Muscle soreness can be a normal response to exercise, especially if you have just started a program, have dramatically increased the intensity or duration of a routine, or attempted a new mode of physical activity to which your body is unaccustomed.  Muscle discomfort that starts about 12-24 hours after an exercise session and lasts for about 3-4 days is referred to as delayed-onset muscle soreness (DOMS).  Symptoms of DOMS are listed below.

• Muscle stiffness
• Mild pain/discomfort 
• Minor swelling
• Decreased strength
• Decreased power
• Decreased range of motion

     The discomfort associated with DOMS is specific to the muscles exercised.  Its severity is related to the intensity and volume of the physical activity.  Eccentric exercise, such as down hill running and the weight lowering phase of resistance training, is associated with the greatest amount of exercise-induced muscle soreness, especially in older individuals.  DOMS appears to be correlated with the following factors:

• Microscopic tears in the muscle tissue
• Damage to the contractile components of the muscle tissue
• Inflammation/Edema - a build-up of fluid in surrounding tissues
• Muscle spasms 

     DOMS is part of an adaptive process that results in a stronger, more resistant muscle.  It has been found that a single bout of exercise has a protective effect, up to 6 weeks, against the development of muscle soreness from subsequent exercise.  Thus, it is suggested that you can minimize muscle soreness by starting with light exercise and slowly progressing the intensity level.  Although the most effective form of treatment is still questionable in alleviating DOMS, the following tactics may provide some relief:

• Selenium and Vitamins E & C supplements (they are thought to protect against disruption of the cellular membrane)
• Protein supplements (for those involved in extremely intense exercise)
• Stretching
• Massage
• Non-steroidal anti-inflammatory medication

     DOMS symptoms should resolve within 10 days.  Stop any physical activity that exacerbates the discomfort and seek medical attention if pain persists longer than expected or if you experience excessive swelling, redness and/or pain. 

Note:  Before beginning an exercise program or increasing the intensity level of a current routine, a physician's approval should be obtained, especially for older adults and those at risk for or who currently have chronic health conditions.

Resources:

Exercise Physiology Energy, Nutrition, & Human Performance, Sixth Edition. pp. 549-552, McArdle, WD., Katch, FI., and Katch, VL.

About.com "Muscle Pain and Soreness After Exercise - What is Delayed Onset Muscle Soreness," Quinn, E. (www.sportsmedicine.about.com/cs/injuries/a/doms.htm)