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.
If you are a fan of baseball, you know that we are well into the playoffs. My Dad asked me a question the other day that I thought would make a good blog post. His question was how could Edgar Renteria, a shortstop for the San Francisco Giants, continue to play with a torn biceps tendon? This is a good question, and from media reports, it seems Renteria tore the tendon in his left biceps brachii while swinging at a pitch against the Braves last week. The biceps brachii is the large (well, large in some people) muscle located on the anterior (front) part of the upper arm. There are actually two parts to the biceps brachii, as biceps means two heads. In the picture to the left, the number 5 is pointing to the tendon that attaches the long head of the biceps brachii to the upper part of the scapula (shoulder), and the number 9 is pointing to the short head of the biceps brachii, which attaches to the lower part of the scapula. The number 4 points to the common tendon of both heads that attaches to the radius (elbow). The biceps brachii actually has 3 jobs, or functions. 1) It helps with shoulder flexion (raising your arm straight up in front of you), 2) it helps with elbow flexion (bending your elbow), and 3) it helps with supination of the proximal radiounlar joint (holding your palms up). I say it helps with these motions because there are other muscles that cause these motions as well, such as the anterior deltoid for shoulder flexion, the brachialis for elbow flexion, and supinator for supination. It is not clear from the article which biceps brachii tendon that Renteria tore, but since there are other muscles that cause the same motions as the biceps brachii, it is possible to perform without the muscle. Renteria is probably in less pain now that the tendon has completely torn than he was when it was only a partial tear.
