The air we breathe contains 21 percent oxygen. If bodybuilders train in a room where the oxygen concentration has been reduced to 16 percent, their muscles grow 4-6 times faster than normal, according to research by Akinobu Nishimura.
Reduce the oxygen supply to muscles when they make intensive contractions, and the anabolic growth stimulus after exertion rockets. You could, for example, train in a room with a lower oxygen concentration. This increases the body's anabolic response, according to a 2010 Japanese study. It's not clear from this study, however, whether bodybuilders gain more muscle mass in this way.
According to Nishimura's study, though, this is the case. He got 7 male students to training their biceps and triceps muscles for 6 weeks under normal circumstance [normoxia], and 7 other students to do the same in an oxygen-depleted room [hypoxia].
The test subjects just trained one arm. The figure below shows that the size [CSA] of the triceps and, underneath, that of the biceps increased by 7.3 and 9.9 percent respectively as a result of training in the oxygen-depleted room [Hyp-Ex]. The untrained arm in the hypoxia group [Hyp-N] did not increase in size, but the trained arm in the normoxia group [Norm-Ex] did, although the increase in the CSA was not statistically significant.
Nishimura speculates that training in a reduced oxygen environment boosts the production of lactic acid. That causes your testes to make more testosteroneand the pituitary to produce more growth hormone. The concentration of free radicals may increase in the muscle cells, and the cells may react to this by boosting the anabolic processes.
"Although further investigation is needed, resistance training under hypoxia appears to offer an effective and practical method of increasing strength and inducing muscle hypertrophy", writes Nishimura.