User: Which swimming stroke is the hardest?
Assistant: Butterfly is considered to be the hardest of all swimming strokes. It engages full body including arms, core muscles, legs and feet. In particular, it strongly depends on core strength and legs. In order to recover the arms over the water the swimmer needs to execute a strong dolphin kick, which lifts the chest and rest of the body. Swimming butterfly requires a lot of energy. Amateur swimmers who can swim long distances with freestyle, breastroke or backstroke typically can cover only a short distance with butterfly e.g. 25 or 50 meters.