Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
dIscussIon
The results showed that a higher foot position of BSFE produces a
higher horizontal wall approximation of the CM in the starting position.
In the same way, BSFE presented a higher HOT. Differences may
certainly be attributed to the foot segment constraint in the starting
position, which seems to imply a more complex lower and upper limb
movement sequence to release handgrip contact at BSFE. Previous values
published without these constraints presented values between 0.42
and 0.52 s (Kruger et al., 2006). Differences may be attributed to the
higher competitive level of the swimmers of the referred study. In accordance
with the analysing movements reported in previous studies
such as vertical, horizontal, drop or rebound jumps (Bobbert and Van
IngenSchenau, 1988; Rodacki and Fowler, 2001), during the backstroke
start (both variants), the CM is initially accelerated by the extension of
the hip joint, although BSFE registered lower time to reach joint peak
angular velocity. We can speculate that the more tucked position of the
BSFE may imply a higher pre-stretching of the subsequent propulsive
muscles. In this situation, the force production capacity of these muscles
can be maximized if the time between the stretch and shortening actions,
and their loading characteristics respect particular limits. This has
yet, however, to be verified.
At BSFI condition a higher knee joint angular displacement was
observed, which seems to contribute to a higher DxCM-HO.
The present results confirmed the hypothesis that the BSFI start
condition might result in a different resultant vector angle with a more
pronounced vertical component as noted by the higher DyCM-TO.
As Counsilman (1971) previously stated, few swimmers can perform
successfully the BSFE due to the longer training required to increase
the vertical component at the instant of take-off. Temporal analysis also
revealed that BSFE obtained higher TOT. It may be due to the specific
placement of the feet above water level, which seems to imply that the
body mass was accelerated through a greater distance after the start signal
until the take-off, which explains the higher impulse.
The analysis of segmental co-ordination indicated that BSFI showed
greater angular velocity of the knee joint at the beginning of the take-off
phase; while, the BSFE condition presented a greater time of the knee
joint maximal extension. Additionally, BSFI imposed a lower time to
reach ankle joint maximal extension, at the end of TO phase.
The paired sample T-test revealed higher FLT and X1 for the BSFI.
In a previous study, Miller et al. (1984) found values of 0.11 ± 0.06 s
(100 m events) and 0.11 ± 0.05 s (200 m events) for FLT during the
backstroke start without foot position specified (but assumed to be
BSFI), values that suggested being lower than those found in our study
for both variants. Additionally, these authors found similar values to
ours for X1, 2.77 ± 0.12 m (100 m) and 2.78 ± 0.12 m (200 m). Nevertheless,
differences may be attributed to the end point measurement
of the flight analysis, which was established by Miller et al. (1984) as
distance to the hand contact with the water instead of the distance to
the CM at the hand touch.
conclusIons
As a performance parameter, the total time spent during the start was
lower for BSFI than BSFE, allowing the conclusion that the first, being
faster than the second should be preferred for competitive use. This
observed superiority of the BSFI may be at least partially justified by
the higher flight time (FLT) and reach of the centre of mass (X1). These
findings seem to confirm the hypothesis that a lower foot position can
determine the CM water reach by constraining the orientation of the resultant
wall reaction vector. Inter-segmental coordinative analysis of the
lower limbs showed that the relative time at which the knee and ankle
peak joint angular velocity occurs might be an important variable to
characterize the co-ordination pattern, and to explain the performance
capacity of the BSFI. It is recommended that coaches begin monitoring
the backstroke start variation strategies to improve inter-segmental
coordination, which can be the determining factor of success of the start.
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