Time and time again, people waste their time doing endless pushups and crunches.
Originally Posted by Poop-Loops
Pushups will not help your sparring and fighting endurance. Since when do your arms get tired when working the heavy bag? It's your respiratory system and willpower that is holding you back. Chances are if your arms are getting tired, you are arm punching and not using rotational force with your entire body.
Crunches will not strengthen your core or "tighten" your midsection to take a punch. Stabilization exercises and rotational exercises will strengthen your core, not 100 rep sets of crunches.
Weighted crunches without any additional core training is simply bodybuilding and will not help develop more rotational power into your punches. You need to do heavy stabilization exercises, like the side press and all types of exercises with a rotational movement, similar to how you would really punch. Try doing common weight movements on one leg or with one arm (or both!). Try one-armed barbell military presses or side presses!
Mental training is key. However, you can do it more specific to fighting. Flurries, and heavy bag training in general will help this more than anything.