Sore Muscles

Stretching Before Swimming

If you don't stretch your muscles before swimming, your muscles can tear more easily as the muscle tries to move through its range of motion. So when you swim freestyle, your arm muscles -- specifically, your biceps, triceps and shoulders -- will feel that burn. Stretching before you get in the water can help prevent muscle tearing. As you stretch, your muscles will loosen up so you're already relaxed and ready to go before you start swimming.

Swimming Warmups and Cooldowns

If it's your first or second time swimming laps, then you should start getting in the habit of warming up and cooling down before and after your workout. A slow warmup will prevent you from swimming "cold," as “cold” muscles that suddenly have to work hard are more likely to become damaged. Warmups also gradually increase your heart and breathing rates while increasing the flow of oxygen to your muscles. Likewise, cooling down after a workout helps lower your heart and breathing rates back to resting levels. DOMS is caused by sudden increases in intensity or duration, so without warmups and cooldowns, your muscles can become unnecessarily tired.

source