First, it’s important to understand that the mechanism of skeletal muscle contraction is still not completely understood, or all the components identified.
Still, the sliding filament theory is the most well-supported model. In simplest terms, myosin acts like a sort of ratchet, bonding to actin and pulling itself along the actin chain, which creates tension in the muscle. This is believed to be what occurs when the muscle is contracting.
Here is a more biological representation of the “sliding filaments”
If we stretch a muscle, does it get stuck in that position long after the stretch or snap back proper? Hopefully the latter, which is defined as the property of elasticity.
Our muscles are similar in this regard, to a rubberband; it quickly resumes it’s position in the absence of any resistance. However, just like a rubberband, a muscle can be stretched to the point of lengthening, which could be at the cost of producing force.
This video is processing – it'll appear automatically when it's done.
So what happens after a muscle is excited by an action potential? This involves the property of contractility, which states that a muscle stimulated above it’s threshold will generate tension between it’s origin and insertion points.
If that tension created is great enough to overcome the resistance of the object opposing it, the muscle will shorten and movement occurs.