Involuntary contractions of a muscle are known as muscle spasms. These are caused by muscle overuse or injury and may manifest as muscle tightness or muscle cramps. Muscle spasms often affect the neck, back, shoulders, or legs, causing pain, discomfort, and tenderness to the touch. They can be caused by dehydration during strenuous activity—as often is the case with athletes—as well as poor posture, emotional stress, or holding muscles in one position for too long. In some cases, spasms produce muscle knots, which are typically very painful and tender to the touch. It is often difficult to use a muscle affected by knots or tightness. One common type of muscle spasm is a charley horse, which is a term used to describe leg cramps, especially those occurring in the calves. Many people experience these cramps while sleeping at night. Treatment for muscle spasms may include application of ice and heat, stretching, massage, and use of certain medications (such as muscle relaxants). Those affected by muscle spasms also can take preventative measures such as staying adequately hydrated (especially during exercise), loosening the covers at the foot of the bed (to prevent calf cramps at night), engaging in a regular stretching routine, and seeking physical therapy to improve posture and muscle strength.