Hmm, I'm a molecular biologist, not an expert at this particularly, what I do know is that damage of muscle fiber may not be necessarily mean new cell growth and thus extra DNA replication. The growth of the muscle cell certainly does.

Exercise is good for the immune system, this helps clear cancer, helps stay away from inflammation (ie auto-immune diseases). Where as a sedentary life style a poor diet will promote this.

But indeed for some things/behaviors, the relation the cancer may be complex.