The main energy expenditure involved in running is the work done by the muscles to accelerate the legs. When a foot strikes the ground, it is momentarily brought to rest while the remainder of the animal’s body continues to move forward. When the foot is picked up, it is accelerated forward by one set of muscles in order to move ahead of the rest of the body. Then the foot is slowed down by a second set of muscles until it is brought to rest on the ground again. The muscles expend energy both when speeding up the leg and when slowing it down. How are thoroughbred horses, deer, and greyhounds adapted so that they can run at great speed?