Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
swim were excluded from the study. All subjects were non-smokers,
non-obese, and free of overt cardiovascular or other chronic diseases
as assessed by medical history. None of the subjects were taking cardiovascular-acting
medications. The Human Research Committee reviewed
and approved all procedures, and written informed consent was
obtained from all subjects.
Procedures. Subjects came into the laboratory after 12 hours fasting.
Each subject performed all 3 experimental sessions (time control,
running exercise, and swimming exercise) randomized and separated by
at least one week. During the time control sessions, subjects sat in a
temperature-controlled laboratory room. The exercise protocol for the
swimming and running sessions consisted of interval training exercises
at an intensity of ~75% of heart rate reserve determined from the graded
exercise test. The target heart rate during swimming was adjusted on
the basis of the observation that maximal heart rate during swimming
is ~10-13 bpm lower than that during running (Magel et al., 1975). The
intervals were five 10-minute bouts of exercise with 1 minute of rest between
each bout (for a total for 54 minutes). During pilot studies, many
subjects expressed difficulty in swimming continuously for a prolonged
period of time. This necessitated the implementation of the interval
exercise format for the present study. All subjects wore a waterproof
heart rate monitor to maintain the desired intensity of exercise as well
as to document exercise heart rate. Subjects used the freestyle technique
in an indoor swimming pool. During the running session, subjects ran
on a treadmill in a temperature-controlled room. During both exercise
sessions, subjects consumed a standard amount of water. We made an
attempt to include a sham control session, in which subjects floated in
the indoor swimming pool using a floatation device. But this session
was abandoned due to excessive heat loss and the resultant reduction in
body temperature.
Blood pressure was measured before each session noninvasively
three times in the laboratory by the arm and ankle cuff techniques (Omron
VP-2000, Bannockburn, IL) after the subject had been quietly lying
in a supine position for at least 10-15 minutes. In order to eliminate
investigator bias, arterial blood pressure was measured automatically
with modified oscillometric pressure sensors incorporated in extremity
cuffs. The validity and reliability of measuring ankle blood pressure
using this automated device have previously been reported by our laboratory
(Cortez-Cooper et al., 2003). The blood pressure measurements
were repeated after exercise at 15-minute intervals to 60 minutes with
the subject lying in the supine position. Brachial-ankle pulse wave velocity,
a measure of arterial stiffness, was measured using the automatic
device (Omron VP-2000). A blood sample was taken before and after
the exercise protocols for later enzymatic analyses of plasma lipid and
lipoprotein concentrations.
In order to provide insight into the physiological mechanisms underlying
the hypothesized reductions in blood pressure, an echocardiogram
using the ultrasound machine (Philips iE33, Bothel, WA) was
performed to assess stroke volume and cardiac output. Stroke volume
was calculated from the product of the cross-sectional area of the aortic
orifice (π*(aortic diameter/2)²) and the mean velocity time integral. Cardiac
output was calculated from the product of stroke volume and heart
rate recorded during the echocardiogram.
Based on the previous finding that the attenuation of heat loss was
associated with the magnitude of postexercise hypotension (Franklin
et al., 1993), core body temperature was measured during exercise
through the use of the CorTemp disposable temperature sensor and the
data recorder (HQ, Palmeto, FL). Body fat percentage was measured
non-invasively by dual energy X-ray absorptiometry (DEXA). Maximal
oxygen consumption was measured using a metabolic cart during a
modified-Bruce protocol.
Statistical Analyses. ANOVA and MANOVA with repeated measures
were used for statistical analyses, and Newman-Keuls post-hoc
tests were used to identify significant differences. All data are expressed
as mean±SEM.
382
results
Baseline blood pressure values were not different between the time control,
running and swimming sessions. Brachial blood pressure did not
change significantly after all 3 sessions. Both running and swimming
produced significant decreases in ankle mean blood pressure compared
with the time control (P