Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
(Dopsaj et al., 2000; Dopsaj et al., 2003). The 2000 N load cell had 4
attached strain gauges and an accuracy of 30 g. One of its extremities
was fixed to a special designed support, attached to the starting platform
or to the pool border, while the other was connected to an inextensible
cable, in which swimmer was tethered. Mechanical deformations in the
load cell, generated by swimmer’s efforts, were recognized by an A/D
interface, which converted the analogue voltage in a digital signal. In
sequence, the force values were obtained through the calibration straight
line. The minor acquisition frequency used was 200 Hz. Raw data was
low-pass filtered with a cut-off frequency of 8 Hz. The system was calibrated
using increments of 5 Kg until the maximum weight of 50 Kg.
It was checked before every testing session using a weight of 10 Kg.
As warm-up, swimmers performed a 10-minute active stretch and 15
minutes of free swimming. After, a specific procedure described earlier
(Dopsaj et al., 2000) was conducted in order to get swimmers familiarized
with the equipment.
Figure 1. Example of a typical force-time curve during the 30-s tethered
test
As test protocol, the swimmers accomplished a 30-s self-chosen cycle
frequency maximal breaststroke effort in tethered swimming, whereas
the beginning and the end were signalized by a whistle. In order to
minimize the effects of swimming intensities transition, the first second
swam maximally was discarded. A typical f-t curve obtained in the test
protocol is shown in Figure 1. Main points of force-time curve in a
complete were marked, as shown in Figure 2.
Figure 2. Visualization of the determinant points used in the force-time
curve’s analysis. F peak = Peak force; I mp F = impulse of force represented
by the curve’s area; F min 1 = beginning of the stroke; F min 2 = end of the
stroke; ∆t = time variation; ∆F = force variation.
Through these points, five independent variables were extracted, as follows:
1) Peak force (F peak ), represented by the maximum force value
found in a complete stroke, expressed in N. 2) Average Force (F avg ),
represented by the average of all force values found in a complete stroke,
i.e., between minimum force 1 (F min 1) and 2 (F min 2), expressed in N.
3) Impulse Force (I mp F), represented by force-time curve’s area found
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in a complete stroke, i.e., between F min 1 and F min 2, expressed in Ns.
Integration was made using the trapeze method. 4 ) Stroke Explosiveness
(RFD), represented by the rate of force development (RFD) in a
complete stroke, expressed in N/s. RFD was calculated using the equation:
RFD = (∆F / ∆t) x 1000, where ∆F = F pea k - F min 1, expressed in N
and ∆t = Time value when F peak was reached - time value found in F min 1,
expressed in ms. 5) Full Stroke Duration (DUR), represented by the difference
between time values found in F min 2 and F min 1, expressed in ms.
The variables’ value retained for analysis was the average of all movement
cycles executed during the test protocol. Time in 50m breaststroke,
in different moments of the periodisation, obtained in competitive conditions,
two weeks before or after the tethered test, was converted in
average swimming velocity (VEL 50m ) to be used as dependent variable.
Mean ( Χ ), standard deviation (SD), minimum (Min), maximum
(Max) and coefficient of variation (CV) were used as basic descriptive
statistics. The relationship model was established through Multiple Regression
Analysis (MRA) with the Backward Elimination Method. An
equation of the regression model was generated using the variables being
statistically significant and most accurately described the criterion.
All statistical analysis was conducted using SPSS for Windows (Version
16.0; SPSS, Inc., Chicago, IL). The significance level was set at 95%
(p