Torque and Force Couples

Torque is a measure of the rotatory strength with which a segment moves. For torque to be acting at any segment, we need two forces that have are equal in magnitude but opposite in direction. This concept is applied to the human body in the understanding of force couples. Torque is also called as a moment of force and is expressed as

Torque= Magnitude of force * Perpendicular distance between the 2 forces

It is expressed in the direction in which it acts. For instance, a force in the clockwise direction is given a -sign, a force in the counterclockwise direction is given a +sign. It is also expressed in the motion which occurs.

It is also called a moment of force and is expressed with the force name for instance a torque in the flexion motion is a flexion torque or moment. Likewise, a torque acting in the extension direction is called an extension torque or an extension moment. It is the transverse plane force couple that acts by centring the head of the humerus Without the centring of the head, the deltoid can translate the head superiorly.

Force couples[edit | edit source]

A pair of forces that are equal in magnitude, parallel to each other and in opposite directions constitute force couples. These forces are usually equidistant to each other.

Force couples in the shoulder[edit | edit source]

In the elevation of the arm, there are mainly 2 force couples acting in different planes. In the coronal plane the deltoid and supraspinatus along with the rotator cuff tendons and in the transverse plane the subscapularis act anteriorly and the infraspinatus/teres minor acts posteriorly.

Deltoid and rotator cuff[edit | edit source]

This force couple produces the most torque. With the arm in the overhead position, the deltoid causes an OUTWARD and an UPWARD force on the humerus during the initial part of the motion. The infraspinatus, teres minor and subscapularis pull on the scapula creating an INWARD and DOWNWARD force, maintaining proper positioning of the shoulder.