Joint Moments

Question 1

What is the definition of the center of gravity?

Answer 1

The center of gravity is the point where the mass of the body is concentrated. It is the point around which gravity acts.

Question 2

What is the definition of line of gravity?

Answer 2

The line of gravity is an action line and direction of force of gravity on a person and/or segments.

Question 3

What is a moment?

Answer 3

Moment is a rotation. It is dependent on forces. A ground reaction force can produce a joint moment. Rotation around the center of the joint. The rotation we talk about is the bone above the joint and the bone below the joint.

Question 4

What is a joint moment in the body?

Answer 4

In the body, it is the tendency of a segment to move one way or the other around its joint axis.

Question 5

Define an internal moment arm?

Answer 5

The internal moment arm is the distance between a muscle acting on a joint and the joint itself.

Question 6

Define an external moment arm?

Answer 6

The moment arm is the perpendicular distance between the line of action of the force and the center of moments.

Question 7

Define the base of support?

Answer 7

The base of support (BOS) refers to the area beneath an object or person that includes every point of contact that the object or person makes with the supporting surface.

Question 8

What can produce joint moments in the body?

Answer 8

• Produced by muscles: named by the motion that is occurring
• Produced by external forces
• Produced by gravity. Where the joint is pulled will depend on where the line of gravity is in relation to it.

Question 9

What is the direction of the joint moment determined by?

Answer 9

Where the line of gravity falls in regards to the joint axis/center of rotation (CoR)

Question 10

Why is the direction the segment would move generally named by the motion that would occur in the sagittal plane or frontal plane?

Answer 10

Gravity only pulls down, so it doesn’t affect the transverse plane.

Question 11

In proper standing posture, where is the LOG at the hip?

Answer 11

It is behind the center of the hip joint.

Question 12

In proper standing posture, where is the LOG at the knee joint?

Answer 12

It is in front of the center of the knee joint.

Question 13

In proper standing posture, where is the line of gravity at the ankle joint?

Answer 13

It is in front of the center of the ankle joint.

Question 14

What counteracts the pull of gravity?

Answer 14

Our posture muscles keep us from falling towards the line of gravity.

Question 15

The line of gravity is in front of the ankle joint. How does this affect the ankle and muscles?

Answer 15

The tibia and fibula will rotate anteriorly which would put the ankle into dorsiflexion. The gastroc and soleus are always fighting this. They are always on at a low level.

Question 16

What is postural sway?

Answer 16

Postural sway is movement of the COM over the BOS. This happens all the time in everyone. You’re always swaying slightly forward and backwards because the gastroc and soleus are fighting gravity. In a healthy person, you can’t see it or feel it. If someone has bad motor control, you will be able to see them move and adjust.

Question 17

The line of gravity passes in front of the knee joint. What kind of joint moment does this promote?

Answer 17

An extension moment. The femur is constantly being pulled anteriorly towards knee hyperextension. This, along with the screw home mechanism, allows us to stand without much quad activity.

Question 18

How do weak quads change the joint moment of the knee?

Answer 18

If a patient has weak quads and they can’t extend their knee fully, the LOG is shifted posterior to the knee joint. This promotes knee flexion and can cause the knee to buckle. This is why you need to block a stroke patient’s knee and help them extend.

Question 19

The line of gravity is behind the center of the hip joint. How does this affect the hips?

Answer 19

There is a naturally occurring hip extension moment. This is helpful because we want our hips to be extended.

Question 20

How do people compensate for tight hip flexors?

Answer 20

Most people have tight hip flexors, so we have to help gravity extend our hips with out gluts. If you don’t have control over your glutes, you can let gravity extend your hips and then just sit with your Y ligament holding you up. This ligament is very strong, and this won’t cause a problem for it. People who have lower body paralysis an do this.