The effect of aerobic fitness level on heart rate and electrocardiographic T-wave amplitude reactivity and conditioning

Aerobic fitness level (AFL) is known to produce cardiovascular (CV) adaptations to physical exercise or stressors. The experiments reported in this thesis were conducted to evaluate the general hypothesis that AFL will also affect psychologically-elicited CV responses. The principal reason why this area of study is of interest in that it has been hypothesized that increasing AFL may be of clinical importance inasmuch as fit individuals are believed to be at lower risk for developing CV disease. One possible mechanism of this putative effect is that increasing AFL reduces excessive CV reactivity to psychological stressors, while also enhancing opposing (protective) parasympathetic (PNS) influences. Increasing AFL may also mitigate psychologically conditioned CV responding, and thus be useful in clinical settings where the intention is to train people to learn how to reduce psychologically-elicited CV reactivity. Both experiments measured phasic (as opposed to tonic) ECG T-wave amplitude (TWA) (a measure of cardiac sympathetic activity) and HR (a measure of combined sympathetic and parasympathetic activity) in order to determine the relative influence of these two branches of the autonomic nervous system.;Experiment 1 used an established associative learning preparation using head down body tilt as the unconditional stimulus and found that higher levels of AFL were associated with a smaller HR UR, a decelerative HR CR that is maintained longer, and a larger magnitude HR extinction response as well as some topographical differences in conditioning and extinction responses. TWA did not show any evidence of conditioning in that there was an attenuative (i.e. SNS) response during conditioning (the opposite direction of the augmentative UR) which was interpreted as resulting from anticipatory anxiety responses. Consequently, caution was recommended in using this preparation to teach HR deceleration as the evident SNS arousal may counteract any beneficial PNS effects.;Experiment 2 found that TWA but not HR responses distinguished AFL groups during a hard but not easy iterative subtraction task, indicating that enhanced AFL is associated with a reduced SNS arousal response to psychological stress. This is consistent with the purported beneficial effects of higher AFL and also suggested that these effects are not lost as stress magnitude increases. Due to the short time frame in which the CV responses occurred in both experiments, it was concluded that the responses were neurally, as opposed to hormonally mediated. Finally, as both experiments noted AFL differences in HR and TWA responses to these two preparations, it was concluded that AFL is an important, if not critical explanatory variable to be taken into account in future studies of psychologically-elicited cardiovascular responding.