Complex sentence: The main limitation on a macropod's ability to leap is not the strength of the muscles in the hindquarters: the greatly elongated foot provides enormous leverage and the key factor is the ability of the joints and tendons to stand up under the strain of hopping.
Split into simpler sentences: The main limitation on a macropod's ability to leap is not the strength of the muscles in the hindquarters. It is the ability of the joints and tendons to withstand the strain of hopping.