Most people can only handle four intense training sessions per week. An intense workout has a systemic effect that impacts the whole body, not just the targeted muscles.
A tough workout will fatigue the nervous system and challenge the immune and hormonal systems – the latter two being responsible for the actual hypertrophy response to training. The immune system is responsible for the macrophage/cytokines and satellite cell activation part of the muscle-building process, which is necessary to initiate repair and growth.
The hormonal system plays a role in protein synthesis by having anabolic hormones (testosterone, IGF-1, etc.) higher than the catabolic hormones during the recovery period. If the immune and hormonal systems are constantly challenged you can’t build muscle optimally – and you’ll be more likely to get sick or suffer from low libido and depression. A drained nervous system means your performance and well-being will suffer.
A hard session challenges all of these systems. Too many of these workouts in a week leads to stagnation. You can do a blitz phase of about three weeks in which you have more intense sessions per week. But you can’t do that for the long haul, and once the blitz is done you’ll need a deloading period to be able to continue progressing.
But I Wanna Train More Often!
You can. This doesn’t mean you can’t train more than four days per week. It means complementing the hard workouts with less demanding sessions. If you like to train almost every day, you can do four hard sessions and add two or three less demanding workouts.
For me, these will either be neural charge sessions or low-volume bodybuilding/isolation sessions of lower weight and higher reps. In these workouts I don’t go to failure and find that they actually help me recover and prolong the growth period from the previous hard session.
Sometimes we want to progress so badly that we make counterproductive decisions. Having more than four hard sessions per week is one example. You can train at a moderate level six days a week too, but these workouts are hard to calibrate properly and can be unrewarding. If you plan on training really hard, stick to the four-per-week rule.