Anatomy Atlas Part 18 – Arm Muscles

I mentioned that hands are a marvel – and so are arms. However the muscle structure is a bit weird.

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If you have ever wondered why biceps are called biceps, now you have the answer. The musculus biceps brachii splits into two parts on the upper end  and each is attached to a different part of the shoulder-blade. Whilst it its the most prominent muscle and its development is seen as a sign of strength, biceps is not the strongest flexor in the arm. That is in fact musculus brachialis which lies underneath, connects to ulna it and generally is not seen very much.

Professor Kos mentioned that this arrangement of these two muscles leads to one peculiar thing – flexing of the arm can exert more force when done palm up, than when palm down. Why? Because when the palm is directed down, the musculus biceps has its load bearing tendon wound around the radius to which it is connected. Therefore it cannot flex without also trying to turn the hand palm up.

So when lifting things by flexing your arm palm-down, only two muscles – m. brachialis and m. brachioradialis – can exert force, whereas palm up the m. biceps can join for more strength.

Why is it like this I do not know, but had it been designed, the engineer would deserve at least a pay cut.

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