Compared with the amount of research available on activities such as jogging, running, and swimming, research on aquatic exercise is still somewhat limited. In many cases it has been difficult to draw conclusions about water exercise in general because of the number of variables that affect exercise response in the water. Most of the studies that have been conducted have concentrated on discerning how exercise in the water compares with similar exercise on land. Investigators have looked at heart rate, ventilatory responses, oxygen consumption, respiratory-exchange ratios, and ratings of perceived exertion using various research protocols. Oxygen consumption and heart rates have been used frequently as indicators of metabolic workload. Heart-rate response to exercise is especially significant to practitioners if target heart rates are used to prescribe and monitor exercise intensity. An analysis of available research seems to indicate that exercise responses in the water vary with the temperature of the water, the depth of immersion, the intensity of the exercise bout (submaximal vs. maximal efforts), the exercise protocols (walking, running, calisthenics, step, cycle ergometry, use of the arms), and the skills and motivation of the participants involved in the study. Water Temperature The question of water temperature is a critical one, especially as it relates to heartrate response in the water. The general consensus is that heart rates are lower in cooler water than in warmer water. Lower heart-rate responses have been reported at rest and during cycle ergometry at temperatures ranging from 18 to 25 °C (65-77 °F; Craig & Dvorak, 1969; McArdle, Magel, Lesmes, & Katch, 1976). Dressendorfer, Morlock, Baker, and Hong (1976) measured physiological variables during cycle-ergometer exercise to exhaustion in men and observed that heart rate was 8 beats/min lower in 30 °C (86 °F) water and 15 beats/min lower in 25 °C (77 °F) water than in 35 °C (95 °F) water. Avellini, Shapiro, and Pandolf (1983) also reported lower heart rates at a given oxygen consumption in their participants who trained on cycle ergometers at 20 °C (68 °F).