Torque and Force Couples

Original Editor - Ashmita Patrao Top Contributors - Khloud Shreif, Ashmita Patrao and Vidya Acharya

Torque

Torque (T) is a measure of the rotatory force with which a segment moves. For torque to be acting at any segment, we need two forces that are equal in magnitude but opposite in direction. This concept is applied to the human body in the understanding of force couples. The perpendicular distance from the the line of action of a force to the rotation axis of joint. Torque is also called a moment of force and is expressed as

Torque= Magnitude of force (F) * Moment arm (Ma).

It is expressed in the direction in which it acts. For instance, a force in the clockwise direction is given a -sign, and 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 that expressed in newton, 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 centering the head of the humerus without the centering of the head, the deltoid can translate the head superiorly.

Force Couples

A pair of forces that are equal in magnitude, opposite in directions acting on opposite sides of an axis of rotation that cause the object or joint to produce rotatory force but without linear translation. These forces are usually equidistant to each other.

Force couples in the shoulder

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

This force couple produces the most torque. 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.

Serratus anterior and Trapezius

The serratus anterior (SA) muscle primarily extends and rotates the scapula upward during arm movement, acting in conjunction with the acromioclavicular (AC) and sternoclavicular joints (SC). The lower trapezius supports this upward rotation and helps maintain the subacromial space, which is critical for shoulder mobility.

The serratus anterior and trapezius muscles (especially the middle and lower muscle fibers) work together as a pair of forces to promote upward rotation of the scapula and balance each other, ensuring stable and efficient shoulder mechanics during upper arm, abduction and other movements.

Force Couple in the Pelivs

Anterior Pelvic Tilting

During the anterior pelvic tilt, the hip flexors (such as the iliopsoas) and lumbar extensors (such as the erector spinae) become tight or overactive, pulling the front of the pelvis downward and pulling the lower back into an exaggerated curve. While, the abdominal and gluteal muscles are often weak or inactive and unable to counteract this pull.

Posterior Pelvic Tilting

With the posterior pelvic tilt, the abdominal muscles and the gluteal muscles become the dominant muscles over the hip flexors and back extensors, that pull the pelvis backward and flatten the natural curve of the lower back.