Biomechanics

Contraction, contraction, what's your function?

Courtesy of the NIH.

This sort of explains it. In response to nerve signals, the muscle cells, which are elongated, change shape, becoming shorter and wider, pulling the ends together. Of course, the ends are connected to bones and the muscles can only pull, so the muscles are always in pairs, opposing each other.

https://quizlet.com/270328607/10-steps-of-muscle-contraction-flash-cards/

"Each strand is made up of sub-units called "sarcomeres." These sarcomeres contain the "contraction proteins” called actin and myosin. The chemical interactions of these two proteins are responsible for the abilities of the muscle to contract (shorten), relax and produce force (necessary to move the body or objects)."

https://www.semc.org/muscle-structure-and-function#:~:text=Each%20strand%20is%20made%20up,move%20the%20body%20or%20objects).

GA

Here is a video that goes into detail.

It's impressive the amount of force generated by a muscle. The tendon attachment in the forearm is relatively close to the pivot point (the elbow). So, when lifting a weight, the actual force generated by the bicep muscle is many times the force exerted by the hand. Of course, the same is true for other skeletal muscles.

When you eat food it immediately begins to be digested or broken down into it's chemical components and absorbed by the blood which transports these chemicals to various internal organs that further process these chemicals into useful components that can then be utilized by the body and turned into various building blocks for all the life processes that take place...The brain and nervous system is the control circuitry that sends electrical signals all over the body controlling movement of the muscles...

https://en.wikipedia.org/wiki/List_of_systems_of_the_human_body

Electricity! If you don't believe me grab a hold of a cattle prod or an electric fence, don't try with live wires, likely to have your heirs vying for your toys.

Galvani’s experiment with frog legs—when electrodes touch a frog, the legs twitch into the upward position. (Image: Luigi Galvani/Public domain)

Luigi Galvani touched the frog leg muscle with two dissimilar metals, creating a battery and a circuit which activated the dead muscle. He wrongly attributed the biological tissue to generating the electricity rather than the metals.

https://www.wondriumdaily.com/how-galvanis-animal-electricity-theory-led-to-the-invention-of-the-battery/#:~:text=Galvani's%20Experiments%20on%20Frogs&text=Galvani's%20most%20famous%20experiments%20involved,even%20from%20a%20dissected%20frog.