Measurements of energy expenditure, while swimming on the surface and underwater, have been made using indirect calorimetry and by prediction from heart rate. These results show that oxygen consumption underwater of more than 3 litres/minute (lpm) is possible, and values greater than 2 lpm are quite common. The diver’s energy expenditure when inactive may be lower than found on land, presumably because the absence of gravitational effects reduces the energy required to maintain posture underwater.
Typical gas consumption and energy expenditure levels are as follows:
For a slow swim, 0.5 knots, the diver would have an air consumption of 20 lpm and an oxygen consumption of 0.8 lpm. A swim of 0.8 knots would cause an air consumption of almost 40 lpm and an oxygen consumption of 1.4 lpm. A fast swim of 1.2 knots would cause an oxygen consumption of about 2.5 lpm and an air consumption of 60 lpm (air consumption measured at the depth the diver was swimming and oxygen consumption at 1 ATA).
Increased gas density increases the work of breathing. This increases the resistance to gas flow through the diver’s airways and breathing apparatus, increases the work of breathing and reduces ventilatory capacity. A maximum breathing gas density (helium) of around 8 g/litre appears to be realistic for practical purposes, thus limiting diving to around 400 to 500 metres for useful work.
Gas density may prove to be the limiting factor for deep diving.
It may be expected that the higher oxygen partial pressures in hyperbaric environments could improve physical performance. However, chamber experiments, in which the subjects exercised while breathing oxygen at 3 ATA, showed that the maximum aerobic work performance was not significantly increased.