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Definition of the main differences between muscle origin and insertion.
Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be discussing one of the basic things that you need to know about a muscle when we're studying anatomy – the origin and insertion of the muscle.
When studying anatomy, one of the first things that we learned about muscles are their points of origin and insertion but what does this really mean? What does it mean when we talk about the origin or the insertion of a muscle? Well, in this short tutorial, we'll explain what the terms origin and insertion really mean when describing skeletal muscles that are attached to the bone. And skeletal muscles can be defined as the muscles that have two or more attachment points and these muscles attach to bone via tendons comprised of strong fibrous collagenous tissue. And skeletal muscles can be seen in any of these muscles illustrated covering this right shoulder on our image on the right.
So let's first begin with the term origin. In anatomy, there are usually two or more ends to a muscle – one end that is located more proximal to the center of the body and one that is located more distal. And of these two ends, the end that is more proximal is often referred to as the origin. The origin often does not move during contraction. In other words, the origin of a muscle is the point of attachment of the muscle that is both the most proximal part of the muscle and the muscle that remains relatively fixed during muscle contraction. So, if we're looking at this image of the short head of the biceps on our right here, the part that is considered the origin is the part that is highlighted in green as the end that is attached to the acromion is the part that is the proximal end of the muscle. It's also relatively fixed during contraction.
Since the origin of a muscle that is more proximal to the center of the body is the origin, it's safe to say that the opposite end – the insertion point – is the end that is more distal to the center of the body. And since we decided that the origin was the end that does not move during contraction, we can also infer that the insertion of the muscle is the attachment site of the muscle that does move during contraction. In other words, the insertion is the end of the muscle that is attached to the movable bone. So as we can see in the image on our right with the distal end of the biceps brachii highlighted in green, we have our insertion of the muscle which inserts into the radial tuberosity.
So up to this point, we've seen the origins and insertions of muscles but now let's have a look at what these means in terms of muscle movements. As we've already seen, the origin is the attachment site of the muscle that does not affect the movement during muscle contraction while the insertion is the attachment site to the movable bone. Now when muscles contract, they usually act as either agonists, antagonists or synergists. But what do these terms mean? We'll have a look at the definition of each of these muscle types over the next few slides.
During muscle contraction, muscles usually work in pairs or groups to effect the movement. The agonist, also known as the prime mover, is the muscle that causes the primary movement. For example, when we flex the forearm, the biceps brachii muscle is the agonist in this case as it causes movement of the forearm or flexion of the forearm when it contracts.
With regards to antagonists to borrow from Newton's third law of motion, we know that for every action there is an equal and opposite reaction. With that in mind, the antagonist is the muscle that works in opposition to the agonist and using with the examples we just saw, when the forearm is flexed, the antagonist muscle would be the triceps brachii which would return the limb to its initial position by working in opposition to the biceps brachii. And often agonist and antagonist muscles work in pairs. That is to say, one muscle facilitates flexion while the other muscle facilitates extension. And we can see a good example of muscles working together as agonists and antagonists by looking at the flexor and extensor muscles of the forearm, the hand or of the lower limb. But do note that not all muscles work in this way as some of them are synergists as we'll see on the next slide.
So as we just mentioned, muscles can also work as synergists during movement to stabilize and control movements keeping them within the desired range of motion and these muscles act on movable joints and they're also known as neutralizers or fixators. They cancel out extra motion from agonists and stabilize the origin of the agonist making sure that the force of the movement generated is within the desired plane of motion. And in this image on the right, we can see the brachioradialis highlighted in green, and the brachioradialis is well known for its role as an assistant to the biceps brachii especially in movements of pronation and supination.