If the pain is felt posteriorly in the left shoulder, capsular tightness or capsulitis is likely. If pain is felt during the downstroke, an underlying cause may be scapular lag, where the scapula does not keep up with the movement. Primarily, the serratus anterior and the rhomboids should be strengthened. If pain is felt with follow-through at the left shoulder, consider posterior labral or posterior rotator cuff impingement. Too much leg flexion may lead to a flat swing.
Top golfers have similar swings.
In general, shorter golfers have flatter swings that taller golfers. Weight shift with change of club is minor. The head remains stationary with no more than two inches of movement during the swing. The backswing and downswing are not the same. The downswing is outside of the backswing.