I well know that none can train exclusively on a concentrated program for too long because the time will surely come when it will cease to get further results.
Some term this the "sticking-point" others call it "staleness" whereas I term it over-taxing the nerves and muscles.
I have found three weeks of concentrated training to be about enough, and the point at which most bodybuilders become bored and stale. And at this point, after three weeks of hard training, I find that one week of rest to be much better than would a change of program, because the softening up of muscle tissue allows for renewed energy by the storing up of vitality and re-stimulation to muscles for the resumption of training.
Rest is nature's method of restoring the nerves and whole body. Surely, this is logical. If anyone robs himself of needed rest and allows his enthusiasm to govern him, he then continues on nerve force which will soon prove detrimental for muscle-growth.
And if one cares to go into precise physiological technicalities he will readily find that the nerves need rest more than do the muscles.
Muscles over bad nerves soon become weaker under the power of mis-directed nerve force. Far too many gymnasium instructors fail to recognize the need of relaxation for the central nervous system, which I feel controls either growth or shrinkage of musculature.
That is why I emphatically advice the application of real hard training, three times or oftener every week for three weeks, only and to then follow with a lay-off of one whole week so as to recuperate and give the muscles and tissues a chance to grow – and they will grow!