With powerful legs, you will be able to jump further in the air, which increases speed because it is easier to move through the air than it is to move through the water.
There are five main muscles that make the largest difference in a swimmer's performance.
The latissimus dorsi muscle or commonly known as “lats” are your middle back muscles. If you push your hands together in front of the lower part of your chest, you will be able to flex your latissimus dorsi muscles.
These muscles play a large role in your ability to pull. You will flex and use these muscles from point of water entry all the way until your hand is just past your chest, at which point your pull becomes dominated by your tricep.