Muscle tone

Muscle tone is the slight continuous tension a healthy muscle maintains at rest, which keeps posture stable and readies the muscle to move.

It is set by a steady low level of nerve signaling from the spinal cord, regulated by the brain and by stretch receptors within the muscle. Clinicians assess tone by moving a relaxed limb through its range and feeling for resistance. Too little tone, called hypotonia, produces floppy limbs and is seen in some infants, in certain genetic and neuromuscular conditions, and as a sign of drug or toxin exposure. Too much tone appears as spasticity, a velocity-dependent stiffness typical of multiple sclerosis, stroke and cerebral palsy, or rigidity, a constant stiffness characteristic of Parkinson's disease.

Muscle tone also drops naturally during sleep, reaching near-complete paralysis in REM sleep, which is why REM sleep behavior disorder, in which that paralysis fails, is defined by abnormal tone. In cannabinoid science, spasticity in multiple sclerosis is one of the better-studied targets, with an oral cannabinoid spray approved outside the United States for that use, and low muscle tone is a recognized sign of THC exposure in children and pets. Beyond those specific points, research on cannabinoids and muscle tone is limited.