When it comes to building new muscle a couple of things to keep in mind:
First, your muscles have to be subjected to load to stimulate growth. The difficulty comes in when it comes to big muscle groups: legs, shoulders, back, etc. These are big muscle groups and require more load levels than say biceps to fatigue enough to create a stimulus. Keep all of your strength exercises moto specific and calculate the load levels according to your season (for example, you don't want to be lifting heavy or super-setting during your race season). If you have any questions or need examples of what exercises specific to your season, please email me directly at email@example.com
Second, clinical research has revealed that the most muscle that you can create on a daily basis in an controlled environment is one ounce per day - this calculates to 23 pounds in a YEAR. So the products that hype excessive muscle growth is nothing more than marketing hype - it is not scientifically backed up (or it is not being achieved in a natural and safe manner!). As a general rule of thumb, I start my clients with .77 grams per pound of body weight. So if my client weighs 150 pounds times .77 equals 116 grams of high quality protein daily. Please keep in mind that this is specific to motocross and supercross. For my off road riders I use .64 grams per pound of body weight due to the increased endurance necessary for 2 and 3 hour races.
[NOTE: THESE CALCULATIONS ARE WERE I BEGIN. ADJUSTMENTS ARE MADE AFTER FOOD LOGS AND PERFORMANCE DATA ARE EVALUATED]
Without boring you with a bunch of blood chemistry, the goal is to keep the body in a nitrogen positive environment. When the body is nitrogen positive, it has adequate amounts of protein to repair and build new muscle on a daily basis. If you are experiencing low energy, excessive residual soreness, delayed recovery time, feelings of weakness, you could be experiencing low protein intake (and/or quality).
If you have any questions or need anything clarified, please don't hesitate to post back here.