This study was conducted to determine whether the heart rate increase at the onset of passive dynamic exercise is related to the amount of skeletal muscle mass engaged in movement. Fifteen healthy male subjects, 18-30 years old, performed, from the 4th to the 8th second of a 12-s apnea, four different 4-s bouts of passive cycling assigned in a counterbalanced order, each one different from the others by the number of limbs engaged in the movement (i.e., 1 arm, 2 arms, 2 arms + 1 leg and 2 arms + 2 legs), while respiratory movements and limb muscle electromyography were recorded. A repeated-measures ANOVA showed that the RR interval at the end of 4-s passive cycling was reduced in all the four different bouts (P < 0.05); the variations (delta values from pre-exercise to the end of 4 s of passive cycling) were directly related, in a non-linear trend, to the amount of muscle mass engaged in movement. These variations were more expressive when extremes were compared (110 +/- 16 vs. 184 +/- 24 ms, respectively, 1 limb vs. 4 limbs, P < 0.05), with differences observed from the first cardiac cycle after the onset of exercise. It was concluded that in healthy subjects, heart rate increase at the onset of passive cycling is directly related to the number of limbs and consequently the amount of muscle mass engaged, which is possibly related to a greater afferent input from stretch-sensitive muscle mechanoreceptors.