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.
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.
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.
The triceps is the antagonist, and its action opposes that of the agonist. The triceps brachii originates on the back of the scapula and humerus, and inserts on the back of the ulna in the forearm.
Due to these attachments, the triceps is stretched during forearm flexing. Stretching the muscle causes the triceps muscle to contract and, thus, slow flexion. It's important to note that the antagonist contraction is minor in comparison to the agonist contraction, and therefore it doesn't prevent the action of the agonist. Rather, antagonist contraction controls the movement by slowing it down and making it smooth.
The antagonist action helps control the muscle movement.
Agonists and antagonists are always functional opposites. Additionally, these muscles switch roles with opposite movements. Let's take a look at an example. The triceps brachii becomes the agonist - while the biceps brachii is the antagonist - when we extend our forearm.
A synergist is a muscle that enhances the action of the agonist. For example, the brachialis is a synergist of the biceps brachii during forearm flexion. The brachialis originates on the humerus, and it inserts on the front of the ulna. As these attachments of the brachialis are similar in nature to those of the biceps brachii, so is its action. Oftentimes, synergist muscles are needed to get a particular action started.
In summary, skeletal muscles are attached to bones on each end by tendons. The origin is the fixed attachment, while the insertion moves with contraction. The action, or particular movement of a muscle, can be described relative to the joint or the body part moved.
Groups of muscles are involved in most movements and names are used to describe the role of each muscle involved. Agonists, or prime movers, are responsible for the bulk of the action. Antagonist contractions are opposite that of the agonist and serve to control the action. Finally, synergist muscles enhance the action of the agonist.
At the end of this video, you will be able to:
- Differentiate between origin and insertion, as well as proximal and distal
- Explain how agonists, antagonists and synergists work together to control muscle movement