5.13 Applied Forces

Question 1

What is a joint reaction force?

Answer 1

The net force inside a joint in response to muscle contractions, external loads, and body weight.

Question 2

What causes joint reaction forces?

Answer 2

Muscle pull, external loads (like gravity or weights), and body weight transmitted through the joint.

Question 3

Example: In an isometric contraction of biceps and triceps, what is the joint reaction force?

Answer 3

An equal and opposite force at the joint that balances their pulls.

Question 4

What two components can a muscle force be broken into?

Answer 4

X component (parallel) and Y component (perpendicular).

Question 5

What does the X component of a muscle force do?

Answer 5

Runs parallel to the limb; stabilizes the joint through compression or distraction; does not create torque.

Question 6

Why doesn’t the X component produce torque?

Answer 6

Because its line of force passes through the axis, giving it a zero moment arm.

Question 7

What does the Y component of a muscle force do?

Answer 7

Runs perpendicular to the limb; produces torque by rotating the joint.

Question 8

What determines the torque generated by the Y component?

Answer 8

The size of the force and the length of the moment arm (perpendicular distance to the axis).

Question 9

What’s the quick rule to remember X vs Y components?

Answer 9

Parallel stabilizes, perpendicular rotates.

Question 10

In elbow extension with the arm straight, what does the X component of the biceps do?

Answer 10

Compresses the elbow joint.

Question 11

In elbow flexion, what can the X component of the biceps do?

Answer 11

Distracts the elbow joint (pulls it apart).

Question 12

What always creates the torque in elbow movement?

Answer 12

The Y component of the muscle force.

Question 13

How does joint angle affect muscle force components?

Answer 13

It changes the balance of compression vs distraction and the amount of torque produced.

Question 14

How does joint angle affect external torque?

Answer 14

A longer resistance arm increases torque demand, while a shorter resistance arm decreases it.

Question 15

Example: Why is a straight leg raise harder than a bent-knee raise?

Answer 15

A straight leg has a longer resistance arm (15 inches vs 8 inches), requiring more torque to lift the same weight.

Question 16

Why is bending the knee useful in early rehab?

Answer 16

It shortens the resistance arm, reducing torque demand and making the exercise easier.