Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
Motor Coordination During the Underwater
Undulatory Swimming Phase of the Start for High
Level Swimmers
elipot, M. 1, 2 , houel, n. 2 , hellard, P. 2 , dietrich, G. 1
1 Université Paris Descartes, Paris, France
2 Fédération française de natation, Paris, France
The aim of the present study was to identify the main motor coordination
involved during the underwater undulatory swimming phase of
the start. Twelve French high-level swimmers took part in this study.
Swimmers were filmed during the entire underwater phase of the start
(i.e. fifteen meters from the start platform). Specific anatomical landmarks
were identified on the body of the swimmers and 2D space coordinates
of these landmarks were calculated. Cross correlation functions
were used to investigate the motor coordination between the actions of
the hip, the knee and the ankle. Results show that high-level swimmers
regulate the leg amplitude thanks to a strong synergy between the hip
and the ankle and an independent action of the knee.
Key words: Motor control, start, underwater undulatory swimming
INTRODUCTION
High-level athletes are characterised by superior tactical, physical and
technical levels. Improving one of these factors will lead to a performance
improvement. As in many other sports, swimming performance
is strongly linked to the technical level of the swimmers. Swimming
performance optimisation requires an accurate analysis of the swimmers’
movement. Biomechanics and motor control sciences give useful tools
to realise such analysis. Motor control theories, and more especially the
joint synergies theories, will indeed help to understand how high level
swimmers are able to produce specific, appropriate and efficient motor
pattern.
Many previous researches have already shown that start performance
is determinant to achieve a good race performance. Depending on the
studies, start performance is defined as the time to complete the 10 or
15 first meters from the start wall (Alves, 1993; Arellano et al., 1996;
Mason and Cossor, 2000). The start consists of 3 phases: the impulsion
phase, the aerial phase and the underwater phase (Maglischo, 2003).
This underwater phase of the start could be divided in 2 phases, i.e. the
glide phase and the underwater undulatory swimming phase (Maglischo,
2003).
During the glide, swimmers aim to keep the supra-maximal velocity
created during the impulsion and aerial phases as high as possible and
as long as possible. A previous study has already shown that during the
glide phase high level swimmers presented a strong joint synergy between
the shoulders, the hip and the knee (Elipot et al, 2009). It seems
that with this synergic action of those three joints high level swimmers
are able to hold a streamlined position during the whole glide phase.
Hydrodynamic resistance are decreased and supra-maximal velocity is
kept longer. During the underwater undulatory swimming, the swimmers’
aim is to produce velocities with leg action. Swimmers have then
to find the optimal compromise between the amplitude and the frequency
of the leg undulatory movements (Arellano, 2008). Swimmers
have to find the optimal compromise between propulsion force creation
and hydrodynamic resistance decrease. Identifying the motor coordination
during this underwater undulatory swimming phase of the start is a
major step to understand and optimise this phase.
The aim of this study was to determine the motor coordination that
high-level swimmers are able to produce during the underwater undulatory
swimming phase of the start.
72
Biomechanics and Medicine XI Chapter 2 Biomechanics
Swimmers have then to find the optimal compromise between the amplitude and the
frequency of the leg undulatory movements (Arellano, 2008). Swimmers have to find
the optimal compromise between propulsion force creation and hydrodynamic
resistance decrease. Identifying the motor coordination during this underwater
undulatory swimming phase of the start is a major step to understand and optimise this
phase.
Methods The aim of this study was to determine the motor coordination that high-level
swimmers are able to produce during the underwater undulatory swimming phase of the
Subjects start. and instructions:
Twelve male swimmers participated in this study. All were informed of
METHODS
the Subjects aim of and the instructions: study and signed a consent form. All participants were
high-level Twelve male swimmers participated and were in members this study. of All the were French informed National of the aim of Swim- the
study and signed a consent form. All participants were high-level swimmers and were
ming Team. Swimmers’ characteristics are summarized in Table 1.
members of the French National Swimming Team. Swimmers’ characteristics are
summarized in Table 1.
Table 1. Swimmers’ general characteristics (n = 12).
Table 1. Swimmers’ general characteristics (n = 12).
Height (m) Weight ( kg)
Best performance (freestyle)
50-m (s) 50-m (%) 100-m (s) 100-m (%)
Mean 1.83 76.1 24.39 116.47 52.26 111.4
SD 4.89 5.18 1.26 6.02 1.68 3.57
Swimmers were asked to perform 3 grab starts as fast as possible. Only the best
start was analysed (i.e. the best time to complete 15 meters from the start wall). All
swimmers were regularly trained to perform this kind of start and use it during
competition.
