why does muscle contraction depend heavily on good nutrition and good respiration?

Answer 1

Well, what's a muscle made of? Protein, for the most part. You need to eat good protein to put on muscle mass, and to repair the normal wear and tear of muscles. Good respiration brings in oxygen, which is necessary to oxidize ("burn") the fuel you need to operate your muscles, and the rest of your body as well. The more you use your muscles, the more you must be sure to get adequate oxygen. Otherwise, you will first experience fatigue and eventually muscle cramps.