Tonuses

Tonuses, or tonus as a singular, describe the residual and passive state of muscle tension present at rest and the resistance a muscle provides to passive stretch. The term is used to describe skeletal muscle tone, a property arising from the combined activity of the stretch reflex pathways, motor neurons, and the intrinsic properties of muscle and connective tissue. Normal tonus supports posture and ready access to movement, while allowing voluntary contraction to occur smoothly.

Tonuses can be categorized as normal or abnormal. Normal tonus provides baseline resistance without interfering with movement. Hypertonia refers to increased tonus and includes spasticity and rigidity, while hypotonia refers to reduced tonus. The distinction between tonus and muscle strength is important: tonus relates to resistance to stretch, not maximal force production.

Clinically, tonus is assessed by passively moving limbs and evaluating resistance. Scales such as the Modified Ashworth Scale (and its variants) measure hypertonia, while objective methods include electromyography and isokinetic or torque measurements. Tonus can also be influenced by posture, fatigue, arousal, temperature, and age, and it may vary across joints and muscle groups.

Physiologically, tonus reflects neural input from the gamma motor system and intrinsic muscle properties that determine baseline muscle stiffness. Alterations in tonus are central to many neurological conditions and are important targets in rehabilitation.

Etymology traces tonus to Latin tonus, meaning tone. See also muscle tone, hypertonia, hypotonia, and related neuromuscular concepts.