Contraction of Muscles

Skeletal muscles attach to bones via Tendons. The older type of naming muscle would call one point of muscle attachment the Origin, the other point of attachment the Insertion.

Muscles produce movements by contracting, which is actually a shortening. The Type of movement which occurs depends upon the Mobility of the Joint and on Where the Muscle is located in relation to the Joint. There are 2 types of Muscular Contractions which can take place:

1) ISOMETRIC CONTRACTION - Here, the muscle develops tension without changing its length. This type of contraction stabilizes and fixes a joint in a certain position;

2) ISOTONIC CONTRACTION - Here, a contraction occurs which will change the length of the muscle fibers. This will produce MOVEMENT. Isotonic contractions can be broken down into 2 types:

A) CONCENTRIC CONTRACTIONS - This type of contraction occurs when the muscle tissues become SHORTER. Ex: When you lift up a spoon towards your mouth, your Biceps Brachii muscle concentrically contracts to allow this movement to occur;

B) ECCENTRIC CONTRACTION - This type of contraction occurs when the muscle tissues LENGTHEN. The purpose here is to STABILIZE the movement of the Concentrically contracting muscle. For example, in the Example cited above, while the Biceps Brachii is producing the movement of FLEXION at the Elbow Joint, the Triceps Muscle is Eccentrically contracting to stabilize the Flexion movement.

When a muscle contracts Concentrically (i.e., shortens) movement occurs. Where the muscle(s) attach, will determine what type of movement will occur. For example, the biceps brachii muscle attaches into the anterior arm at specific sites. The Primary action of the biceps muscle is Flexion of the elbow. While performing this motion, there is a muscle or muscles which will oppose (try to stop) this motion. In this example, the Triceps brachii muscle will be the ANTAGONIST muscle. This type of relationship is noted throughout the body. In the lower extremity, the prime mover for knee extension are the Quadriceps group of muscles (Vastus medialis, Vastus intermedius, Vastus lateralis, and Rectus femoris). The antagonist group of muscles are the Hamstrings (Biceps femoris, Semimembranosis and Semitendinosis.)