When Not to Do Your Cardio

Aerobics is a useful addition to any bodybuilding program for a number of reasons. It positively affects several cardiovascular risk factors, including total cholesterol, high-density-lipoprotein cholesterol, low-density lipoprotein and C-reactive protein. That last factor is a measure of body inflammation, which is now recognized as a primary symptom of cardiovascular disease. Aerobics also reduces elevated blood pressure and provides a relaxation effect in blood vessels through an increase in nitric oxide synthesis and secretion.

Another reason to do aerobics is to control body composition. Bodyfat burns, or oxidizes, only when oxygen is present, and aerobics is the type of exercise that uses the most oxygen. Aerobics is still the best exercise for losing bodyfat, especially in conjunction with a good diet program designed for fat loss.

Critics contend that aerobics isn’t recommended for those who want to build muscle. They claim that aerobics leads to overtraining, with a consequent increase in the catabolic hormone cortisol and a drop in such anabolic hormones as testosterone. Various studies show that aerobics can interfere with strength and muscle gains because of a blunting of muscle protein synthesis; however, negative effects are caused by exercise volume and frequency, as well as style.

Engaging in hours of aerobics, often more than once daily, is likely to lead to overtraining and muscle loss, but smaller doses of aerobics, never exceeding one hour, are unlikely to adversely affect muscle and strength gains in most bodybuilders. If you’re very lean, you must use common sense in regard to aerobics, since you can indeed lose muscle if you do excess cardio.

Most bodybuilders avoid problems by doing aerobics and weight training at different sessions. In other cases the aerobics workout follows the weight session. That makes sense physiologically, since weight training is powered by muscle glycogen stores. After a weight session, much of the stored glycogen is depleted. Aerobics, with its higher reliance on oxygen intake, will tap into fat faster if glycogen is somewhat depleted. That’s the concept behind doing aerobics on an empty stomach in the morning.

A recent study disclosed another reason not to do aerobics before a weight workout.1 Ten men did low-intensity cycling for an hour, then a weight workout. On another day the same men did only a five-minute-warmup cycling session before their workout. When they did an hour of aerobics first, their growth hormone response to the weight session was nil. Other hormones, such as cortisol and testosterone, weren’t affected by the aerobics. That’s the good news, since it shows that moderate aerobics doesn’t negatively affect hormones related to muscle growth.

But doing the aerobics first did blunt growth hormone release. What is it about aerobics that would block GH?

Aerobic exercise uses greater amounts of fat as fuel, especially as the exercise continues beyond 30 minutes. The use of fat as an energy source elevates levels of free fatty acids in the blood, which, like elevated blood glucose levels, blunt the release of GH. Elevated free fatty acids also promote the release of somatostatin, a protein produced in the brain’s hypothalamus that opposes GH release.

Clearly, it’s just plain dumb to do an extended aerobics session before an intense weight-training session. Not only do you deplete limited energy stores (glycogen), but you also block GH release during the workout. Save the aerobics for afterward. IM

1 Goto, K., et al. (2005). Prior endurance exercise attenuates growth hormone response to subsequent resistance exercise. Eur J Appl Physiol. 94:333-338.