Some golf instructors will tell you the reason that amateurs restrict their body turn during the backswing is because they're afraid of not making solid contact, but there's actually a more typical reason: A lot of amateurs simply can't make a full, athletic turn.
"It's the most common physical limitation I see," says Randy Myers, a Golf Digest fitness advisor who trains several players on the PGA Tour, including Dustin Johnson and Brandt Snedeker. "Even if they have the range of motion to turn farther, a lack of stability or coordination prevents them from doing it." The body moves in three planes: Front to back (sagittal), side to side (frontal) and rotationally (transverse). A big part of the swing takes place in the transverse plane, Myers says, yet most people don't do any exercises to improve their rotation.
The ability to rotate the lower body while keeping the upper body relatively still (or vice versa) is something all golfers need to be able to do. The mid-back region controls upper-body rotation, Myers says, and the hips turn the lower body. Stronger, more pliable muscles around these regions help golfers not just swing faster, they help provide balance and more control of the golf club. From a swing-mechanics perspective, a bigger turn helps eliminate timing issues caused by starting the downswing before the backswing is complete.
Myers offers three exercises (below) that will help you move properly throughout the swing. Master the par exercise before moving on to the birdie and eagle movements.