How does the muscle grow. Does the pump cause it to grow or does it grow thicker through heavy exercise. In other words does it matter if the person who lift weights does low rep (less blood) exercises or should He do high reps exercises. Does low rep thicken the muscle and cause it to grow. I have a problem with my arms. I have done both High rep and low rep (Strength) but my arms stay the same. Lately I have been pumping them up and have seen results.![]()