Differences in autonomic modulation of heart rate during arm and leg exercise

Heart rate (HR) is higher during dynamic arm exercise than during leg exerc ise at equal oxygen consumption levels, but the physiological background fo r this difference is not completely understood. The vagally mediated beat-t o-beat R-R interval fluctuation decreases until the level of approximate to 50% of maximal oxygen consumption during an incremental bicycle exercise, but the vagal responses to arm exercise are not well known. Changes in auto nomic modulation of HR were compared during arm and leg exercise by measuri ng beat-to-beat R-R interval variability from a Poincare plot normalized fo r the average R-R interval (SD1(n)), a measure of vagal activity, in 14 hea lthy male subjects (age 20 +/- 4 years) who performed graded bicycle and ar m cranking tests until exhaustion. Seven of the subjects also performed the dynamic arm and leg tests after P-adrenergic blockade (propranolol 0.2 mg kg(-1) i.v.). More rapid reduction occurred in SD1(n) during the low-intens ity level of dynamic arm exercise than during dynamic leg exercise without beta-blockade (e.g. 11 +/- 6 vs. 20 +/- 10 at the oxygen consumption level of 1.21 min(-1); P<0.001) and with beta-blockade (e.g. 13 +/- 4 vs. 25 +/- 10 at the level of 1.01 min(-1); P<0.05), and the mean HR was significantly higher during submaximal arm work than during leg work in both cases (e.g. during beta-blockade 81 +/- 12 vs. 74 +/- 6 beats min(-1) at the level of 1.01 min(-1); P<0.05). These data show that dynamic arm exercise results in more rapid withdrawal of vagal outflow than dynamic leg exercise.