Why do mu muscles "lock" ot tighten?

this Q is a little hard to explain so ill give an example.... if you stretch out your arm your elbow stops it from going any further but if you keep pushing you will feel muscles in your upper arm start to hurt and lock or tighten and the only way to "unlock" it is to pull your arm back i can also do that with a calf muscle a place in my foot and neck..SO my question is why do they tighten up and should i stop doing it