Biomechanics and Medicine in Swimming XI

front-crawl stroke. This mechanical arm has a restrict movement, since the
medio-lateral direction was not considered as there was little or no hand
motion in this direction due to the mechanical constraints.
Therefore, the purposes of this study, which involved synchronized
swimmers and swimmers in a real situations, were (i) to verify the agreement
between the attack angles calculated using different combinations
of vectors, described in the literature and proposed by this study, to
define the plane of the hand and (ii) to verify the variation in vector
length of the methods found to be agreement in order to establish which
method is most recommended when estimating the attack angle during
sculling motion performed in a stationary vertical position (head above
the water’s surface).
Methods
The sample consisted of 16 participants (10 female synchronized swimmers
and 6 female swimmers; age 13.7±2.3 years; height 1.56±0.11 m;
weight 51.7±4.2 kg). In order to participate in the study, swimmers were
required to have at least 6 months of exercise including sculling actions
during their training program. All the participants or their guardians,
in those cases where the participants were not legally capable, signed
informed consent forms. The Ethics Committee of the university where
the study was undertaken approved this study.
Recording took place in an indoor 25 m swimming pool. Two digital
video cameras ( JVC GR-DVL 9800) were positioned behind two glass
windows in the side of the pool beneath the water level. The distance
between these windows was 11.2 m. The sampling frequency of the video
cameras was 50 fields-per-second. The Digital Video for Windows
(Dvideow) software was used to track the markers. Each camera was
connected to a computer, which was linked to an intranet in order to
start the recording at same time for both cameras. The spatial resolution
of the video system used was 1024 x 768 pixels.
A cube (0.80 m x 0.80 m x 0.80 m) was used as a control object with
12 control object points. The distance between the control object and a
point equidistant between the windows was 6.7 m.
Landmarks were placed on the distal end of the third finger (1),
metacarpophalangeal joints of the second (2) and fifth fingers (3), on
the centre of the wrist joint (4) and the elbow (5) of the right limb, thus,
the movement was considered symmetrical. After this, each participant
performed a warm-up with sculling actions and familiarization with the
experimental conditions with the right side of the body directed toward
the equidistant point between the windows, in the position to be used
during recording. All tasks were performed at the calibrated volume.
Each participant was asked to perform 20 seconds of sculling actions,
maintaining a stationary vertical position with the head above the
water surface and with the water at chin level in order to maintain all
landmarks submerged during recording. This length of time was chosen
in order to allow the participants sufficient time to stabilize the movement,
while maintenance of the chosen position was the only criterion
for standardization of sculling movement performance. Based on a qualitative
criterion of stability, three cycles of sculling motion were chosen
for digitalization.
Most of the studies found in the literature analyzed only one cycle
of sculling motion. By contrast, in the present study, it was decided to
analyze three consecutive cycles of sculling, chosen when the movement
seemed to be stable. An experienced digitizer, using Dvideow software,
manually digitized these. By analyzing three cycles, the influence of
any random error that may occur during the digitalizing procedure was
minimized.
Three-dimensional coordinates were obtained using a direct linear
transformation method. The accuracy of the measurements was calculated
and was equal to 0.004 m. The coefficient of variation was 0.5% when
the distance between two rigidly linked markers at the known distance
was reconstructed. All three-dimensional coordinates were smoothed
using a seventh order low-pass Butterworth digital filter with cut-offs
around 4 and 5 Hz, according to Residual Analysis (Winter, 2005).
chaPter2.Biomechanics
The calculations of the attack angle were made using the Matlab
software (version 7.1). Figures 1 and 2 show the vectors that were used
to reconstruct the hand orientation and Table 1 shows the different
combinations of these vectors described by Schleihauf (1979), Berger et
al. (1995), Lauder et al. (2001) (Lauder 1 – 5) and the new combination
(NC) proposed by this study.
Figure 1. Vectors on hand that can be used to reconstruct the hand orientation.
Figure 2. Vectors on hand and forearm that may be used to reconstruct
the hand orientation.
The angle of attack was defined as the angle between the plane of the
hand and the velocity vector of the hand (Payton and Bartlett, 1995).
The plane of the hand was defined by the cross-product of vectors 1 and
2, which define a vector perpendicular (VP) to the plane of the hand. The
velocity vector (v) was calculated using the raw coordinate data from
the midpoint between the distal end of the second and fifth finger. The
attack angle was calculated as 90° minus the angle between v and VP.
Once the angles were calculated, (1) the time of each cycle of sculling
motion was normalized (from 1 to 100%), (2) each participant was
represented by the average of the three cycles and (3) the average cycle
of all participants was calculated and used to verify the agreement between
the attack angles calculated using different combinations of vectors
described in the literature and proposed by this study. The degree
of agreement between the attack angles calculated using the different
combinations of vectors used to define the plane of hand was established
based on graphical techniques from Bland and Altman (1986),
in which the data were presented graphically by plotting the difference
between each method versus their average obtained by step 3. Thus one
hundred points were plotted for each normalized time. The mean differences
(bias) and standard deviation (SD) of the differences between the
values obtained with the methods, expressed in degree, were calculated.
The limits of agreement were set at bias ± 2SD. All statistical procedures
were performed using Matlab software. Thus, the analysis of the results
comprised of two steps: (1) the agreement between the attack angles
calculated was verified using different combinations of vectors, which
were used to define the plane of the hand; (2) the means and standard
deviations were calculated of these vector lengths in the methods that
were found to agree. Therefore, from these values, the variation in vector
length of the methods that were found to agree was estimated.
Table 1. Different combinations of vectors for the reconstruction of the
plane of the hand (see Figures 1 and 2 for location).
Combinations Vector 1 Vector 2
Schleihauf VE VC
Berger et al. VB VE
Lauder 1 VD VC
Lauder 2 VD VI
Lauder 3 VH VD
Lauder 4 VD VG
Lauder 5 VF VD
NC VH VI
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