Muscle pulls can happen to any muscle in the body at just about any time. The most common ones for runners are in the upper and lower leg muscles--calves, quads, adductors, hamstrings, thighs, etc. You get the idea. No matter how hard you try to warm up, cool down, or stretch conscientiously, there’s still overuse and even fatigue that can cause a muscle pull. Accidents can also happen.
A muscle pull means that a sudden force has been applied to the muscle and its fibers were stretched beyond their capacity. A muscle pull is not as severe as a muscle tear--only a few fibers are involved, not all of them.
Muscle pulls are all treated in the same way: ice the injury and rest the muscle until swelling and pain subside. An anti-inflammatory can be helpful to reduce pain and inflammation. Apply the ice for twenty minutes on and twenty minutes off as frequently as you can in the days immediately following the injury. The pain should disappear in a few days. If not, it’s time to see a medical professional.
Try gentle stretching as soon as the muscle will tolerate it. It the muscles are not gradually re-lengthened, you may pull the muscle again because it will heal in a shortened state. Be careful not to overstretch as it might cause the muscle to spasm. When the muscle can be stretched pain free as far as the healthy one on the other side of the body, you can return to normal activity.
Depending on the muscle and the severity of the pull, it can take anywhere from a week to a month for the injury to heal sufficiently to return to your training program.