shown in Table 2 and Figure
1. The x-axis was defined to be the horizontal axis in the main plane of
movement. The y-axis was defined to be the vertical one.
results
At the starting position, the coordinate of CM in BKP was
(-0.205±0.054 m, 1.367±0.029 m). It was significantly forwarded and
at a higher position than in CON (-0.253±0.054 m, 1.355±0.031 m).
The knee joint angles of front and rear leg in BKP were respectively
140.1±5.7º, and 84.3±11.3º. The knees were extended significantly
narrower than in CON (145.5±8.0º and 97.1±11.4º). The ankle joint
angles of front and rear leg in BKP were, respectively, 140.6±8.4º, and
104.1±8.4º. The front ankle was dorsal-flexed significantly narrower
than that in CON (147.1±10.5º). On the other hand, the rear ankle in
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