An imbalance of autonomic activity to the heart is a risk factor for adverse cardiovascular (CV) events that are potentially fatal. It is established that exercise training can alter autonomic activity thus be a treatment for those with CV disease. However, the alteration of the autonomic control of the CV system after an exercise bout has not been extensively researched. The effects of 3 intensities of supine exercise on cardiac autonomic control during supine recovery were studied using simple non-invasive time domain measures.

Following University of Essex ethics committee approval, in agreement with the Declaration of Helsinki, fourteen moderately fit subjects, (7 female), mean age 21.4 ± 3.9 years completed 3 intensities of supine cycling (Low Work (LW), 2 mM Blood Lactate (La); Moderate Work (MW), 3 mM La; and High Work (HW), 4 mM La) on 3 separate occasions. The intensities were calculated from La data obtained in an initial fitness test. Non-invasive ECG and blood pressure (BP) tracings were recorded continuously for 5 min of paced breathing, before (baseline) and post exercise (P) at P5, P15, P30, P45 and P65 min. The subjects remained in the supine position throughout all procedures. Heart rate variability was reported as the standard deviation of successive differences (SDSD) which gives an indication of vagal activity (Task Force, 1996).

A two-way ANOVA revealed that there were both time and intensity effects (P ▓le│ 0.05) for R-R interval and SDSD. Post-hoc t tests with Bonferroni correction, revealed that R-R interval decreased significantly from baseline at P5 for all intensities -93.2 ms ± 91.6-179.8 ms ± 98.1, -237.3 ms ± 166.7 LW, MW and HW respectively, mean ± SD) and also at P15 for MW and HW intensities (p ▓le│ 0.01). With LW R-R interval increased above baseline at P30 and the increase was graded with time. At all intensities SDSD decreased significantly at P5, at MW for P15, and at HW for P15 and P30 (p ▓le│ 0.01), remaining below baseline for all time points except for P65 for MW where vagal activity rose above baseline in 9/14 subjects. When compared to HW this did not reach significance (P = 0.062). Diastolic BP was below baseline for all readings. Clearly there is a change in autonomic activity following an exercise bout. High intensity reveals a reduced vagal activity for up to 30 min during recovery. However, moderate exercise results in a small post-exercise increase in vagal activity after 1 h potentially providing some cardioprotection. This study may go some way to alluding to the cardiac benefits following moderate intensity exercise.