I've been trying to make a simple simulation of the human body executing some motions in Mathematica (the details are irrelevant), but I need to know some more information about muscles and how they exert forces. To simplify the problem/question that I am asking, take just the bicepts for example. Suppose that you have just the chest, upper arm, and forearm attached, and ignore the rest of the body (and by that, I mean either remove or treat it as rigid, with the exception of the joints in question, which would therefore be just the shoulder and elbow joints).

One relevant idea that I should point out is that I'm simulating the guy basically jumping, and, whilst in the air, your body is in an inertial frame, meaning that Newton's Third law applies. Under the assumption of a constant moment arm, the usual simplifying assumptions that are made here, etc.. (actually, see the hyperlink below), does contracting JUST your bicepts cause an equal and opposite force at the shoulder, or just the elbow, and is this typical of muscles in the human body?

When the bicepts contract, if they're attached to the shoulder/chest they will (theoretically) pivot the upper arm towards you, while if they're attached to the upper arm they may pivot it backwards, the other way (I think). If they're attached to the upper arm, then the diagram here is essentially what I'd use, but if the muscles went all the way through to connect I'd have to basically "add" an additional segment to the diagram depicted in the link, and the muscle would pass directly over the upper arm, instead, for the purposes of contraction, attaching your forearm directly to your shoulder. This is why I think it's a problem of where the muscles seem connected.

Which of these is a more accurate representation of how human muscles accurately attach things, and therefore exert equal and opposite forces when they contract (and is what I've said so far even correct??)? Alternatively, how accurate is the image depicted in the hyperlink in the above paragraph?

Thank you in advance, so so much

Added TLDR; does (-from a conceptual viewpoint-) flexing purely your bicepts function to bring your forearm and UPPER ARM closer together, or is it really bringing your forearm and SHOULDER closer together? Is this how muscles work in general?

  • 1
    $\begingroup$ Does it help you to envision muscles (with tendons) as springs? The third law applies to the forces at the two connection points, like with any spring. $\endgroup$
    – Bryan Krause
    Jan 5, 2021 at 19:11
  • $\begingroup$ @BryanKrause kind of, I think that's a clarifying question though. I guess that what I'm asking is more like where the springs are attached; for example is the equal and opposite force at the other end of the spring exerted on the upper arm or shoulder; one moves the shoulder, the other moves the upper arm, too. $\endgroup$ Jan 5, 2021 at 21:45
  • $\begingroup$ Muscle groups can be really complex, but individual muscles really aren't. I think you just need an anatomy reference and look where the tendons are anchored for any particular muscle you're looking at. It won't be attached to the "upper arm" or "shoulder" but to a specific place on a specific bone which will produce a specific degree of motion. $\endgroup$
    – Bryan Krause
    Jan 5, 2021 at 21:49
  • $\begingroup$ For major muscles you can probably find enough just on Wikipedia. $\endgroup$
    – Bryan Krause
    Jan 5, 2021 at 21:50


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