Fasted exercise won't build you more muscle, but it will make it possible to add lean muscle mass more efficiently. In other words, through fasting (which improves insulin sensitivity), and by being leaner (which means you typically have improved insulin sensitivity than someone who is overweight and/or insulin resistant), you have the ability to utilize nutrients more efficiently, specifically protein.
Fasting basically helps to reset the mechanisms behind protein synthesis, which is the biological process of muscle building and rejuvenation, allowing you to potentially build more lean muscle with fewer available nutrients.
The human body has a mechanism known as the mammalian target of rapamycin, or mTOR for short, which is the body's primary muscle-building gene. Fasting primes the muscle-building machinery by increasing insulin sensitivity and suppressing mTOR.
You can almost think of mTOR as a spring. When it's suppressed, it gains potential and the more it is suppressed, the more potential it gains. When mTOR is unleashed, it does its job much more powerfully.
You can also hack into your hormonal hardwiring and jump-start the muscle-building process with intense exercise. When you suppress mTOR, you increase the potential for efficiently building lean muscle.
When you combine fasting with intense exercise, you can imagine the types of muscle-building results you can expect. So, in simple terms, if you fast through the morning hours, lift hard and heavy for 30 to 40 minutes, and then consume your largest meal as your post-workout meal about 30 to 60 minutes after your intense workout, you can bet that just about all of that meal is going to go directly to building lean, dense muscle tissue.
If you're following the micro-fasting protocol, then the best time to train is the late morning or early afternoon hours, so that when you finish your workout, you may go directly into breaking your fast. The same goes for the Warrior Diet.
Sprinting, because it is a form of intense exercise, is a perfect way to activate your muscle-building genes while fasted. So, when in doubt, sprint! Sprinting is fun, primal, and effective. It boosts natural growth hormone, scorches body fat, and strengthens the heart muscle in a way that no other form of exercise can.
If you're following the 5:2 Diet, then the best approach is to either work out first thing in the morning before you break your fast or cram as much time in between your breakfast and workout session as possible.
If you're following intermittent fasting, then try to time your workout around the end of your fasting period, as you would if you were micro-fasting. The difference, though, is that your workout may fall at any hour, depending on when you started your fasting period.
Time your most intensive bouts of exercise in such a manner that you may resume eating upon the completion of your workout and work in micro-bouts of exercise throughout your fasting period. These small bursts of exercise work to boost the positive effects of fasting.
For example, a short bout of sprints, two minutes of kettlebell swings, or even a set or two of pull-ups are all fine examples of exercises you can do to get better results in building lean muscle.