Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
Critical Velocity and the Velocity at Maximal Lactate
Steady State in Swimming
espada, M.A., Alves, F.B.
Faculty of Human Kinetics - Technical University of Lisbon, Portugal
The purpose of this study was to compare critical velocity (CV) to the
velocity at maximal lactate steady state (MLSSv) in swimming. Eighteen
well-trained male swimmers performed a maximal 400 m front
crawl to estimate maximal aerobic velocity (V400) and two to three 30min
constant velocity swims in order to directly determine MLSSv. CV,
estimated from 200 m and 400 m maximal swims, was highly correlated
to MLSSv (r = 0.94 and p < 0.01) and V400 (r = 0.95 and p < 0.01).
However, CV was significantly faster than MLSSv, confirming that this
parameter does not represent a steady metabolic rate in long distance
swimming. Nevertheless, CV still seems to be a useful tool for aerobic
conditioning evaluation, due to the simplicity of its determination.
Keywords: exercise; swimming; Maximal lactate steady state; critical
Velocity
IntroductIon
The maximal tolerated duration of a constant-load high-intensity swimming
bout has been shown to be a hyperbolic function of the power (or
velocity) of the exercise (Wakayoshi et al., 1992), similarly to other forms
of human locomotion (for a review, see Morton, 2006). According to this
model, the velocity-duration relationship has an asymptote on the velocity
axis, termed the critical velocity (CV) that has been considered as a valid
fatigue threshold and a marker of the transition between the heavy and
the severe intensity domains (Poole et al., 1988), since it has also been
shown to be a close correlate of the highest metabolic rate associated with
pulmonary oxygen uptake (VO2), acid-base status and blood lactate concentration
that can be sustained for a certain time at a constant level.
The power output or the velocity corresponding to the CV has also
been considered as expressing the maximal lactate steady state (MLSS)
(discussed in Pringle and Jones, 2002), although this equivalence has not
been confirmed in many experimental studies, especially in swimming
(Dekerle et al., 2005b).
The MLSS corresponds to the highest workload or velocity that
can be maintained over time without a continuous blood lactate accumulation
(Billat et al., 2003) and its measurement demands several
subsequent constant load tests that have to be performed with different
workloads on different days, until it is possible the determine an
individual workload intensity above which the rate of lactate production
exceeds lactate clearance. However, the number of studies directly
assessing this parameter is surprising limited in swimming, due surely
to the time-consuming and cumbersome procedures required, when
compared to the performance of an incremental graded test for lactate
threshold determination.
The purpose of this study was to compare critical velocity (CV), estimated
by a two-component model, using 200 m and 400 m maximal
swims, to the velocity at maximal lactate steady state (MLSSv) in welltrained
swimmers.
Methods
Eighteen male national and international level competitive swimmers
volunteered for this study. They were aged 17.1 ± 2.8 years, with a height
of 177.6 ± 5.7 cm, a body mass of 65.8 ± 9.1 kg and a% body fat of 10.1
± 2.4%. Subjects have trained regularly for at least 6 years and took no
drugs or medicaments during the study. Most of the swimmers were
familiar with swimming pool exercise testing procedures. They were informed
of the nature of the experiments and gave written consent to
participate in this study.
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Athletes were instructed to avoid exhausting exercise the day before
the tests and to retain their normal nutritional habits. A standardize
warm-up of 600 m was completed in every testing session. A maximal
400 m front crawl was performed in order to use the average velocity
between 50 m and the 350 m (V400) as an estimate of the maximal
aerobic velocity (Lavoie and Leone, 1988).
CV was calculated from the slope of the regression analysis between
the averaged velocity of the 400 m trial previously referred to and a 200
m front crawl maximal trial performed for this purpose.
In a first round of testing, six subjects performed 30-min constant
velocity swims at 75 and 80% of V400. The physiological impact of these
bouts was clearly low so it was decided to continue the study using higher
intensities. All eighteen subjects performed, then, in random order,
30-min at constant velocity at 85, 90 and 95% of V400. Split times for
each 50 m were determined and used by two investigators positioned at
7.5 m and 17.5 m of the pool to control the athletes swimming pace. The
swimmer was asked to maintain the pre-established swim pace as long
as possible. The test was interrupted when the swimmer could no longer
match the required swimming velocity. Each subject was stopped every
400 m (30 to 45 seconds) for blood sample collection and record of the
rate of perceived exertion (MLSSrpe) (6-20 scale).
Maximal lactate steady state blood lactate concentration (MLSSc)
was defined as the highest blood lactate concentration that increased by
no more than 1 mmol.L-1 during the final 20-min of a 30-min constant
workload (Beneke, 2003; Baron et al., 2005). When this criterion was
not accomplished, the test was stopped. MLSSv was the intensity associated
with MLSS. Tests were conducted at similar times on separate
days (1 day of total rest between tests) at a 25 m pool with the water
temperature at 28.2º C. No polyurethane suits were used.
Stroke rate (SR) was measured from three stroke cycles taken in the
middle of the pool for every 50 m and stroke length (SL) calculated.
Blood lactate concentrations were analyzed using a Lactate Pro LT device
(Arkray, Kyoto, Japan). A Polar Sport Tester (S410) recorded the
heart rate frequency every 5 seconds during all tests.
Paired-samples t-test was used to compare CV and MLSSv. Pearson’s
linear coefficient was used to test correlations. Statistical significance
was accepted at p < 0.05.
results
Mean and standard deviation of V400, MLSSv, CV and MLSSc are
shown in Table 1. MLSSv corresponded to 89.7 ± 1.2% and CV to 94.0
± 1.5% of V400. Only one swimmer achieved MLSS at 85% of V400. At
95% of V400 no one achieved stabilization in blood lactate concentration,
confirming that work bouts at this intensity were performed above
MLSS. Extreme values of 2.6 and 7.8 mmol.L -1 were found associated
to MLSSv. Mean SR at MLSSv was 32.9 ± 4.3 (cycles.min -1 ) and mean
SL was 2.49 ± 0.34 (m.cycle -1 ). Mean heart rate frequency at MLSSv
(MLSS HR ) was 177.0 ± 6.9 beats.min -1 and the mean MLSSrpe expressed
by the swimmers was 13.39 ± 1.33.
Table 1. Mean and standard deviation of V400, MLSSv, CV and
MLSSc.
N=18
V400
(m·s-1)
MLSSv
(m·s-1)
CV
(m·s-1)
MLSSc
(mmol·L-1)
1.49 ± 0.07 1.34 ± 0.06 1.40 ± 0.08 5.3 ± 1.6
CV was significantly faster than MLSSv (p < 0.01) and both expressed
velocities significantly different from V400 (p < 0.01).