Stress fractures — small cracks in bones, most often in the feet and lower leg — are common injuries in both serious and recreational athletes. Here are trainers' tips on protecting yourself:

Alternate your activities. Stress fractures are overuse injuries, caused by repeated shock to the same body part. Balance high-impact exercise such as running and aerobics with low-impact choices like swimming and biking.

Don't do too much. Many stress fractures occur with sudden increases in activity. Gradually build up time, speed and distance over a period of weeks. For runners, a general rule is no more than a 10 percent increase in distance per week.

Wear the right shoes. Replace old, worn sneakers and avoid stiff, uncomfortable footwear. Aim to buy shoes designed for the activity you do most.

Be aware of risk factors. People who regularly run or play sports on hard surfaces such as asphalt are more prone to stress fractures. So are runners, dancers, gymnasts and tennis and basketball players.

Do some strength training. Stronger muscles are better able to absorb shock and protect bones.

Take pain seriously. If you have discomfort during activity, swelling and tenderness, apply an ice pack, elevate your foot above heart level and rest for a few days; otherwise you risk a complete bone break. If the pain lingers, see a doctor.

Change your gait. There's some evidence that runners who slightly shorten their stride can lower their risk of a stress fracture. If possible, work with a trainer or running coach.

Eat the right foods. Calcium and vitamin D are especially important to bone strength, but the usual advice holds: plenty of fruits and vegetables, lean proteins, whole-grains and low-fat dairy products.