Swimmers work a huge range of muscles during a session. However, unlike activities such as weight training or boxing, swimming doesn't tend to bulk your muscles up. As an aerobic exercise, swimming creates lean, tight muscles designed for endurance and regular use. Weight training may give you the extra strength required for fast swimming bursts. But, in general, swimming regularly is the best way to develop your muscles as a swimmer.
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Swimming works your body from the toes to the head, with barely a muscle left out in between. For example, RICE University points out that during a standard front crawl a swimmer uses muscles in the neck, chest, arms, shoulders, abdomen, lower back, thighs, calves and even the feet and ankles. A perfect swimming stroke requires propelling the body using both the legs and arms. It also helps to move your body in a way that keeps you streamlined as you move through the water. This undulating movement uses lots of smaller muscles along the body.
In general, swimming is a highly aerobic exercise. That means that it lasts for a long time and uses muscles at a relatively low intensity. Muscles in the human body come in two main forms, slow and fast. Each refers to the type of muscle fiber. Slow-twitch fibers contract for longer because they're needed for prolonged periods. Fast-twitch fibers contract faster and feature in more explosive motions. So, a swimmer's muscles tend to include lots of slow-twitch fibers, according to "Human Biology." Swimmers who specialize in short, fast races may have a higher percentage of fast-twitch fibers.
A classic swimmer's physique has lean but strong muscles over most of the body. Not all swimming strokes work the same muscles, or at the same rate. For example, front crawl tends to put more strain on the shoulders and arms than breaststroke. So, swimmers focusing on front crawl or butterfly tend to have larger arm muscles than specialists in other strokes. Swimmers work their shoulders and thigh muscles more than most. It's therefore not unusual for swimmers to have large thighs and broad shoulders.
Because water supports your weight, muscle and joint injuries tend to occur less often than in more high-impact activities. However, one muscle injury common in swimmers is the rotator cuff strain -- often known as "swimmer's shoulder." This affects around 70 percent of all competitive swimmers, according to the Hospital for Special Surgery. Having tired muscles from overuse increases the chance of developing a shoulder injury. Also, poor technique with your swimming strokes may put extra pressure on your shoulder muscles.