Muscles tighten up for several reasons. These may include periods of prolonged inactivity, during and after exercise. Managing and preventing tight muscles will help keep you exercising.
During periods of prolonged inactivity, such as sitting at your desk, muscles get tight due to their restricted movement. When you sit for long periods of time, your hips are flexed which puts the hip flexors in a shortened position and your glutes in a lengthened position. Also, as you reach forward to work on your computer, your chest muscles (pectorals) are in a shortened position while your back muscles (rhomboids) are in a lengthened position. This results in muscle imbalances with the shortened muscles becoming tight and the lengthened muscles becoming weak. These imbalances cause people to have rounded shoulders and underdeveloped glutes. Maintaining proper posture while sitting, strengthening the muscles that are lengthened and weak and stretching tight muscles (chest and hip flexors) will help alleviate tight muscles.
Muscles can become tight during exercise. A good example is a muscle cramp. During a muscle cramp the muscle fibers remain shortened and are unable to lengthen due to fatigue or improper hydration and nutrition. Forcing the muscle to stretch when it is in a tight, contracted form can tear the muscle fiber and cause injury. Allow the spasm to relax before attempting to stretch it out. To prevent these cramps, make sure you are properly hydrated and have a small snack before you start your workout.
Muscles also tighten after exercise. This is called delayed onset muscle soreness (DOMS) which can be felt as pain and stiffness for 24-72 hours post-exercise. This soreness and tightness is a result of small ruptures in the muscles. To prevent this, gradually increase the intensity of a new exercise. DOMS usually goes away within 72 hours. Increasing the blood flow to the area by doing moderate exercises or a massage will help alleviate the soreness.
To help prevent muscle soreness and tightness be sure to increase intensity slowly, stretch and massage post workout and make sure to hydrate and fuel prior to your workout.