In the upper body, when swimming front crawl, you’ll use the deltoids, latissimus dorsi (down the side of your back), trapezius, triceps and biceps muscles.  The muscles of the shoulders and around shoulder blade (including the deltoids) will help ‘hold’ the ‘paddle’ [your arm] in place as your body moves past it. Your core muscles, including your trapezius and latissimus dorsi, help you hold a streamlined torso. 

In the lower body, front crawl works the hip flexors, quads, hamstrings, glutes and calves. Your abdominal muscles will also tone up from stabilising you in the water.

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A schematic showing some of the skeletal muscles of a Homo sapiens. The schematic was based on an image in the book "The human body" by Linda Gamlin

(Licensed under Wikimedia Commons https://commons.wikimedia.org/wiki/File:Skeletal_muscles_homo_sapiens.JPG)

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