You would
think that the contracts would include clauses regarding dangerous activity, weight issues, etc. but they are very rare. In fact, I can only think of one or two players who had those kinds of clauses in their contracts.
Think of all the "dangerous" activities professional athletes participate in - hunting, motorcycles, fast cars, etc. If a team wants to impose a "no hunting" clause in their contract, I'm sure the player could go to one of 30 other teams willing to sign him without the clause.
And what does a "contract" really mean in professional sports these days anyways?

Call it what you will, but it's pretty much "at will" employment these days.