Biomechanics and Medicine in Swimming XI

chaPter2.Biomechanics
Biomechanics and Medicine in Swimming XI Chapter 2 Biomechanics b 169
forward unsteadiness. This unsteadiness permitted to obtain a couple
of opposing forces and to influence the angular momentum. The aim
of this study was to analyze how the start style influences the angular
stretched upward and forward instead of arms) and flight start (short block phase but
momentum around the mediolateral axis, as this is an important char-
long flight phase due to great leg power).
acteristic McLean of et the al. start. (2000) calculated the angular momentum of the aerial phase, which is
generated during the block phase in order to quantify rotation. They analyzed several
Methods
start relay techniques, with and without one or two steps before taking off from the
Five block. elite Taking male steps front-crawl before leaving sprinters the volunteered block resulted to in participate greater takeoff in this and entry angles
(like a pike start) and each step start was characterized by greater body rotation than the
study no-step (22±2 start. years; Pike 180±0.1 trajectory cm; or 74.7±7.9 arm swing kg; during mean the time flight for phase a 100-m (Volkov style) thus
front has crawl an effect in a on 50-m body pool: rotation 53.6±1.8 and consequently s). Each swimmer on the aerial performed part of the a start. Indeed,
25-m the swim front start crawl cannot three times be reduced at the to 50-m generate race the pace greatest with the forward grab start impulse; the initial
position on is the a blocked block. A forward camera unsteadiness. (50 Hz) was This placed unsteadiness at the edge permitted of the to obtain a
couple of opposing forces and to influence the angular momentum. The aim of this
pool and videotaped the block phase to determine block phase duration,
study was to analyze how the start style influences the angular momentum around the
the mediolateral takeoff angle axis, (calculated as this is an from important the centre characteristic of gravity, of the foot start. extremity
and horizontal axis), the wrist/shoulder/hip angle (body angle at takeoff
METHODS ), and the duration and distance of the flight phase. A second camera
(50 Five Hz) elite was male mounted front-crawl on a specially sprinters volunteered designed support to participate placed in at this the study lat- (22±2 years;
180±0.1 cm; 74.7±7.9 kg; mean time for a 100-m front crawl in a 50-m pool: 53.6±1.8
eral wall 3 m from the edge of the pool deck to videotape the entry phase
s). Each swimmer performed a 25-m front crawl three times at the 50-m race pace with
(entry the grab angle, start calculated position from on the the block. centre A of camera gravity, (50 finger Hz) was extremity placed and at the edge of the
horizontal pool and axis videotaped at water the contact). block A phase third to camera determine (50 block Hz) was phase placed duration, 15 the takeoff
m angle from the (calculated edge of from the pool the centre deck to of determine gravity, foot the extremity moment the and swim- horizontal axis), the
mer’s wrist/shoulder/hip head reached the angle 15-m (body mark angle (placed at takeoff), on the and swimming the duration line). and Im- distance of the
flight phase. A second camera (50 Hz) was mounted on a specially designed support
ages placed were at processed the lateral using wall Simi-Motion 3 m from the (Simi edge of Reality the pool Motion deck Systems to videotape the entry
GmbH, phase (entry City, angle, Germany). calculated The from spatial the model centre of comprised gravity, finger 20 anatomical extremity and horizontal
landmarks axis at water digitalized contact). in A each third frame, camera defining (50 Hz) a 14-body was placed segment 15 m model from the edge of the
(de pool Leva, deck 1996). to determine To compute the moment the angular the swimmer’s momentum head for reached each body the seg- 15-m mark (placed
on the swimming line). Images were processed using Simi-Motion (Simi Reality
ment, we evaluated the mechanical parameters such as the segment mass
Motion Systems GmbH, City, Germany). The spatial model comprised 20 anatomical
and landmarks inertia. For digitalized this purpose, in each we frame, assumed defining that each a 14-body body segment, segment ex- model (de Leva,
cept 1996). the head, To compute could be the modelled angular momentum as a homogeneous for each cylinder. body segment, The head we evaluated the
was mechanical considered parameters as a sphere. such To obtain as the the segment body mass segment and inertia, inertia. we For used this purpose, we
the assumed radius of that gyration each body computed segment, by except Zatsiorsky the head, and could Seluyanov be modelled (1983). as As a homogeneous
cylinder. The head was considered as a sphere. To obtain the body segment inertia, we
the motions were in two dimensions, we computed the angular momen-
used the radius of gyration computed by Zatsiorsky and Seluyanov (1983). As the
tum motions along the were transverse in two dimensions, axis as follows: we computed the angular momentum along the
transverse axis as follows:
i
⎛
L GZ / F * = [ Ii
] wi
z + mi⎜
( GGix
* VGiy
/ F * ) − ( GGi
y * VGix
/ F *
⎝
⎞
)⎟
⎠
where G is the centre of gravity of the whole body, mi is the mass of the ith segment, Gi
where G is the centre of gravity of the whole body, m
is the centre of mass of the ith segment, F* is the barycentric i is the mass of the
frame, Ii is the inertial
i matrix of the ith segment, and wi is the angular velocity. The total angular momentum in
the global frame was then obtained by adding the local angular momentum of each
segment.
