Complex sentence: The flow of lymph from the legs towards the heart is the result of the "calf pump"--during walking the calf muscle contracts, squeezing lymph out of the leg via the lymphatic vessels.
Split into simpler sentences: The flow of lymph from the legs towards the heart is the result of the "calf pump". During walking the calf muscle contracts, squeezing lymph out of the leg via the lymphatic vessels.