Cardiovascular kinetics during moderate-intensity arm and leg exercise: a preliminary report

The phase I cardiovascular response to exercise implies an instantaneous cardiac output (Q̇) increase, due to the effect of sudden vagal withdrawal on heart rate (fH) and of sudden venous return increase, due to muscle pump action, on stroke volume (SV). If the latter is the case, we would expect that, when exercise is performed with small active muscle mass, the cardiovascular responses at exercise are depressed. On 8 healthy young subjects, we measured beat-by-beat fH, SV and Q̇ during arm ergometer and cycle ergometer exercise transitions, from rest to 50W. A double exponential model was applied to the transient phase, and we computed amplitudes and time constants of phase I (A1 and T1). For arm cranking, steady state fH was 65.27, and 102.37.8 bpm, at rest and 50 W exercise, respectively; corresponding SV was 106.116.5, and 112.913.4 mL, so that Q̇ was 6.60.8, and 11.81,4 L/min. For leg cycling, fH was 68.47.8, and 92.76 bpm, SV was 101.814.4, and 117.116 mL, and Q̇ was 6.90.6, and 10.81.2 L/min, at rest and exercise, respectively. For fH, A1 and T1, for arm exercise (18.48.1 bpm and 7.55 s, respectively) were greater (p<.05) than the corresponding values for leg exercise (9.12.2 bpm and 3.22 s, respectively). No significant differences appeared in A1 and T1 for SV and Q̇ between the two exercise types. Exercises with different muscle masses acted on the kinetics of fH, but not on that of SV, and thus essentially on the vagal withdrawal mechanism