Assuming nutrition is adequate, how long should one rest between intense weight training sessions?

There are a few factors to consider here:

First, how intense is 'intense'? Let's say you've worked your arms all day and can barely lift the groceries; that's too intense. Give it 48-72 hours. If you were more careful and worked them hard, but could probably squeeze out another set if you really really pushed it (a good point to stop at), then it depends on the muscle groups you're working. In general, my advice is to give the single-purpose muscle groups a full day's rest in between working them. That's also About.com's advice, so whoever told me that was on to something. [1][2]

Second, which muscle group(s) are you working? Almost every exercise works almost every muscle in your body, but most lifts work a particular set of muscle way harder than the rest. So, if you're doing curls, then to me you're focusing on the upper arm - biceps and triceps. These muscles aren't used to constant use and regeneration, so they need more time to repair. Again, 48 hours seems to be an average. However, other muscle groups, like the abs or the lower/mid back are constantly in use all day every day; they support your frame. Because of this, these muscle groups are multipurpose (in my mind) and they are used to constantly repairing themselves. I work these groups every day: running, surfing, sit-ups, pilates, whatever. Everything I do works these muscles, so they're constantly in flux. For these muscles you don't need to worry much, a night's rest is all you really need to get the inflammation down and hit them again the next day. [2][3]

As for how many times per week: my rule of thumb is to switch between upper and lower body every time I go to the gym. So if you're going every day, I would say you could probably get 3 days in each week. That's considering only moderate to heavy exercise - not the "I can't move my arms or legs" exercises.

Finally, you can speed the recovery process a little with supplements. I don't use them, but a lot of people swear by the NOx route. If it works for you, great. Still, I'd give my muscles a day off regardless, as you can't speed up the recovery too much - not to mention the variation in muscle physiology between people.

Just don't hurt yourself

[1] http://sportsmedicine.about.com/...
[2] http://ajpregu.physiology.org/co...
[3] http://sportsmedicine.about.com/...
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