We use up to 200 muscles just to talk a step forward! Though any of these muscles can result in problems with our walking, soreness is indicative of an overused muscle.
First, you want to rule out any change in activity or weight, and any strength training as the possible trigger for the soreness. After these have been ruled out, the most likely problem is with your gait (walking form). Because we do not think about firing the 200 muscles in a coordinated motion every time we take a step, our bodies have developed patterns that make the thought process easier. On occasion, faulty movement patterns can become a part of our routine which can lead to a change in the forces the body applies to various joints and muscles. All of these changes can lead to muscles that are overworked and become. A chiropractor specializes in musculoskeletal condition and can help you find the source of the problem. If you do not have a chiropractor, here is a map of evidence-based chiropractors. http://www.forwardthinkingchiro.com/member-map-1/#!map
Sometimes you can also have cramping if you are low in vitamin D, low calcium, dehydrated, and if someone is taking a diuretic pill (sometimes the potassium is too low). This can be determined by a complete blood count. Lab work is needed for this.
In your case, if it's just exercises, increase your H2O. See if that helps.
Got your back,
Dr. Todd Gewant