Although the hands are the main connection we have with the golf club, the arms play a major role connecting our hands (small muscles) with our body (large muscles). When learning golf from a qualified professional, you can reference the arms and their position during the swing. In a mechanical sense, the arms act as a lever in the golf swing.
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In order to be a proficient golfer, you must have an efficient golf swing. One major way is to have great "connection" throughout the swing. What this means is having both your upper arms in contact with the area between your armpits and the outer pectoral muscles. By staying connected, your hands and the grip will stay closer to the centre of your body. By keeping your hands in front of your chest longer, you encourage better body pivots on the backswing, downswing and follow through.
Width of Arc
"Width" is a major checkpoint for both distance and consistency in golf. Width is obtained by creating a large radius between your sternum, or body centre, and the handle of the golf club. For a right-handed golfer, the left (or lead) arm should be extended straight to maximise width. The right (trail) arm should also be extended slightly to keep the hands in front of the chest and maintain your connection. If the trail elbow bends too much, you become disconnected and lose your width. For highly skilled golfers, a tendency is to become too "narrow," or lose the width on the downswing, creating pushes to the right and quick hooks to the left.
One of the most popular terms with golf instructors is "swing plane." Swing plane is influenced greatly by the arm angle. A lot of great teachers will also refer to "left arm plane." A desirable position for the modern golf swing is to have the left arm at the same angle as the shoulder tilt at the top of the backswing. Too flat an arm swing at the top will promote a laid-off position where the shaft is pointing left of target. A vertical arm swing leads to an "across" position where the shaft points right of the target. Understand that the wrist position also influences swing plane greatly.
"Make sure your left arm is locked." That is a lesson that has been taught too many times, usually to junior golfers. The fact is, there are many great golfers who have some bend in their left elbow during their swing. Some have bend in their elbow on the backswing (Andres Romero from Argentina), some bend their elbow on the downswing (South African Retief Goosen) and at least one player, England's Lee Westwood, bends his elbow through impact and into the follow-through. A better lesson might be that you should let your arm lengthen as it swings, like a rope.
Rhythm is important to golf, and by allowing your arms to swing freely you will be able to improve your rhythm and balance. One lesson a beginning or high-handicapped player might take from his golf pro is to let your arms swing your body. Most novice golfers try to do too much work with their body (big muscles) and do not let their arms swing the club. In order for the club to rotate properly, the arms must be well-synchronised with the body motion.