Swimming does tone your legs by engaging your leg muscles, especially when you swim using the dolphin kick and breaststroke which builds strength and definition in your hamstrings, quads and you calves.
The resistance of swimming in the water also leads to muscle toning.
The hardest way to swim through water is using the butterfly stroke as it requires demanding coordination, upper body strength and specialized technique.
Swimmers using the butterfly stroke need to engage their core and maintain rhythm while they propel themselves forward through the water which can be very exhausting.
The reason why butterfly is the hardest stroke when swimming is because it's a demanding technique when swimming and also requires good coordination and precise timing as well as a high level of endurance and strength.
The butterfly stroke when swimming also have unique movements which engage all the muscles in your body which makes it a challenging stroke to master.
The strength and endurance that the butterfly stroke requires helps your body to propel through the water efficiently which also engages every muscle in your body.
To swim with the butterfly stroke you need a great deal of practice and repetition to perfect the position and timing along with other technical aspects of the butterfly stroke.
The breaststroke swimming technique is also a hard swimming stroke which also requires precise timing and coordination although the butterfly stroke is still the hardest.
The most powerful swimming stroke is the front crawl, which is also the fastest swimming stroke and is often chosen by swimmers to maximize speed and efficiency when swimming.