neural control of muscle tone
If you take a relaxed person's arm and gently bend and straighten it, you feel a slight, even resistance — not floppy like a wet noodle, not stiff like a board. That background springiness, present even at rest, is muscle tone. It is what keeps you from collapsing in your chair and gives a limb its 'ready' feel before you move it.
Muscle tone is partly the natural stiffness of muscle tissue, but mostly it is a low-level setting of the stretch reflex maintained by the nervous system. The spinal cord keeps a quiet hum of activity in the muscles, and the brain continuously turns this up or down through descending pathways. When you move someone's joint and feel resistance, you are feeling this neural setting at work. Tone that is too low (hypotonia) makes a limb feel loose and slack; tone that is too high (hypertonia) makes it feel stiff, and one velocity-dependent form of hypertonia is spasticity.
Tone is one of the most important things a rehabilitation clinician feels with their own hands. Low tone after a fresh stroke or spinal injury, then rising tone over weeks, tells a story about the nervous system's state. High tone can both help (a little stiffness can support standing) and hinder (too much blocks movement and causes contractures). That double edge is exactly why the goal is rarely to abolish tone — over-treating spasticity can strip away useful supporting stiffness and leave a limb weaker for function.
A therapist passively flexes a stroke survivor's elbow and feels it 'catch' and resist more when moved quickly than slowly; that velocity-dependent resistance tells her the high tone is spasticity from an upper motor neuron lesion, not a fixed, mechanical stiffness.
Whether resistance changes with speed helps separate spasticity from rigidity and contracture.
More tone reduction is not always better. Some patients lean on a degree of leg stiffness to stand and transfer; wiping it out can leave them less functional, so tone is managed toward goals, not toward zero.