Biomechanics and Medicine in Swimming XI

BiomechanicsandmedicineinswimmingXi
results
Table 1 shows changes in heart rate during arm cranking exercise during
gradually increasing and decreasing workload. During the calibration
test (20, 60 and 40%VO 2peak ), heart rate in the W-condition was significantly
lower than on the L-condition (p< 0.05, respectively). Heart
rate in the W-condition was significantly lower than in the L-condition
during triangular exercise (ANOVA; p< 0.05). Figure 1 shows the mean
response time of heart rate (phase lags) to (a) the top of the work rate
and (b) the bottom of the work rate in both conditions. The phase lags to
the top the work rate in the W-condition was significantly shorter than
on the L-condition (p< 0.05). Furthermore, phase lags to the bottom of
the work rate in the W-condition were significantly shorter than on the
L-condition (p< 0.05). The amplitude (difference between maximal and
minimal values) was no significant different in each condition (Figure
2). The ln HF in the W-condition was significantly higher than on the
L-condition during triangular exercise (p< 0.05) (Figure 3). Figure 4
shows T30 in both conditions. T30 in the W-condition was significantly
lower than on the L-condition (p< 0.05). During 1 minute after exercise,
the ln HF in the W-condition was significantly higher than on the Lcondition
(p< 0.05) (figure 5).
dIscussIon
The results of the present study are as follows; 1) the heart rate phase
response to gradually increasing and decreasing arm cranking exercise
was shorter in the water than on land, but no differences in heart rate
amplitude were observed, 2) the reactivation in cardiac parasympathetic
tone after arm cranking exercise was greater in water than on land. These
results may expand our understanding of heart rate responses modulated
by the autonomic nervous system to gradually increasing and decreasing
exercise in water.
In the present study, heart rate in water exercise was significantly
reduced as compared to land exercise. This fact may depend on immersion-induced
bradycardia, and be caused by increased venous return due
to water pressure. It is thought that the heart rate response to exercise
under the anaerobic threshold level is mainly due to the attenuation
of cardiac parasympathetic nervous activity (Xenakis et al., 1975). In
the present study, since peak heart rate values in W-condition and Lcondition
were 113 bpm and 131 bpm, respectively, it is speculated that
exercise intensity was below the anaerobic threshold level. Thereby, heart
rate response to gradually increasing and decreasing exercise can be also
explained by activation of the cardiac parasympathetic nervous system.
Indeed, the ln HF (an index of cardiac parasympathetic nerve activity)
was higher in the W-condition as compared with on the L-condition,
supporting our speculation.
Sone et al. (1997) have suggested that the contribution of the withdrawal
of cardiac parasympathetic activity to the increase in heart rate
with increasing exercise intensity was greater at lower heart rate, and
that the cardiac parasympathetic system was more activated during
heart rate decreases than during heart rate increases at the same heart
rate. The density of the high frequency spectrum of heart rate during
light work rates is higher than during heavier workloads (Nabekura et
al., 2007). In this present study, the phase lags at the top and bottom
of the work rate in the W-condition were significantly shorter than in
the L-condition, suggesting that immersion-induced bradycardia might
have coursed an increase in the phase lags at the top and bottom of the
work rate.
In the present study, T30 in W-condition was greater than in Lcondition
suggesting that the cardiac parasympathetic nervous system
rapidly reactivates in the water. It has been reported that an increase
of central blood volume due to water immersion might accelerate the
recovery process of cardio-respiratory system (Miyamoto et al., 2001).
Given this, the increased venous return shown in the present investigation
may contribute to the rapid reactivation of the cardiac parasympathetic
nervous system. In fact, the ln HF in the W-condition was
significantly higher than in the L-condition 1 minute after exercise.
210
conclusIon
The results of the present study suggest that the activation of the cardiac
parasympathetic nervous system caused by water immersion may produce
an attenuation of heart rate time response to gradually increasing
and decreasing exercise. This attenuation of the phase lags during exercise
in the water may contribute to a reactivation of cardiac parasympathetic
nervous system after exercise. Thus, exercise and recovery in water
may enhance the stability of the autonomic nervous activity not only
during exercise but also after exercise.
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