Angular momentum was calculated at takeoff (H, kg.m²/s) and the mean standard
deviation for H (∆H, kg.m²/s) was also calculated. For dynamic analysis, starts were
th segment, Gi is the centre of mass of the results
The takeoff angle differed significantly with the start styles and ranged
from 19±3.3° for flat start to 27.8±2.4° for Volkov start (26.8±3.6° was
measured for the pike start). Three swimmers used a flat start defined
by a low takeoff angle (19±3.3°) and one swimmer used the pike start
(26.8±3.6°). The body angle at takeoff (wrist/shoulder/hip) was significantly
different between the three start styles. The Volkov start showed
negative values because of the position of the arms behind the trunk.
The entry angle was significantly greater for the pike start (34.3±3.9°,
39.7 ±2.8° and 37±3.1°, respectively, for the flat, pike and Volkov starts).
Although these differences were not statistically significant, greater entry
angles were observed for the pike and Volkov vs. the flat start. No
significant difference in performance to the 15-m mark was observed
between the start style groups. The takeoff angle, flight distance, and
the total and standard deviation of the angular momentum were significantly
lower for the flat start than for the two other start styles (with a
significantly longer flight phase for the Volkov start in comparison with
the two other starts). The time to 15-m was significantly and negatively
correlated with the vertical impulse (r=-0.507) and positively correlated
with Biomechanics ΔH (r=0.461) and Medicine (Table in Swimming 1). XI Chapter 2 Biomechanics b 171
Table 1. Start parameters as a function of start style
Table 1. Start parameters as a function of start style
Start Styles Flat Start (n=3) Pike Start (n=1) Volkov Start (n=2) Correlation
with T15m
Block Phase (s) 0.95±0.04 0.94±0.03 0.89±0.08
Flight Phase (s) 0.26±0.02 0.32±0 a 0.38±0.03 a, b
Flight Distance 3±0.06
(m)
Takeoff Angle 22.1±3.5
(°)
Entry Angle (°) 23.4±2.2
3.2±0.01 a
30.0±2.7 a
24.6±4.8
3.2±0.03 b
34.6±5.2 b
28.2±2.5
Body Angle at
Takeoff (°)
Horizontal
Impulse (N)
Vertical
Impulse (N)
145.3 ±7.6
788.4±48.7
188.7±26.2
27.1±10.5 a
698.9±60.8
171.5±21
- 55.4±13.9 a, b
829.2±61.9 a
205.4±31 r= -0.507
H Takeoff
(kg.m²/s)
∆H (kg.m²/s)
14.7±2.92
0.8±0.13
18.0±0.67 a
1.1±0.2 a
17.5±0.4 b
1.1±0.15 b r= 0.461
Time to 15m (s) 6.5±0.2 6.6±0.08 6.6±0.07
a: significantly different from the previous style on the table; b: significantly different
ith segment, F* is the bary- a: from significantly the flat start different from the previous style on the table; b: significentric
frame, Ii is the inertial matrix of the ith segment, and wi is the cantly different from the flat start
DISCUSSION
angular velocity. The total angular momentum in the global frame was The aim of this study was to analyze how the start style influences angular momentum.
