Linear Motions
Now let’s look at a different component of movement. So far, we have been looking at rotations, now let's look at the linear motions. First let's clarify each type of movement.
Linear and Rotational Motions: What are they and why are they important?
Imagine someone standing in a room. When the person moves across the floor, but continues facing the same direction, that is linear motion. When the person stays in the same place in the room, but turns and is now facing a different direction, that is rotation.
Can they happen at the same time?
Absolutely. The person can do either or both at the same time.
But what about when the person walks in a curved path across the room? Is that still linear motion?
Yes it is, though the confusion is understandable. First, let’s assume the person is facing the same direction the whole time, so the person is not also rotating – we are only talking about moving across the room in a curved path.
Linear motion is measured instantaneously; meaning that the linear velocity of the person is how fast and what direction he is moving at that exact moment. At any given time, he is going a certain speed and a certain direction – that is his current linear velocity. It may change the next time he takes a step, and when the person is done moving, we can measure the total linear distance moved in all directions (forward/backward as well as left/right.)
Sometimes the term translation is used instead of linear to avoid this confusion. So linear motion or translation means a change in location, while rotation means a change in the direction something is facing.
How does the body create motions?
Virtually all movements of the body begin as rotations, because with very few exceptions, the joints of the body rotate only – they do not translate. The shoulder allows for rotation in all directions, but it does not allow the arm to move in a straight line – to do this, the arm would have to separate itself from the shoulder.
So can we ever move a body part in a straight line?
Yes. We can move the hand in a straight line to pick up a golf ball. We create this straight line motion through a careful combination of rotations – we rotate the shoulder, the elbow and the wrist, all just the right amount at the right time, and the hand can then move in a straight line. So even though the end result is to move the hand in a straight line through the room (a global motion), we control it with a series of joint rotations (relative motions). All body movements are created by a careful orchestration of relative motions, that is why looking at relative motions and not just global motions, can be so important in the understanding the golf swing.
Pelvis Linear Motion
Let's look specifically at how much the pelvis and upper torso move or slide, toward the target. Most golfers do shift toward the target during the downswing, but how much can vary substantially and have a considerable effect on impact.
In the first example we're going to focus on side to side motion of his pelvis. Very early in the backswing, this golfer shows a small shift of about half an inch away from the target. By the time his lead arm is parallel he has already shifted back toward the target to where he was at address. You can see that he begins a more aggressive slide late in his backswing, before his pelvis starts to rotate toward the target. By the time he begins his downswing, his pelvis will be an inch and a half closer to the target than it was at address and he will continue moving toward the target until club parallel.
At that point, his pelvis will have shifted three inches closer to the target and he will stay there through impact. This golfer is an example of a reasonably good side to side movement of the pelvis. You may have noticed, however, a lack of vertical movement in his pelvis. He lowers his pelvis about an inch through his backswing and brings it back up to its height at address at impact.
In our second example, you'll see a golfer with a much more dynamic vertical movement pattern. Once again, you can see a small early shift away from the target in the backswing. In this case, right about an inch. Like the first golfer, he starts shifting back toward the target at this point in his backswing, so that he too is about an inch and half closer to the target at the top of his swing. And about two and a half inches closer at impact. However, this golfer also displays a noticeable drop in the second half of his backswing.
Early in his downswing his pelvis is an inch and a half lower that it was at address. He will then start driving his pelvis up aggressively, almost five inches by impact. Both examples shown here are good golfers who show similar lateral pelvis motions and both do a good job of shifting the pelvis toward the target before the pelvis starts rotating in the downswing. However, the second golfer is able to generate substantially more club head speed partly because of the vertical motion of his pelvis, which is a result of better use of the combination of horizontal and vertical forces that he applies to the ground.
| Address | Top | Impact | |
|---|---|---|---|
| Pelvis | Designated as the Zero Point | 1 inch toward or away from target | 2-6 inches toward target |
| Torso | Designated as the Zero Point | 1 inch toward or away from target | 1 inch toward to 2 inches away from target |
| Address | Top | Impact | |
|---|---|---|---|
| Pelvis | Designated as the Zero Point | 1 inch back to 2 inches forward (toward ball) | 0-2 inches toward ball |
| Torso | Designated as the Zero Point | 0-2.5 inches toward ball | 0-3 inches back away from ball |
| Address | Top | Impact | |
|---|---|---|---|
| Pelvis | Designated as the Zero Point | 0-2 inches down | 0-2 inches Up |
| Torso | Designated as the Zero Point | 0-2 inches down | 1 inch down to 2 inches up |
Kinematic Takeaway #1: What if I see the pelvis sway away from the target on the backswing?
Excessive or continued sway (linear movement) of the pelvis away from the target in the backswing can create several problems in the down swing. It is often either caused by or accompanies restricted pelvis and torso rotation, and it almost always leads to a poor pressure shift to the lead (left) side. The end result will typically be a lack of ability to create club head speed. Also, an excessive sway often leads to an overly steep angle of attack and a path that is out-to-in (to the left for a right-handed golfer).
Kinematic Takeaway #2: What if I see the head and upper torso lifting during the backswing?
Extra rise, or lift, of the head is typically not just the head moving, but the torso lifting as well. The head and torso lifting in the backswing is often caused by a lack of rotational flexibility, especially through the upper part of the spine. It is also a possible indicator that the golfer is not able to maintain good stability on one leg. When lifting up the torso and head, the golfer will typically not properly shift pressure to the trail (right) leg, which will limit how well the golfer can then shift pressure to the lead side, which is important for generating power.