Predicting a muscle's action from the joint it crosses
A muscle can only pull, so its likely action at a joint can be predicted from the joint it crosses and the side of that joint on which it lies.
Remember
A muscle can only pull, so to predict its action, find the joint it crosses and which side of the joint it lies on: a muscle in front of the knee extends it, and one behind the knee flexes it.
Memory aid
a muscle is a rope, not a rod
You can pull on something with a rope, but you cannot push with one, and a muscle behaves in the same way.
A skeletal muscle (the type attached to bones) works like a rope, not a rod. When it is activated, a muscle will try to shorten. That creates tension, a pulling force, and the tendons (the strong cords of connective tissue joining muscle to bone) carry it to the muscle's attachment points. Nothing in this process can generate a push, so a muscle can only pull.
This one principle lets you work out what a muscle does. First identify the joint it crosses. Then ask on which side of the joint its line of pull (the direction in which its force acts) lies, relative to the joint's axis (the line about which the bones turn). A muscle in front of the knee pulls the tibia up towards the femur and straightens the leg, which is extension. A muscle behind the knee does the opposite and bends it, which is flexion. So the quadriceps at the front of the thigh extend the knee, and the hamstrings behind it flex the knee. The knee is mainly a hinge joint (it opens and closes like a door hinge), so this front-or-behind rule predicts its main actions.
The attachments have conventional names. The origin is the less mobile attachment, usually the more proximal one (nearer the trunk). The insertion is the more mobile attachment, usually the more distal one. Which end actually moves depends on which bone is fixed, so the names describe the attachments, not the direction of movement.
The prediction gives the direction of the turning effect, called a moment, rather than a guarantee of movement. An opposing load or another muscle may balance it. In an isometric contraction (tension that holds a joint still against an opposing load), the muscle does not need to shorten at all.
Key facts
- Pull only
- a muscle can only pull on its attachments and cannot push.
- Both ends
- the force is transmitted to both attachments equally.
- Which end moves
- this depends on which attachment is fixed, and the names origin and insertion do not change.
- Method
- identify the joint the muscle crosses, then decide where its line of pull lies relative to the joint axis; at a mainly hinge joint like the knee, that means in front of or behind the joint.
- At the knee
- a muscle anterior to the knee extends it, as the quadriceps do, and a muscle posterior to the knee flexes it, as the hamstrings do.
- Limit of the rule
- the prediction gives the direction of a muscle's moment, not a guarantee of movement, because an opposing load or another muscle may balance it.
Exam traps
- Calling an attachment the origin does not mean it is the end that moves, because which end moves depends on which part is fixed, and the origin and insertion keep their names.
- A muscle does not push bones apart at a joint, because a muscle can only pull: a bone moves away from another when a muscle on the opposite side pulls it that way, or when gravity or another force moves it.
- A muscle does not pull more strongly on the end that moves, because muscle tension is transmitted equally to both attachments.
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