Step up to denser bones.

A 6-month fitness-and-health study in college women turned up an unexpected finding that could prove useful in tailoring workout routines to fight osteoporosis.

Bones react to the pull of muscles attached to them by building up density and strength, notes Sara Arnaud, a retired NASA physician and a coauthor of the new study. That's why trainers routinely recommend that women build bone density in their legs and hips through muscle-strengthening resistance exercises, such as squats, leg presses, leg curls, and calf presses, she says.

It may not always be the best advice. Arnaud's colleague, exercise physiologist Michael T.C. Liang of the California State Polytechnic University in Pomona, divided 139 recruits into three groups. One group followed a resistance-exercise regimen that included three weekly sessions of 30 minutes each. Another substituted step aerobics for resistance exercise. The final group didn't exercise at all.

To their surprise, the researchers found that resistance exercise boosted leg-muscle strength but had no effect on bone density in the lower body. Step aerobics, however, boosted heel-bone density by 3.3 percent and the density of bone in the lower spine by 1.2 percent. The sedentary group showed no bone or muscle improvement.

The acceleration and impact of step aerobics may offer the quickest route to building bone in young women and to preserving it in postmenopausal ones, says Liang, whose team presented the findings in early April at the Experimental Biology meeting in San Diego.

"The message here is that to improve bone, you want high-impact aerobics," he says.--J.R.