trapezius and serratus anterior muscles

Force couple relationship refers to a biomechanical concept in which two or more muscles act together to produce a coordinated movement around a joint. In a force couple, each muscle produces a force at a different angle than the other muscles, but the combined effect of all the forces is a rotational movement around the joint. The muscular forces are often synergistic, meaning the main muscle performs a specific action while the other muscles stabilise the joint to prevent injury or displacement of the bones.

An example of a synergistic force couple relationship is that of the trapezius and serratus anterior muscles acting around the scapula (shoulder blade) to stabilise it during movements of the arm. The upper fibers of the trapezius elevate the scapula, while its lower fibers protract and rotate it medially. The upper fibers of the serratus anterior protract and suspend the scapula, while its lower fibers pull the lower angle of the shoulder blade forward. Together, these muscles perform scapular rotation to allow the arm to be raised above the head.