This stroke uses the muscles in the lead shoulder. Contracting muscles at the front of the shoulder pull the arm across the chest. Contracting muscles at the back of the shoulder pull the arm back to its start position, and then onward into the follow-through.
To get a sense of the motion, stand straight and hold your arms straight out in front so they are horizontal to the ground and ninety degrees to the spine. Now, move both arms to the left, and then to the right. To move the arms toward the centre of the body, muscles connecting the upper arm and the shoulder contract, pulling the upper arm toward the centre of the body. To move the arms away from the centre of the body, muscles connecting the upper arm and the shoulder contract, and pull the upper arm toward the back.
The trailing arm is essentially along for the ride. The reason has to do with body geometry. As the lead arm moves across the body, the distance between the trailing shoulder and the grip gets smaller. Eventually the distance is so small that the trailing arm must bend at the elbow. This elbow bend creates unpredictability in the stroke. For this reason, one does not want to use the trailing arm as a power source to push the putter along. The reverse occurs in the forward stroke, with the lead arm bending. By this point, however, the ball is already on its way, so the bending does not matter.
The shoulders remain horizontal to the ground throughout the stroke.
Care should be taken to not move the lead shoulder socket while moving the upper arm in the lead shoulder socket, particularly toward the end of the range of motion of the upper arm in the shoulder socket. In our daily lives, we often move both the upper arm in the shoulder socket and the shoulder socket together. One can tell whether the shoulder socket is moving by sensing the muscles that are contracting. Muscles that move the upper arm in the shoulder socket are close to the shoulder socket. Muscles that move the shoulder socket are in the chest.
To practice the stroke, putt with only the lead arm. While it may feel awkward at first, one should eventually get the feel of the stroke.
The graphic below illustrates key aspects of the stroke.
In this stroke, only two muscles groups are involved, specifically those that are attached to the upper arm and shoulder socket. They contract to pull the lead upper arm to centre of the body on the back stroke and away from the centre of the body on the down-stroke and follow-through.
Only the muscles in the lead shoulder are involved, so the degree of coordination is high.
The shoulder joint is a ball and socket joint, which allows a wide range of movement. In this stroke, the joints will not provide automatic control to the stroke through limited range of movement.
The shoulder muscles are of medium size.
With this stroke, the shoulder muscles will be moving both the putter and the arms. There is more mass being moved than with the wrist stroke.
Eye sockets are still, since only the arms are moving. The head, spine and lower body remain motionless.
The rotation plane is vertical when the player leans forward, lets the lead arm hang straight down, executes the stroke from this initial position, and lets the shoulder muscles move the arm across the body on the back stroke and away from the body centre on the forward stroke. With a vertical plane, the putt will be on line with proper alignment and a stroke that goes directly away from and toward the target.
The rotation radius is the distance from lead shoulder socket to the ball. This distance will be directly related to the length of the putter, the length of the lead arm, the extent to which the lead hand is down the putter grip, and the angle between the putter and the arm. The rotation radius will be considerably larger than with the wrist stroke. In practical terms, the larger circle radius in combination with the vertical rotation plane will mean the putter will stay closer to the ground on the back and forward strokes.
The rotation centre will remain steady in the properly executed stroke.
The comments here are the same as for the wrist stroke. The key aspect of the stroke is achieving the vertical rotation plane by leaning forward, letting the lead arm hang through gravity to the vertical position, and executing the movement from this vertical position. Whether the eye position is directly over the ball will depend on a number of factors, but particularly the amount of forward lean and the angle between the arm and the club.
The length of the putting stroke will be directly related to the amount of rotation of the upper arm in the shoulder socket. A 45 degree rotation is possible and similar to the rotation in the wrist stroke. However, the rotation circle radius will be larger, and this will allow a longer stroke.
The amount of forward leaning is not excessive, so the stroke should not cause problems in terms of back health.
Here are some ideas about performing the stroke:
The lead upper arm stroke has several strengths. From a muscular point of view, the stroke is simple, and therefore offers the potential for consistent results. It delivers a vertical plane and the potential for on-line putting. It presents no significant problems in terms of back health or eye alignment over the target line. There is ample rotation potential in the stroke; however, the lead upper arm stroke does not generate a lot of acceleration. The stroke will rely on rhythm and momentum. This may be positive, since rhythm and momentum reduce the potential for the "yips".