the human body is stronger than what you think. human bone is strong, yet flexible. with repeated stressing and healing, a bone can be very strong.
the human brain got the ability which can function to block off pain - analgesia .
the human muscle can produce so much that can break off from it's attachment to the bones.

so far, this "chi" is unproven. but the techniques to harness chi seems to be sort of preparing the mind and body by breathing and focusing which can reduce pain and increase force. this has been used in many types of sports.

well, there are some possible answers to the question at the comment section.
i can stand on ostrich egg without breaking them
well if you want to post proof, post more reliable source.