Adding a little extra challenge to your regular workout can keep you motivated and push your body for greater fitness. Running stairs works your quads, your butt and your calves, and adds intensity to your regular run. However, running can already take a toll on your knees, and running stairs adds even more strain on your knee joints.

Running and the Knees

Knee injuries are unfortunately very common in runners because of the major stress that pounding the pavement can put on your joints. Running consistently over many years can cause the bones and cartilage in the knee joint to wear down from the heavy strain of supporting the body and taking the impact of running. One wrong step can also sprain or strain a muscle, ligament or tendon in the knee, causing injury and pain that requires rest to help with healing. Runner's knee, also known as patellofemoral pain, can occur when the knee joint becomes irritated or injured from overuse or very heavy training, says the American Academy of Orthopaedic Surgeons.

Stairs and the Knees

Climbing stairs adds considerable extra strain on the knees. According to a study by Queen's University researchers that was published in "Gait & Posture," the strain on the knees when climbing stairs is somewhere between three and six times the force of normal body weight. When you're running up stairs, your knees are under a strain equivalent of up to eight times that of your body weight.

Osteoarthritis

Over time, running stairs can wear down the cartilage in your knee joints. But if your kneecap is out of alignment, the strain of running stairs can cause even heavier wear and tear and stress on the cartilage in the knee, causing a breakdown of the cartilage. Deterioration of cartilage in the knee is one of the causes of osteoarthritis, which results in stiffness, pain and inflammation in the knee.

Strain and Injury

While running stairs can cause strain on even a very fit athlete, it can really do some damage for someone who exercises too hard too fast. If you are trying to kick-start your physical activity regimen with running stairs, slow down and strengthen first. The weaker your muscles that support the knee joint, the more likely you are to suffer an injury because the knee doesn't have the support that it needs. Running stairs when you've recently been sedentary or exercising only lightly can cause significant knee pain.