Experimental set up and data analysis:
Swimmers were filmed by 4 mini-DV camcorders (576x720 pixels) during the
whole underwater phase of the start (i.e. fifteen meters from the start platform) (Fig. 1).
The camcorder 1, 2 and 3 were placed the swimming pool portholes and the camcorder
4 was placed in the water in a waterproof housing. The camcorders position has been
chosen so as to minimise to reconstruction error due to optical distortions (Snell low).
All camcorders were synchronised using a light signal and sampling frequency was
25Hz. Images were deinterlaced and odd and even fields were both used.
Nine anatomical landmarks were identified on swimmer’s body: the toe, the
lateral malleolus, the knee, the iliac spine, the acromion, a finger hip, the wrist, the
elbow and the centre of the head. To minimise the error during the digitising process,
only the right side of the swimmers has been identified. Both sides were supposed to be
symmetric. Using a modified DLT 2D technique (inspired from Drenk et al., 1999) and
the Dempster anthropometric data (Dempster, 1959), joints’ positions and swimmers’
centre of mass position have been calculated. Reconstruction error was calculated as
indicated by Kwon and Casebolt (2006). Mean reconstruction error was 6.2 mm and
maximal reconstruction error was 12.2 mm. Data were filtered using a Butterworth II
filter (Winter, 1990). Cut-off frequencies were included between 5 Hz and 7 Hz.
Swimmers were asked to perform 3 grab starts as fast as possible. Only
the best start was analysed (i.e. the best time to complete 15 meters from
the start wall). All swimmers were regularly trained to perform this kind
of start and use it during competition.
Experimental set up and data analysis:
Swimmers were filmed by 4 mini-DV camcorders (576x720 pixels)
during the whole underwater phase of the start (i.e. fifteen meters from
the start platform) (Fig. 1). The camcorder 1, 2 and 3 were placed the
swimming pool portholes and the camcorder 4 was placed in the water
in a waterproof housing. The camcorders position has been chosen so
as to minimise to reconstruction error due to optical distortions (Snell
low). All camcorders were synchronised using a light signal and sampling
frequency was 25Hz. Images were deinterlaced and odd and even
fields were both used.
Nine anatomical landmarks were identified on swimmer’s body: the
toe, the lateral malleolus, the knee, the iliac spine, the acromion, a finger
hip, the wrist, the elbow and the centre of the head. To minimise the
error during the digitising process, only the right side of the swimmers
has been identified. Both sides were supposed to be symmetric. Using a
modified DLT 2D technique (inspired from Drenk et al., 1999) and the
Dempster anthropometric data (Dempster, 1959), joints’ positions and
swimmers’ centre of mass position have been calculated. Reconstruction
error was calculated as indicated by Kwon and Casebolt (2006). Mean
reconstruction error was 6.2 mm and maximal reconstruction error
was 12.2 mm. Data were filtered using a Butterworth II filter (Winter,
1990). Biomechanics Cut-off and frequencies Medicine XI were included Chapter 2 Biomechanics between 5 Hz and 7 Hz.
Camcorder 1
Camcorder
2
Camcorder
3
Figure 1. Experimental set up
Figure 1. Experimental set up
To avoid between-subjects variations due to the slope of the trajectory to reach the
To water avoid surface, between-subjects data were expressed variations in a (O’, due n, t to ,b) the local slope reference of the frame trajectory attached to the
swimmers’ centre of mass and with n collinear to the velocity vector and t perpendicular
to reach the water surface, data were expressed in a (O’, n, t ,b) local
to n (Fresney frame of reference). During the whole underwater phase, the velocity of
reference the centre of frame mass, attached the joint positions, to the swimmers’ velocities and centre angles of for mass the hip, and the with knee n and the
collinear ankle, the to angles the velocity between vector the x-axis and and t perpendicular the limbs (angle to of n (Fresney attack) were frame calculated.
of Only reference). the joints longitudinal During the and whole vertical underwater positions were phase, expressed the velocity in the original of the global
frame (O, x, y, z).
centre Motor of mass, coordination the joint was positions, investigated velocities by computing and angles the cross for correlation the hip, the functions
knee between and all the ankle, calculated the kinematics angles between variables. the These x-axis kinematic and the data limbs were (angle represented
of in form attack) of were time-dependent calculated. signals. Only the Results joints were longitudinal obtained using and Matlab vertical software posi- (The
Matworks Inc., USA). Level of confidence was set at 95% (p