How do sesamoid bones function as pulleys in our body?



Hi everyone! I couldnt find anything that answered this question, so here goes: Im doing osteology right now and I came across this phrase on Wikipedia:

“Sesamoids act like pulleys, providing a smooth surface for tendons to slide over, increasing the tendon’s ability to transmit muscular forces.”

I think I kind of understand how pulleys work: You use less force over a longer distance, yet the amount of work is the same.

Im confused as to how this is applied on sesamoid bones. I understood they are “simply” embedded into our tendons. How exactly would they be creating a larger distance, as seen by the mechanism of pulleys? And how would that put less strain on the tendons?


If I got it wrong please let me know! (Will flair this under physics since I believe my problem is understanding its biomechanics.) Thank you so much in advance!

In: Physics

The ability for pulleys to multiply force is when you chain them together in a “block and tackle” arrangement. That’s useful, but not the only reason to use pulleys and it’s not what those bones are doing.

The *other* reason to use pulleys is to reduce friction when a “cable” would otherwise rub on something, typically when it changes direction or has a long unsupported span. This doesn’t do the force multiplying thing but, by reducing friction, let’s more force get to the end of the “cable” where it’s needed. If I want 100N at the end of the cable, with good pulleys, I might only need to pull with 101N of force. With no pulleys I might need to pull with 150N to overcome
friction going around corners.

My understanding is that they act as a fulcrum. When you think of a simple machine the sesamoid bone is like the wheel and the tendon is the shaft that slides across the smooth surface of the bone. When multiple pulleys work together the force required to move the load decreases. That’s why often you see a couple tendons with sesamoid bones at joints like the toe.

Moreover, though, is that like the quote says, it provides a smooth surface for the tendon to move across and reduces friction or wear down of the tendon

They aren’t providing mechanical advantage. They are simply allowing force from one direction, to be applied with minimal loss, to another direction. If the tendons were wrapped around a bone or something ( doesn’t exist in the human body to my knowledge) then possibly it could provide a mechanical advantage.

Say you were sitting down, and want to stand up. Your patella for example, the largest sesamoid bone, will translate the force of contraction of your quads, x direction, to your tibia, y direction.

Bam, a pulley which allows you to change the direction of a force, without providing a mechanical advantage. Movement is 1:1

I think it’s better to say the kneecap (a sesamoid bone) is acting as a fulcrum, which is modifying the existing pulley system (muscles, long bones, tendons).

Here’s a [1:30 YouTube video demonstrating the effect ](