Muscle growth occurs after you traumatize them in the gym. After you workout, your body repairs or replaces damaged muscle fibers through a cellular process where it fuses muscle fibers together to form new muscle protein strands or myofibrils. These repaired myofibrils increase in thickness and number to create muscle hypertrophy (growth). Muscle growth occurs whenever the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown.
So this means you actually grow the muscle when you rest. In order to activate the muscle growth process you must.
Create Muscle Tension
Your body adapts to a certain amount of tension. To create growth you must add more tension than your body has adapted to. The smartest most sustainable way to do this is by adding an ever so slight amount of tension week after week.
Create Muscle Damage
Soreness after a workout is an indicator of muscle damage but not needed for actual damage to have occurred. As long as you created enough stimulus from your program, then you would have damaged the cells to enable them to grow back stronger ready to take on the new stress.
Eat enough to facilitate growth
In order to bulk (adding more muscle) without gaining too much fat. You want to be in a caloric surplus to allow for the growth of new muscle tissue. Under great conditions, you can gain about half a pound of muscle per week. That ends up being roughly 180 calories over maintenance per day. This is because there are 1,250 calories in half a pound of muscle. 1,250 split up over 7 days ends up being 178 calories. Take a look at how many calories you should be consuming with my bulking tool Super Hero Jacked
Hormones also play a factor in muscle growth
Insulin and testosterone are the two biggest hormones that facilitate muscle growth. This is why it can be quite effective to eat a high carb meal using simple carbs like white rice after a workout to spike insulin. Because it increases protein synthesis.
You can learn more and check how many calories you need to create muscle growth Super Hero Jacked