A muscle strain, or pulled muscle, occurs when your muscle is overstretched or torn. This usually occurs as a result of fatigue, overuse, or improper use of a muscle. Strains can happen in any muscle, but they’re most common in your lower back, neck, shoulder, and hamstring, which is the muscle behind your thigh. In a mild strain, a torn muscle may feel slightly stiff but still flexible enough for use. A severe muscle strain is when the muscle is severely torn. This results in pain and very limited movement.
An acute muscle strain is when your muscle tears suddenly and unexpectedly. Such tears can occur either from injuries or trauma. This can be due to:
-not warming up properly before physical activity
-overexertion and fatigue
-acute muscle strains are also more common in cold weather.
So it is important to warm up before exercise like swimming and running; to have good form/posture to avoid injuries; and to stretch after exercise before the muscles cool down.