From Ergo Log
Just after you’ve trained your legs, you have much more testosterone and growth hormone in your blood. If you then train a small muscle group, it’ll grow faster. Norwegian sports scientists at Lillehammer University College tested this theory.
The 11 male test subjects that the Norwegians used were aged between 20 and 34 and had never done weight training before. For eleven weeks they had to go to a bodybuilding gym 4 times a week. On two days they trained only their left biceps, doing biceps curls, hammer curls and reverse curls. [A]
On the two other days they trained their legs. The men did leg-presses, leg-extensions and leg-curls. Immediately afterwards they trained their right biceps using the same exercises as they did for their left biceps. [L+A]
The subjects trained using weights for which they could just manage 10 reps. Before, during and just after a training session they drank shakes containing proteins and carbohydrates.
When the researchers studied the men’s biceps at the end of the 11 weeks, they saw that the L+A biceps that had been trained after the leg training were all significantly bigger than the A-biceps that had been trained separately from the leg training. What’s more, the L+A biceps were stronger than the A-biceps.
Why biceps grow faster if you train them after doing a leg training session first is shown above. After any L+A training session there is more testosterone and growth hormone present in the body than after an A-training.
T-0 = before training; t-1 = after the biceps training; T-2 = after the leg training (in the A group: half an hour after the training; T-3 = half an hour after the training (in the A group: one hour after the training).
Danish researchers showed a decade earlier that biceps grow faster if you train them together with your legs. By the way, the Danes got their subjects to train their biceps first and then their legs. [Scand J Med Sci Sports. 2001 Dec;11(6):347-54.]
The Danish approach may be even more effective than that of the Norwegians, going by other research in which subjects train several muscles in the upper body in different sequences. This research showed that the muscle group you train first grows faster than the muscle groups you train later in the workout.
Eur J Appl Physiol. 2011 Sep;111(9):2249-59.