July 29, 2011 -- What's the difference between the way a golf pro swings a club and the way a duffer does it?

Many amateurs have swings that can land more than their golf balls in the rough, according to a biomechanical study of 10 professional golfers and five amateurs. Those flawed golf swings can also cause low back pain and elbow problems.

Stanford University researcher Jessica Rose, PhD, and colleagues attached 42 reflective markers to the golfers' bodies and three to their 5-irons. The golfers then stood on a special plate that recorded the forces in their feet. An eight-camera system captured their every movement as they whacked a reflective practice ball off a mat of artificial grass.

'We examined which factors were most highly consistent among the professional golfers and what contributed the most to hitting the ball hard," Rose tells WebMD. "There were some biomechanical factors that varied among the pros and some that were indistinguishable among pros. The factors that didn't differ were the motions that we thought essential to a proper swing."

The essential components of a professional golf swing were:

X factor: The difference in horizontal rotation between the hips and the shoulders. At the top of their swings, pro golfers' shoulders rotated 55-60 degrees beyond the rotation of their hips. Amateurs' tended to over-rotate, exposing them to back injury.

S factor: The tilt of the shoulders right after impact. Pro golfers' lead shoulders tilted up quickly at impact to a 48 degree angle; amateurs were slower to tilt and tilted less.

Clubhead height. Pros dipped their clubheads only slightly at the top of their swings, just before their hips began their downswings. Amateurs dropped their clubheads too low at the top of their swings, putting extra torque on their wrist and exposing them to injury.

O factor -- the tilt of the hips during the swing -- varied quite a bit among the pros.

"There is a lot of variation here in what you can do and still have a good shot," Rose says. "The S and X factors need to be very precise, but not so much the O factor."

These factors may help explain why, according to the American Orthopaedic Society for Sports Medicine, some 40% of amateurs suffer an injury while golfing. The most common golf injury is low back pain, followed by elbow problems.

Rose suggests that golfers stretch their muscles before teeing off. The AOSSM recommends that golfers get regular exercise that includes core strengthening as well as stretching all the major muscle groups.

The Rose study appears in the July 29 issue of the Journal of Applied Biomechanics.