then obtained by adding the local angular momentum of each segment. dIscussIon
The results regarding the kinetic momentum showed that significantly less rotation
generated during impulsion induced a flat aerial trajectory and permitted the swimmers
Angular momentum was calculated at takeoff (H, kg.m²/s) and the The to enter aim the of water this more study quickly was (as to demonstrated analyze how by a significantly the start style shorter influences flight phase). an-
mean standard deviation for H (ΔH, kg.m²/s) was also calculated. For
dynamic analysis, starts were evaluated with a Bertec 4060-15 force plate
gular The pike momentum. and Volkov starts The showed results significantly regarding more the rotation. kinetic The momentum arm swing in showed the
pike (forward) and Volkov (backward then forward) increased the quantity of rotation.
that In the significantly Volkov start, the less backward rotation swing generated during impulse during accelerated impulsion the forward induced rotation a flat
(Bertec Corp., Columbus OH, USA) mounted on a specially built support
fixed to the pool wall to allow a start position that conformed with
the FINA rules. The sampling rate was 1000 Hz. The analogue signal was
transmitted to a PC through a Biopac A/D converter (Biopac Systems
Inc., Goleta CA, USA). The start signal was in accordance with the swimming
rules and was produced by the starter device (ProStart). After filtering
and smoothing (Hamming, low pass, 80 Hz), the reaction time,
impulse time and impulse values were obtained in 3D from the force
plate. Only the horizontal and vertical axes were analyzed because they
aerial of the body trajectory but the swimmer’s and permitted head was in the complete swimmers extension, to thus enter in opposition the water to the more
arms. The opposite was observed for the pike start. The body angle at takeoff indicated
quickly that the arms (as were demonstrated in continuation by of the a body significantly for the flat style shorter (angle flight close to phase). 180° with The
pike 145.3±7.6°). and Volkov The Volkov starts start showed significantly a negative value more due rotation. to the fact The that arm the arms swing
were behind the trunk, and the values were low for the pike start (27.1±10.5°),
in indicating the pike that (forward) the swimmers and were Volkov able (backward to swing the then arms forward forward) during increased the flight the
quantity phase. The of takeoff rotation. and entry In angles, the Volkov distance start, to hand the entry, backward and phase swing duration during were in im-
the range of the results provided by the literature: flight distance was 3.09±0.14 m in
pulse our study accelerated vs. 3.42±0.16 the m for forward McLean rotation et al. (2000), of the and the body angular but momentum the swimmer’s was
head 16.2±2.5 was kg.m²/s in complete in our study extension, vs. 16.4±3.2 thus kg.m²/s in opposition for McLean et to al. the (2000). arms. The The
vertical impulse was significantly and negatively correlated with time to 15-m,
opposite indicating that was an observed efficient vertical for the impulse pike needs start. to The be high body enough angle to ensure at takeoff that the indicated
flight distance that is the sufficiently arms were long in (this continuation variable was not of significantly the body for different the among flat style
made up the plane in which the swimmer moved. The swimmers’ starts (angle close to 180° with 145.3±7.6°). The Volkov start showed a negative
were classified according to the four start styles presented by Seifert et al. value due to the fact that the arms were behind the trunk, and the values
(in press). The parameters defining the start styles were: block and flight were low for the pike start (27.1±10.5°), indicating that the swimmers
phases, takeoff angle (arms/trunk and body/horizontal axes), body angle were able to swing the arms forward during the flight phase. The takeoff
at takeoff (wrist/shoulder/hip), and entry angles (body/horizontal axis and entry angles, distance to hand entry, and phase duration were in
at hand entry). After verifying the normality of the data (checked with the range of the results provided by the literature: flight distance was
the Shapiro-Wilk test), ANOVA tests analyzed the variables that signifi- 3.09±0.14 m in our study vs. 3.42±0.16 m for McLean et al. (2000), and
cantly differentiated the start styles. Pearson correlation analysis was also the angular momentum was 16.2±2.5 kg.m²/s in our study vs. 16.4±3.2
applied to study the relationships between the start parameters and the kg.m²/s for McLean et al. (2000). The vertical impulse was significantly
total 15-m start time (T15m). Statistics were performed with Minitab and negatively correlated with time to 15-m, indicating that an effi-
14.10 (Minitab Inc., 2003) and the level of significance was set at p< 0.05 cient vertical impulse needs to be high enough to ensure that the flight
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