If one socket moved up and the other down, the resulting movement by itself would create a partial rotation centred approximately around the mid-point between the shoulder sockets. If the player stood erect, this rotation would be relatively vertical to the ground.
The spine can bend forward and backward, and can tilt to the left or the right It can also twist clockwise and counter-clockwise.
Muscles contractions cause the movements of in the spine. Muscles at the front of the body in the abdomen and chest contract to bend the top of the spine forward, causing the forward bend. Muscles at the back contract to cause the top of the spine to bend backward. Muscles at the side of the body contract to pull the top of the spine to the left or right. These movements are relatively straight forward. The spinal twist is less so.
To get an intuitive feel of the spinal twist, sit on a bench or chair in front of a mirror, rest a club across your upper back below your shoulders (to ensure you are not moving your shoulder sockets), hold it in place with your two hands, and gently try to rotate your shoulders. Keep the club parallel to the ground to ensure that you are rotating your shoulders. Sitting in a chair ensures that your hips are stationary so that any rotation occurs in the spine between the hips and the shoulders. The rotation of the club that you see in the mirror demonstrates the spinal twist.
To figure out what muscles are causing the spinal twist, twist back and forth with a club on your shoulder a number of times. Eventually, the active muscles will fatigue and you will know what muscles are involved.
The spinal twist creates another basic putting stroke. The player grips the putter, leans forward, and twists the spine using the aforementioned muscles. No other muscles contract. This action will cause rotation of the upper body, including the shoulders sockets, around the spine. As the shoulder sockets move, the arms and putter will also rotate around a circle with the same axis in a parallel plane. The arms and club are like appendages fixed to the circle radius that move when the shoulder sockets move. The radius of the rotation circle of the putter head will not necessarily have the same radius as the rotation circle of the shoulder sockets, because of the way the upper arms relate to the rotation circle of the shoulder socket and the angle between the arms and the shaft of the putting club.
While vertebrae in the spine will twist slightly relative to each other, the spinal column stays in the same place.
The rotation will be at right angles to the spine. If the spine is leaning forward so that it is horizontal to the ground, the rotation circle will be vertical to the ground. More likely, the spine will be leaning forward a bit, and the rotation circle will not be vertical to the ground.
While there are primarily three muscles groups involved, these groups are accustomed to working together. For practical purposes, they make up one group.
As there is only one group of muscles involved, there is a high degree of coordination.
When the spine is twisting through the action of the aforementioned muscles, there is relatively little range of movement in the joints. Contract the muscles in the appropriate way and one will get a rotation that is perpendicular to the spine. The spinal twist yields a consistent putting stroke.
The muscles groups involved are relatively large.
The movement involves the movement of the upper body, the shoulder sockets, the arms, and the putting club. Relative to other putting strokes, the spinal twist requires the movement of the largest mass, and therefore has the most momentum.
The eye sockets will not move in aproperly executed stroke, because the spinal column does not change is position. The head sits at the top of the spinal column, and it does not move. Consequently, the eye sockets will also be still.
The rotation plane depends on the amount of forward lean. If the spinal column is horizontal to the group, the rotation plane will be vertical. If the forward lean puts the spinal column at 45 degrees to the ground, the rotation plane will be 45 degrees off the vertical.
There are advantages to a vertical rotation plane. Consequently, the player is likely to perform better with a larger forward lean.
Because the rotation plane is not vertical to the ground, ball position at impact is critical to hitting putts on the intended line.
The rotation centre will be the mid-point between the two shoulder sockets. The rotation circle radius will be the distance from the rotation centre to the ball. This will depend on factors such as the amount of knee bend and forward lean, the length of various body parts, and the length of the putting club. Relative to other putting strokes, the rotation circle radius with the spinal twist will be in line with other strokes, but longer than with the wrist stroke. However, this stroke benefits from a large forward lean, which has the effect of reducing the rotation circle radius. A larger rotation circle radius can mitigate the potential to hit off line putts when the rotation plane deviates from the vertical.
The rotation centre in this stroke is steady throughout the stroke, since the spinal column does not change position while twisting.
An eye position over the ball can be achieved through a combination of adjusting the angle at which the arms fall from a forward leaning upper body, and the angle between the arms and the putting club. In particular, there is no requirement for the arms to hang straight down (i.e. perpendicular to the ground) from the shoulder sockets.
The spinal twist can generate a lot of power. A normal person who should be able to twist the spine 60 degrees in both directions. This translates into a powerful putting stroke.
The forward lean is important to good performance; the more the better. Unfortunately, the greater the forward lean, the greater the risk of back problems.
To get the best results from this stroke, try the following:
This is a reliable, consistent, simple stroke, with limit opportunities for error in the stroke. However, the stroke has a non-vertical rotation plane. To get puts on line, the impact point needs to occur at the precise point in the rotation when the club face is perpendicular to the target line.