One example of a pair of muscle movements is flexion and extension.
Flexion refers to the movement of a limb that decreases the angle between two bones, such as when you bend your elbow or knee. This movement is typically caused by the contraction of the flexor muscles, which are located on the anterior (front) side of the limb.
Extension, on the other hand, refers to the movement of a limb that increases the angle between two bones, such as when you straighten your elbow or knee. This movement is typically caused by the contraction of the extensor muscles, which are located on the posterior (back) side of the limb.
Together, flexion and extension work as a pair to allow for the full range of movement in a joint. The muscles that are responsible for flexion are located on the anterior side of the limb, while the muscles that are responsible for extension are located on the posterior side. This pair of muscle movements allows a limb to move through a complete range of motion, from fully flexed to fully extended.
For example, in the elbow joint, when you want to bend the arm the biceps muscle on the front of the arm contracts and flexes the elbow, this muscle movement is flexion. Then when you want to straighten the arm the triceps muscle on the back of the arm contracts and extends the elbow. This movement is extension.
In summary, flexion and extension refer to two opposing muscle movements that work together to allow for a full range of motion in a joint. Flexion is the movement of a limb that decreases the angle between two bones, whereas extension is the movement of a limb that increases the angle between two bones.