Biomechanics and Medicine in Swimming XI

Effect of Stroke Drills on Intra-cycle Hip Velocity in
Front Crawl
Arellano r., domínguez-castells r., Perez-Infantes e., sánchez
e.
University of Granada, Granada, Spain
Competitive strokes have been widely analysed, while kinematics of coordinative
drills, used to train or teach swimming techniques, have been
scarcely studied. The aim of this work is to evaluate the differences in the
intra-cycle hip velocity among freestyle swimming drills. Thirteen national
and regional level swimmers performed five 25-m freestyle trials,
using different stroke drills in a random order. The propulsive phase of
each stroke was analysed. Mean and peak (one or two) velocity were obtained,
and their corresponding phase percentage. The first peak recorded
corresponded to the inward movement, which coincided with the final
propulsion or downward movement of the opposite arm. The second
one corresponded to the final backward-upward movement, which, is in
most cases, the highest value of velocity while the non-propulsive arm
is extended horizontally in front of the shoulder. The freestyle stroke
drills applied to teach or train different types of inter-limb coordination
reduce the mean and peak values of intra-cycle hip velocity while the
percentage location of peak hip-velocity value during the underwater
stroke phase is similar without significant statistical differences.
Key words: swimming kinematics, swimming technique.
IntroductIon
Current teaching and competitive swimming programs are composed
of the correct combination of skill acquisition and conditioning exercises.
Many swimming books or papers describe or classify these swimming
exercises proposing guidelines to use them properly or in a skill
assessment context (Goldsmith et a. 2007; Guzman, 1998; Laughlin &
Delves, 1996; Maglischo, 2003; Sweetenham & Atkinson, 2003). The
classifications used to include coordinative drills: arm-arm coordination,
arm-breathing coordination and arm-leg coordination. Lately, armarm
coordination drills have become very popular applied to develop
the crawl-stroke swimming technique in master swimming, triathlon
and competitive swimming while they were and are broadly used in the
advanced learning stages of technique development in ‘learn to swim’
programs (Figueiredo et al., 2009; Seifert & Chollet, 2009).
While hip or centre of mass intra-cycle velocity has been studied
in the current competitive strokes using observational or biomechanical
methods (Barbosa, et al., 2005; Figueiredo, et al., 2009; Psycharakis et al.
2009), this has not been the case with the stroke drills applied to teach or
train swimming technique. Stroke coordination, bi-dimensional or tridimensional
kinematics, intra-cycle velocity, body-trunk rotation, stroke
frequency and stroke length have been some subjects of study frequently
applied to formal strokes but missed out in these drills. A first attempt
was performed to analyse the differences in body rotation and 3D hand
swimming path between freestyle swimming and one arm crawl stroke
drills (López et al. 2002). Less body rotation and hand depth were found
during the practice of formal one-arm and catch-up crawl stroke, while
a modified one-arm stroke drill obtained similar values to that recorded
during no breathing freestyle swimming.
The purpose of this study is to reveal the differences in intra-cycle
hip velocity between formal front crawl and four other front crawl
swimming coordination drills.
Methods
Thirteen national and regional level swimmers (five males and eight
females, aged 19.58±2.23) participated in this study as volunteers. All
of them had trained in swimming for 6-8 years. Every participant had
chaPter2.Biomechanics
previous experience of the exercises. The protocol was fully explained
to them and they provided written consent to participate in the study,
which was approved by the university ethics committee. This investigation
was performed in September, at the beginning of the competitive
season. The characteristics of the participants are presented in Table 1.
Table 1. Anthropometric characteristics of participants in the study
(n=13).
Body mass
(kg)
Height
(cm)
Max. body length
(cm)
Arm span
(cm)
Mean (SD) 67.4 (10.5) 171 (7) 228 (11) 181 (11)
After a standard warm-up, swimmers randomly performed five 25-m freestyle
trials, using a different stroke drill and no-breathing freestyle. They
always started in the water and were instructed to swim as fast as possible,
keeping to the stroke drill technique. All trials were performed during a
specific testing session. Each participant performed every trial with a rest
interval of more than 5 minutes (see Table 2 for stroke drill definitions).
Table 2. Stroke drills applied in the study.
A No-breathing formal freestyle swimming (reference technique)
B Crawl catch-up stroke, kicking some seconds after each stroke
C One arm front crawl with the resting arm extended in front,
breathing on the arm-moving side
D One arm front crawl with the resting arm close to the body,
breathing on the no-moving side
E Controlled two-arm freestyle, swimmer perform arm-pull and
arm-recovery simultaneously finishing each half cycle with
one arm resting close to the body and the second one resting
extended in front, kicking some seconds after each stroke.
During each trial the swimmers` time and intra-cycle velocity were recorded
synchronized with underwater video recording. With this information,
peak and mean velocity for each stroke (and its phases) were
obtained.
The behaviour of the swimmers during the experiment was recorded
with two underwater cameras, which were attached by a metal device to
the lateral edge of the pool. Both cameras were placed 50cm below the
water surface and were positioned 10 and 15m from the starting wall of
the pool, respectively. The swimmers swam in lane 3, so that the cameras
were able to record at least 15m of their performance from a lateral view.
An electronic timer connected to a touch pad was used to measure
the swimmer’s times. Velocity was measured by a linear encoder,
attached to the swimmers` waist using a belt. The associated software
enabled us to receive the data and videos in the computer in real time,
ready to be processed through two USB 2.0 ports.
Velocity data were sampled at 200Hz and the video signal synchronized
at 50 Hz. After selecting the stroke-phase sections of the videos,
and before further processing, data were filtered using a Butterworth
filter, with a cut-off frequency of 6 Hz.
For every participant, three strokes were selected and represented, in
order to compare the velocity variations among them. A complete underwater
stroke phase was considered from the beginning of the entry of
the hand into the water until it was completely out. All the underwater
stroke durations (100%) were normalized to a percentage scale, making
it easier to compare strokes of different duration.
A statistical analysis was performed using Microsoft Excel 2007 and
Statistica/Mac Plus 5.1. Then, the raw data was checked for normal distribution
characteristics (p=0.96 for velocity). And finally, an analysis of
variance with repeated measures was applied to determine variability between
exercises using Scheffé post-hoc tests for intra-group comparisons.
45