CAMBRIDGE -- Every winter it happens -- people hurting themselves shoveling snow from their sidewalks and driveways with injuries ranging from minor aches and pulled muscles to fatal heart attacks.

Shoveling snow is physically stressful. The bending, lifting and twisting motions can take a serious toll on the body, and back injuries are among the most common injuries. According to the Weather Channel's website, shoveling snow is equivalent to lifting weights. The average shovel of snow weighs 16 pounds and the average driveway requires 100 shovels to clear it. That's 1,600 pounds of snow to clear one driveway.

Shoveling can be made more difficult by the weather. The risk for hypothermia, a decrease in body temperature, is increased if one is not dressed correctly for the weather conditions. According to the American Heart Association, the cold air also makes it harder to work and breathe, which adds extra strain on the body, especially the heart. When exposed to the cold, the body's natural reflex is to tighten arteries and blood vessels. That, in combination with the physical demands of snow shoveling that cause the heart to pump blood faster, is a recipe for a heart attack.

Those most at risk for a heart attack include: Anyone who has already had a heart attack; individuals with a history of heart disease; those with high blood pressure or high cholesterol levels; and smokers.

Older or out of shape people also have a greater chance of a heart attack while shoveling snow. Even people who exercise regularly can find shoveling to be strenuous if they try to tackle the job quickly without taking breaks.

Snow shoveling is hard work, but it can be good exercise when performed correctly and with safety in mind. By understanding your physical condition and taking appropriate precautionary measures, you can help reduce the risk of injury. The American Heart Association and the American Physical Therapy Association offer the following tips for safe snow clearing:

* Avoid caffeine or nicotine before shoveling. These stimulants may increase heart rate and cause blood vessels to constrict, which places extra stress on the heart.

* Drink plenty of water to avoid dehydration.

* Dress in layers and be sure to wear a hat, gloves and footwear.

* Do some basic warm-up exercises before shoveling, such as walking for a few minutes or marching in place. Stretch the muscles in the arms and legs. Warm muscles will work more efficiently and are less likely to become injured.

* Shovel fresh snow rather than partially melted and packed snow. Lift small amounts at a time using your legs, not your back. Scoop snow in a forward motion and step in the direction as you throw the snow.

* Avoid twisting. Bend the knees and keep the back as straight as possible to lift with the legs.

* Avoid tossing snow over the shoulder or to the side. If possible, try pushing the snow forward rather than lifting.

* Stop frequently to rest. Five minutes rest for every 15 minutes of shoveling is recommended.

* If you experience any pain in the chest or arm, shortness of breath or profuse sweating, stop shoveling immediately and seek immediate medical attention.

* If conditions are icy, spread salt or sand over the area to avoid slipping and falling.