There is evidence that ape muscle is intrinsically stronger than human muscle and that their maximum shortening velocity is faster. I.e., if you have a given cross-section of muscle (say 1 cm^2), the ape muscle will create more force when maximally activated than the human muscle. Apes also have larger moment arms (so a given muscle force produces a larger joint torque) but this can impair muscle power too. I also suspect there are some neural factors that makes apes able to produce more joint torque.