Of three 45-min weight circuits at 70% of 15RM, the one alternating free weights with aerobic exercise drew the most oxygen (29.9 ± 3.6 ml/kg/min, against 24.2 ± 2.8 for free weights and 20.4 ± 2.9 for machines) with the lowest lactate and perceived exertion; the anaerobic share of energy was small in all three (2.3-6.2%)

Benito and colleagues measured the oxygen uptake and the total energy expenditure, aerobic plus anaerobic, of three circuit weight-training sessions of equal length.

In random order, participants completed three circuits of 3 sets of 8 exercises lasting 45 min 15 s at 70% of their 15-repetition maximum: one of free weights, one of machines, and one of free weights alternated with aerobic exercise. Oxygen uptake and blood lactate were measured through each circuit to estimate the aerobic and anaerobic energy.

Oxygen uptake, which the abstract reports as energy expenditure, was highest in the combined circuit (29.9 ± 3.6 ml/kg/min), then free weights (24.2 ± 2.8) and machines (20.4 ± 2.9). The combined circuit also gave the highest total energy expenditure, yet the lowest lactate and perceived exertion.

The anaerobic share of total energy was 6.2% with machines, 4.6% with free weights and 2.3% in the combined circuit.

The authors conclude that the combined circuit gives the highest oxygen consumption, that total energy expenditure varies with the type of exercise in the circuit, and that the anaerobic part, though modest, should be estimated, since leaving it out may underestimate total energy expenditure.