Continuing on with some research review today I'll look at two related studies. Both are from the Journal of Strength & Conditioning Research.
The first is a study of physiological and neuromuscular responses during a "body pump" class (for you John! ;).
The second is a study on the physiological and metabolic response to a "functional" training circuit.
The two studies look at similar issues. Namely how the body responds to continuous circuit-style exercise.
*disclaimer* I am reporting results of two independent studies to which I have no connection. The comparison of the studies, due to many factors inherent in such research, are not necessarily proof of anything, but may just give us clues into improving our training.
First, the fundamental difference in the two studies are that in the body pump class the primary tools are very light barbells and dumbbells (approx 10% of 1 rep max, or 10 lbs). The second study uses Freemotion cable machines, which employs higher resistance levels and less volume (reps). The first study used 15 untrained women, the second used 10 men and 10 women (19-27 yrs).
The Body Pump class is a group fitness "aerobic" class in which participants do hundreds of repetitions of light weights, the idea being to improve aerobic capacity and metabolic rate. In other words to improve one's fitness level and body composition (primarily fat loss).
The study concluded that for the 15 untrained women body pump "generated a fatigue condition that is sufficient for the strength improvement of inferior limbs in untrained subjects." However (and this is the biggie) "the metabolic and cardiovascular stimulus seem not to be efficient to improve aerobic capacity."
In other words the untrained women showed some strength improvement in smaller muscles due probably purely to the deconditioned state they were in. Their cardio fitness however showed no significant improvement, which one could stipulate means that such classes are not the most efficient means by which to improve one's fitness nor create a rise in metabolic rate (a big factor in fat loss).
Regarding the second study the workout "did not result in oxygen uptake values that meet the ACSM recommendations for improving cardiovascular fitness". However the relatively high intensity of the cable circuit workout resulted in elevated blood lactate and fairly significant caloric expenditure.
These studies are by no means exhaustive, but are similar in result to other research I've read, and personal experience with client's results. The bottom line is that body pump-type classes are not very effective for fat loss nor improving conditioning. Yes it's certainly better than nothing and you'll be tired afterward, but a key mistake many people make is mistaking being fatigued for getting results.
The "functional circuit" type of workout appears to be a better option, though not optimal, concerning overall fitness. And if done with good form does provide a means by which to learn how to fire the right muscles at the right time, and reinforce good movement patterns.