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First I make a drawing of a muscle that lifts a weight. Muscles respond to nerve impulses from motor neurons which are Electrical. The other type of nerve signal comes from a sensory neuron which gives us a sense of touch or pain. To make it less complex, I will assume that the muscle contracts (gets shorter) when a positive voltage is applied along its length and extends (gets longer) when a negative voltage is applied.
To provide greater physical reality, if the muscle gets shorter by, say 10%, then it also becomes larger in in cross-sectional area by the same factor 10%. This just reflects the fact that flesh is nearly incompressible because its bulk modulus is very large which is similar to that of water.
I show a voltmeter that provides a fluctuating voltage along the length of the muscle. The voltmeter dial is animated so that it varies between a negative and positive voltage. And, of course, the muscle contracts when it receives a positive voltage and relaxes (lengthens) when it receives a negative voltage. There are two muscles shown and the one on the left is 1/4 of a cycle out of phase with the one on the right. Applied to the arms, the motion would resemble the act of juggling.
Muscles work only by contraction along their lengths: i.e. muscles pull but they do not push. Therefore each joint that permits a hinging action in both directions must have flexor muscles on one side of the bone being hinged and extensor muscles on the other side of that bone. The muscles that bend your forearm toward of away from your shoulder are a good example of this. At the front of the upper arm we have the very strong bicep (flexor muscle) and on the back we have the less strong tricep (extensor) muscle. It is the tricep that helps us do a pushup. The bicep is what we use to lift weights from our waist toward our shoulder. See Forces in joints Figure 1 for a more elaborate illustration.
In the human body, muscles must work with the rigid skeletal bones. At least one end of the muscle must be attached above the joint of the rigid bone which it will be picking up when the muscle contracts: Below is a simple sketch of the way the bicep picks up the forearm and whatever is in the hand attached to the forearm.
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