Considering the forces involved in many sports, it's no surprise that professional athletes sustain serious injuries to their muscles, ligaments, tendons and bones. A spate of bone fracture–related injuries seems to be dogging professional teams this year. The Houston Rockets of the National Basketball Association lost seven-time all-star Tracy McGrady to season-ending microfracture surgery in February. And on Monday, Rocket’s team physician Tom Clanton announced in the Houston Chronicle that all-star center Yao Ming's fractured foot, which he sustained in a play-off game against the Los Angeles Lakers in May, has worsened over time and may end his career. The possibility that New York Mets centerfielder Carlos Beltran might have a microfractured knee turned fans and fantasy baseball owners into nervous wrecks. Such an injury ended the career of NBA star Jamal Mashburn.

So how do these fractures develop? And why can they have such impact on athletes' careers, in some cases forcing players into early retirement? To find out, we turned to Howard Palamarchuk, a former Olympic-class race walker who is the director of sports medicine at Temple University School of Podiatric Medicine.

[An edited transcript of the interview follows.]

What is a stress fracture, and how does it develop?
It's actually a very small, microscopic fracture that occurs in the outside portion of a bone called the cortex. A bone that is constantly under stress will eventually weaken or give. The body keeps up with these stresses by generating osteoblasts, cells that make bones. At the same time there are osteoclasts—cells that take away older diseased or broken bone, or bone that is worn out.

You have a balance between osteoblastic activity and osteoclastic activity. Eventually osteoclastic activity wins out, and that is literally the breaking point.

Speculation was that Carlos Beltran might have a microfracture in his knee. What is the difference between a microfracture and a stress fracture?
That’s a good question. I almost see them being the same except maybe the location where they occur. You don't see too many stress fractures in the knees. Stress fractures are more an issue of overuse.  A microfracture could be caused by trauma, like getting hit by something.

An MRI can generally give you an answer between a bone bruise [which Beltran was diagnosed with] and a microfracture without second-guessing.

Yao Ming fractured the tarsal navicular bone of his left foot. Which bone is that exactly?
If you look at the foot from the side, it is the bone that sits at the top of the arch. It is a bone you could equate to a keystone of an arch—the stone that keeps an arch from falling in on itself. It carries a lot of weight. The bone is in the mid-tarsal region, where the heel bone and forefoot meet. There is a lot of stress and force on that bone.

One possible treatment for Yao is to let the fracture heal naturally. Can all fractures heal naturally?
No—specifically, not in bones that do not have a good blood supply. Without a good blood supply you will not get the osteoblast cells. These areas are "nonunions"—they will not heal—due to the poor blood supply.

The tarsal navicular bone has poor blood supply. Yao injured the foot in May, and we are in July. They did another bone scan, and they can tell it is not healing. So now they have to ask, what will be the difference if we let it rest another six months or should we do more aggressive treatments, like a bone bridge or a bone graft, where you take a different region of bone and insert it into the gap?

How long does it take to recover from such injuries?
Under the best conditions, say you were a relatively healthy and nonsmoking young person and you had no big competition in front of you, the textbook would say eight to 12 weeks. A problem is that pros rush back to play. Sometimes they put titanium screws and plates in to take the stress, but the bones may not heal.

Yao is at the crossroads of making a decision now. Do I let it heal by itself, or do I try another method? Rest is the healer. If you keep disrupting the healing, by constantly putting the same load and stress on the foot, it is not going to heal.

Yao has had bone fractures in the past. Are some people more prone to getting fractures?
They are. One reason could be foot mechanics. Is he a flatfoot? Does he have a higher arch? Does he have maybe a short leg and a long leg? If you upset the mechanics, you can concentrate the stress in a way that it is not normally focused. These are things we look at to explain why people that are healthy and built to play these sports keep getting hurt.

Another thing is that Yao is seven feet, six inches [2.28 meters] and has a foot size of 18. That is a small foot for someone so tall, so the pressure his body exerts is distributed over less area. We have players at Temple that are a foot shorter than him and wear a size 20.

Presumably, Yao would be forced to retire if he kept getting fractures or the fracture never heals properly. So how did he most likely injure his foot?
I have a general observation that people may be under the assumption that this is something you might get from jumping. Yao has over a two-foot- [60-centimeter-] high vertical leap. Yet it is not so much causing injuries as the running up and down the court. Due to the soft tissue of the foot and the nature of the arch, there is not as much impact coming down from a layup shot as there is running up and down the court.

What are the risks for everyday or recreational athletes?
Runners tend to develop more stress fractures than walkers. In running there is a flight phase where you are off the ground with both feet and landing on one foot.

We have the rule of three: three times your body weight on impact. So, if you are 100 pounds [45 kilograms], you are hitting the ground with 300 pounds [135 kilograms] of force. A walker always has one foot on the ground; therefore, the force is divided between legs, and only 1.5 times the body weight goes to a foot.

Overdoing it is the high risk. You know when training for a marathon you can only run 100-mile weeks for so long before the osteoclast cells begin acting. The bones are constantly remodeling to adapt to the stress and get to the point where the osteoblast cells cannot keep up with the osteoclast cells, which are removing bone tissue.

Yao reportedly is feeling no symptoms and has no physical signs from the fracture. Is it possible a person might not know if they have a fracture?
Yao is probably not training. The minute he would go back and train he would feel it. You cannot play through it. In other words, something like tendonitis starts to feel better after you warm up and while you play, but a stress fracture does not get better. It gets worse.