Every Sip Costs Your Muscles Seven Cups

Holding a cup of coffee still, arm bent, feels like the easiest thing you will do today. Inside your arm it is not.

Your elbow is a lever with the pivot at the joint. The cup sits in your hand, roughly 30 to 35 centimetres out. The muscles that bend your elbow attach only a few centimetres from the pivot - for the biceps, the peak lever arm measured in cadaver arms is 4.2 to 5.4 cm. To balance a load that far out from that close in, they have to pull about seven times its weight. The biceps does not do it alone: brachialis and brachioradialis share the work, and how they split it changes with the position of your hand.

That short lever is not a design flaw. The same ratio works the other way for movement: a small shortening near the joint swings your hand through a much larger arc, several times faster. Your arm traded force for speed.

Sources:
https://nmbl.stanford.edu/publications/pdf/Murray2002.pdf
https://pmc.ncbi.nlm.nih.gov/articles/PMC4526813/