When the body is exposed to more of a stimulus that it is equipped to handle the body will makes a positive adaptation as a form of self-protection. That change will occur if the body has the capacity to change plus the needed time and resources to recover. Too much of the stimulus and too little time to recovery can produce a negative result – tanning versus burning or an increase in strength versus over-training and resulting weakness. Just like everyone has a different tolerance to exposure of the sun, each person has a different tolerance for high intensity strength training frequency.
With high intensity training there are those for whom twice a week will produce ongoing results while others would suffer under such a regime. For those who need more recovery time working out twice a week results in ruining two workouts, If you will not have fully recovered from your first workout, the second workout will be sub-par as you are not at full capacity. You will not improve, and you will have wasted valuable time and effort.
Working all the major muscle groups in a high intensity fashion once a week works best for most people. One of our clients trains every nine to ten days. You say surely that person can’t achieve peak strength training that infrequently. While it might not be optimal for others, it really works for him. He does 1000 pounds on the leg press, and he uses the stack on several other machines. A person that strong can put greater stress on the body that may require a longer recovery time.
Through trial and error you can eventually find out what works. I spent years figuring it out. At Austin TX Personal Trainers and New Orleans Personal Trainers our personal trainers have developed a high intensity training program (HIT) with special attention paid to recovery to insure that the improvements are ongoing – our business depends on it. We cannot afford to have clients come in and ruin two workouts in a row by not being adequately recovered.