When moving large blocks, or carrying heavy loads, the legs exert most of the force. If you look at most mammals, the rear legs are much thicker and more muscular. When the legs are stressed by a heavy load, the body releases anabolic hormones (e.g., T, GH, IGF-1, mGF) to promote muscle growth so that the load can be more easily handled next time it is encountered. This is the effect that is being simulated in the gym. The joints and digits of the upper body are not designed to handle equally heavy loads and the relative muscle mass is less than the legs, perhaps explaining why working the arms and chest do not stimulate a similar increase in T and GH. It is possible that the hormone response is mass-dependent, and that men with considerably greater upper body muscle mass might be able to generate a T and GH peak.8
Numerous studies, many co-authored by Dr. Kraemer, have documented the T and GH response to specific weight-training protocols.9, 10 The general pattern is for multiple exercises, multiple sets, heavy loads, and short rest periods between sets. Logically, boosting T and GH, especially in conjunction with exercise, should increase the mass and strength gains. However, some studies have shown no effect, leading many to state empirically that the anabolic hormone response seen during exercise provides no added benefit.11 Dr. Ronnestad and colleagues reviewed the literature, noting that the protocols used in studies that showed no benefit peformed the leg exercises (the anabolic signal) after the arm exercises; also, one study used a separate group of subjects as controls, rather than the opposite arm in the same person.12-14 Differences between individuals can blur the significance of many outcomes.
In their study, Ronnestad, et al. had the subject perform the leg exercises first so that the maximal anabolic signaling was occurring as the arms were being exercised, rather than after. Also, the design used the other arm as the control (performing the same workout on non-leg training days) so there was no between-individual variation in diet, lifestyle, or genetics.
Having said all that, what was the degree of benefit to the subjects? By performing the prescribed leg exercises first, the increase in 1RM increased by 21 percent in 11 weeks (the subjects were not trained lifters or athletes).1 By comparison, the opposite arm increased 1RM by 14 percent, still an impressive gain, but 50 percent less than the “anabolic group.” It appears that the timing of the hormonal release is a more significant factor than had been previously appreciated. The short window of opportunity suggests the effects may be dependent upon non-genomic androgen responses, though that was not addressed in the study. Incidentally, the legs also benefited from the training with a 23 percent increase in 1RM in the leg presses.
Size matters, and while both arms saw a similar increase in cross-sectional area (girth) of 10-12 percent, the pattern of increase was different. Both groups had size gains in the mid-portion of the biceps and brachialis, but only the “anabolic” group had an increase toward the distal end of the muscles (toward the elbow). This is the largest portion of the muscles, and possibly the most dependent upon anabolic signaling for growth.1 The measures were taken by an MRI (an X-ray like machine) with the arms extended. It would have been interesting to compare the flexed measures of the two groups.