There are none. If you break your hand, you work around it.
Extreme Scenario: Train for when the zombies come. Do you think the flesh eaters are going to have pity on you and wait until you are full go to eat you? No. You have to be able to work through difficulties. (Ok, probably not going to have Resident Evil happen any time soon, but you might as well be ready.)
You have to adapt and thrive under adversity. Over coming difficulties build confidence. Knowing - not thinking or wishing or hoping - you can overcome what life throws at you is the essences of confidence. Getting up one more time than you fall.
Anyone can train under optimal conditions. Any one can stay snuggly in their comfort zones. Anyone can go to their favorite bar and platform and perform. How about when the bar sucks, the ground is uneven and you have to perform? Character is developed by such adversity.
Excuses - There are none.