Using Correct Terminology

An issue that came up recently in class was the use of incorrect terminology to describe muscle actions and joint actions. Many times, people will say things such as "flex" your muscle, or he/she "over-extended" a muscle. When we describe motion, there are two things to look at; how the joint angle changes, and how the length of the muscle changes. To keep things simple, I will use the knee as an example. The primary motions, or joint actions, at the knee are flexion and extension. When a persons straightens his/her knee, this is called knee extension. When a person bends his/her knee, this is called knee flexion. The terms flexion and extension refer to changes in a joint angle, not changes in a muscle. Flexion and extension also occur at other joints, such as the hip, shoulder, and elbow. Now, let's examine what happens to the large muscle group located on the anterior (front) portion of the thigh. This is the quadriceps muscle group. Muscles either shorten, lengthen, or stay the same length. When you extend your knee, such as during the upward phase of a squat, the quadriceps muscle group shortens. This shortening of a muscle under tension is known as a concentric muscle action. When you flex your knee, such as during the downward phase of a squat, the quadriceps muscle group lengthens under tension. This is called an eccentric muscle action. If you were to squat down and hold your knee at a constant joint angle (90 degrees of knee flexion), the length of the quadriceps muscle group would remain constant under tension. This is known as an isometric muscle action. To summarize, terms such as flexion, extension, abduction, and adduction refer to changes in joint angles, terms such as concentric, eccentric, and isometric refer to changes in muscle length. It is correct to say a person hyper-extended his/her knee or elbow, but it is not correct to say that he/she hyper-extended a muscle.