Complex sentence: The hindlimbs act as stablizers and orient the body over the pectoral limbs and the thumbs or pollices stabilize the pectoral limb and contribute to extending the time over which vertical force is exerted.
Split into simpler sentences: The hindlimbs act as stabilizers and orient the body over the pectoral limbs. The thumbs stabilize the pectoral limbs, contributing to the time over which the vertical force is exerted.