I feel like this question is very broad. The majority of writing (especially fiction) is about writing things you've never experienced.
I would say the majority of answers will be the same thing: research.
Read books and stories of a similar genre, see how they handle those situations. You may be able to get recommendations for books from people, or just search for similar tag/plot lines. A local library is a good place to start, but the internet has almost everything in eBooks you'd need.
Watch videos that include people being injured, to see how it looks (movies are great for this). Do they look panicked? How can you describe the pain? How do people around them react? Ask yourself questions, even write notes! :)
Consider similar circumstances you have been in, or someone you know has been in, then exaggerate them. You may not be as strong as your character and hurt someone by accident, but you probably have poked someone in the eye or thrown something and accidentally hit someone. How did that effect you? If exaggerated to the point of severe injury, how would it effect you then?
Along the same vein, try asking someone. Ask the strongest person you know if they've ever hurt someone by accident how it made them feel. Ask them how it happened.
Go out and live it! Join a wrestling club and find out what it feels like to get thrown around. Join rugby, or american football, or just watch some matches. Experience and see what strength is, then try describing that.