What is happening in a muscle that is “stiff”? That is, it’s not especially sore but the range and speed of motion seems to be limited. This usually happens after exercising it in a way that it’s not used to, or overuse. Like your back when you wake up after shoveling two feet of snow the day before, or if you spend 4 hours raking leaves.
Is it a low level of pain such that your brain subconsciously and autonomically limits the range of motion? Is is that the fibers become shortened and inflexible temporarily? If so why? Is it fatigued muscle fibers that cannot fully respond until they recover?
I have never studied muscle biology so if you are an expert in this area please try to discuss in layman’s terms.