Muscle Origin and Insertion: Definition and Actions

Did you know that groups of muscles are needed for various body movements? Skeletal muscles attached to bone are responsible for movement and support. Muscles work in groups to produce a particular movement. This lesson describes how muscle origins and insertions dictate the type of movement that occurs when a muscle contracts.
Origin and Insertion
Most skeletal muscle is attached to bone on its ends by way of what we call tendons. As the muscles contract, they exert force on the bones, which help to support and move our body along with its appendages.
In most cases, one end of the muscle is fixed in its position, while the other end moves during contraction. The origin is the attachment site that doesn’t move during contraction, while the insertion is the attachment site that does move when the muscle contracts.
The insertion is usually distal, or further away, while the origin is proximal, or closer to the body, relative to the insertion. For example, one could say the wrist is distal to the elbow. Conversely, you can say the elbow is proximal to the wrist.
Muscular contraction produces an action, or a movement of the appendage. We will use examples to describe how the origin and insertion affect the action of a skeletal muscle.
Action Nomenclature
Muscle contraction results in different types of movement. The particular movement is a direct result of the muscle attachment. Most of these movements are realized when we run. Each of these actions can be described in one of two ways.
The first describes action in terms of the bone to which the muscle is attached or the appendage that is moved. For example, the biceps brachii performs flexion of the forearm as the forearm is moved. The second way to describe a muscle’s action is based on the joint, or the articulation. For example, that same muscle, the biceps brachii, performs flexion at the elbow, in which the elbow is the joint.
One way to describe muscle action is by the bone that is involved.
Action Described by Bone
Muscle Functional Roles
The human body has over 500 muscles responsible for all types of movement. Each of these muscles has a name; for example, again, the biceps brachii and now the triceps brachii, responsible for both forearm flexion and forearm extension, respectively. When movement of a body part occurs, muscles work in groups rather than individually. Working together enhances a particular movement. During that particular movement, individual muscles will play different roles depending on their origin and insertion. These different roles can be described as agonists (or prime movers), antagonists, or synergists.
Let’s take a look at forearm flexion and identify the roles of the different muscles involved. The biceps brachii is the agonist in forearm flexion. An agonist, or as I said before, a prime mover, is the muscle that is primarily responsible for the movement described: forearm flexion. The action makes sense when you consider the muscle’s points of attachment.
The biceps brachii originates on the front of the scapula of the shoulder and inserts on the front of the radius in the forearm. Due to these attachments, contraction and muscle shortening of the biceps flexes the forearm.
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