What does tonus mean?
Tonus is the slight, sustained contraction or tension present in a muscle when it is not actively performing a movement. It is therefore more than complete relaxation: muscles retain a baseline level of tension that helps maintain posture, stabilize joints, and prepare the body for movement. In medicine and physiology, tonus is assessed when evaluating muscle function, particularly in cases of weakness, nerve injury, spasticity, or reduced mobility. A clinician might describe a limb as having normal, increased, or decreased tonus, while physical therapists often discuss tonus in relation to rehabilitation and muscle activation. The word is formal and technical, but it names an important everyday bodily fact that is usually described more generally as muscle tone. Tonus should not be confused with the unrelated musical sense of tonic or with the broad medical adjective tonic.
nounTonus is the state of slight, sustained contraction or tension in a muscle, especially when the muscle is at rest. It refers to a muscle's natural baseline level of tension, as distinct from the forceful contraction produced during movement or voluntary effort.
- The slight, sustained contraction or resting tension of a muscle.
- The state of tension or firmness in a muscle or other physiological tissue.
"The physiotherapist assessed the patient's muscle tone and found a normal level of tonus in the affected limb."
"Good hydration and regular activity helped preserve muscle tonus after the injury."
"The clinician noted reduced tonus in the patient's shoulder, suggesting possible nerve involvement."
The plural is used when referring to different instances or measurements of muscle tonus.
"The test recorded changes in the tonuses of several muscle groups."
Even at rest, your muscles are never completely slack: a subtle background tension known as tonus keeps the body alert and ready.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does tonus come from?
Tonus comes from Latin tonus, meaning tension or a stretched condition. Latin tonus was borrowed from the Greek tonos, a word associated with stretching or tension and the pitch or tone of an instrument. The same root developed into English words including tone, tonal, and tonic.
How tonus is actually used
Tonus is primarily a formal medical, physiological, and anatomical term. It is especially common in discussions of muscle function, rehabilitation, neurology, and physical examination; in general contexts, tone or muscle tone may sound more accessible.
Easily confused with tonus
Tonus is the resting tension of a muscle, while tonic more broadly means relating to sustained activity or contraction and also has uses in medicine and music.