It is suggested that heart rate (HR) recovery for the first 30 sec after exercise is primarily mediated by vagal reactivation. We examined the difference in vagally mediated HR recovery after exercise between resting on land and in water. Seven and 6 healthy non-smoking college students were participated in the experiments 1 (Exp. 1) and 2 (Exp. 2), respectively. In Exp. 1, students ran on land or swam in water for 4-5 min at three HR levels (125, 140 and 155 bpm) then rested on land or in water to the level of the xiphoid for the first 30 sec after exercise. In Exp. 2, students ran on land or swam in water for 4-5 min at HR 147 bpm then rested on land or in water to the level of the navel or clavicle for the first 30 sec after exercise. The time constants of the beat-by-beat HR decay for the first 30 sec after exercise (T30) was used as an index of vagally mediated HR recovery after exercise. T30 after exercise in water was significantly lower at HR 125 and 155 bpm and slightly lower at HR 140 bpm than that after exercise on land in Exp. 1. In Exp. 2, T30 after exercise in water at the level of the clavicle was significantly lower than that after exercise on land. However, there was no significant difference in T30 after exercise between that in water at the level of the navel and that on land in Exp. 2. These results suggest that the vagally mediated HR recovery for the first 30 sec after exercise is accelerated in water compared with that on land. Augmentation of the vagally mediated HR recovery after exercise in water is thought to depend on the level of water immersion during recovery. When we use the heart or pulse rate counted for 10-15 seconds immediately after exercise for estimating the HR during exercise, these results must be taken into account.

DESCENTE SPORTS SCIENCE Vol.23/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE