Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
Analysis of Swim Turning, Underwater Gliding
and Stroke Resumption Phases in Top Division
Swimmers using a Wearable Inertial Sensor Device
Vannozzi, G. 1 , donati, M. 1 , Gatta G. 2 & cappozzo, A. 1
1 Department of Human Movement and Sport Sciences, University of Rome
“Foro Italico”, Italy
2 Faculty of Exercise and Sport Science, University of Bologna, Italy
Improving performance is a difficult task for élite swimmers. The study
of turning, underwater gliding and stroke resumption kinematics may
lead to the reduction of the relevant durations. Common video analysis
is sometimes inadequate to investigate motor tasks. This work aimed
at describing the mentioned phases in top division swimmers using a
wearable inertial device. Eight élite swimmers were selected so as to
cover all the four swimming styles. The device was positioned on the
lower trunk and a 50m trial at the maximum velocity was executed. The
angular velocity about each axis was post-processed and both time and
kinematic parameters were extracted for each phase, characterizing each
swimming style. Strength points of the approach are: simple description
of the turning kinematics; possibility to extract performance-related parameters;
simplicity of use for the operator; the minimal encumber for
the athlete.
KeY-Words: inertial sensor, angular velocity, turning, kinematics
IntroductIon
Improving athletic performance is one of the main goals of sport biomechanics,
in general, and of swimming research, in particular. This goal
is more difficult to obtain as the elite swimming level increases, thus
chances for improvement are relatively limited. However, closely examining
swimming aspects such as turning, underwater gliding and stroke
resumption phases, and investigating how the relevant timing might be
optimised for improving the overall swimmer performance, may provide
valuable information to the coach. This circumstance was remarked by
Lyttle et al. (1999) who stated that “little changes on the turning action
performance can imply substantial improvements of the final event
time”. The goal of deepening knowledge into the mentioned swimming
phase can be obtained if an accurate way to describe the kinematics of
the abovementioned swimming phases is made available.
Among the mentioned swim phases, turning is one of the most critical
to be analysed. Turning mechanics is usually described using force
plates embedded in the pool wall (Takahashi et al., 1982), to obtain
forces and torques during the push-off phase that follows the rotational
phase. Both flip-turns and open turns are complex movements, usually
investigated - with great difficulty - using video analysis. In fact, due to
the aquatic environment, refraction of the water, as well as actions of
several body segments, which are moving in different movement planes
(Pereira et al., 2008), it is not rare to experience serious difficulties in
tracking the selected skin markers. Moreover, video analysis usually
requires a complex instrumental set-up and a massive computational
effort to obtain the desired biomechanical quantities. For these reasons,
this important phase of a swimming competition is scarcely dealt with
in the scientific literature.
Following the recent trend of implying wearable inertial devices in
human movement analysis, also swimming research recently started to
include such sensors in experimental setups. Ohgi and colleagues (2003)
used a tri-axial accelerometer positioned on the wrist to carry out a
phase segmentation of breaststroke trials. The feasibility of using a 3D
accelerometer mounted on the pelvis was demonstrated by Slawson et
al. (2008), who succeeded in characterising pelvis linear accelerations in
different swimming styles. At this stage of research, the available applications
dealt only with linear accelerations and there is no evidence
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on how the mentioned swimming phases might be characterized using
inertial sensor devices.
This work aimed at quantitatively describing the swim turning, underwater
gliding and stroke resumption phases in top division swimmers
using a wearable inertial device composed of a tri-axial accelerometer
and gyroscope. This objective entailed the definition of appropriate
parameters that could be effectively used by the coach to compare different
swimming techniques towards total-time minimisation.
Methods
Eight elite swimmers (four males and four females) volunteered to participate
in the study. The athletes are part of a top division Italian team
and were selected so as to cover the four swimming styles (freestyle,
backstroke, breaststroke and butterfly).
Participants wore the FreeSense device (Sensorize, Italy) which is
a wearable measurement instrument equipped with a tri-axial accelerometer
and two bi-axial gyroscopes, weighing 93g and fully portable
(size: 8.8 x 5.1 x 2.5 cm), allowing to perform the swimming task in
the most natural fashion (as depicted in Figure 1). Once water-proofed,
the device was incorporated into an elastic (neoprene) belt which was
fastened around the participant’s waist, at the dorsal side of the trunk
at the fourth lumbar vertebra level. In Fig.1 the device positioning is
represented together with the definition of the local reference frame.
The device output were three linear accelerations along the three local
axes and three angular velocities (ω x , ω y and ω z ) about the same axes.
Figure 1: Device positioning on a swimmer low trunk and definition of
the local frame of reference.
Data were collected with a sampling frequency of 100 Hz and were
saved directly on board of the device. A commercial video camera and
a chronometer were also used to video-record each trial and to record
the relevant time duration (t chr ), respectively. All data collection was performed
in a 25m swimming pool.
After familiarisation with the instrument, each participant was
asked to perform a 50m trial at his/her maximum velocity as during a
real competition.
In this study, temporal and kinematic parameters were mainly obtained
using angular velocities about the three axes of the local frame
of reference. For each swimming style, the following parameters were
extracted for the different phases:
� Gliding phase - duration of first and second lap, respectively
t glide25 and t glide50 ;
� Stroke phase - duration of first and second lap, respectively
t stroke25 and t stroke50 ;
� turning phase - duration, t turn ; peaks of angular velocities
- pω x , pω y and pω z ; time interval between the main two
angular velocity peaks, t pp .
The parameter selection procedure is depicted in Figure 2 considering a
randomly selected backstroke trial that includes the execution of a flipturn
technique. The same procedure was also reported in Figure 3 for a
butterfly trial, including an open-turning execution. Even if only angular
velocities were used in this paper, similar considerations can be made to
extract appropriate parameters from measured local accelerations.