How can someone encourage muscle growth in weak muscles?

Resistance excercise: Do anaerobic resistance exercise, usually weight lifting and eat well. Whether you should increase your protein intake is controversial i say yes, but there have been vegetarian body builders. My guess is that there is a genetic component to muscle development and these individuals just have a propensity.

Exercise: Assuming that there are no pathological problems you can only build muscle with protein and exercise(assuming no use of any medications).