Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. Skeletal muscle has an intrinsic resistance to stretch resulting from the elastic properties of the tendons, connective tissue, and the muscle tissue itself. Therefore muscle behaves much like a spring. Reflexes also function to counteract the active or passive stretch of the muscle tissue via the monosynaptic connections from the spindles to the alpha motor neurons, and work with the elastic components of muscle to resist stretch.
Normal muscle tone serves three important functions. First, it assists in maintaining posture, or the resistance of the muscle to the forces of gravity. Muscle tone helps to ensure that the center of gravity is aligned over the base of support. Second, because of a muscle’s inherent ability to act as a spring, it can store energy and release it at a later time. This is particularly important for movements such as walking. When a leg pushes off, some of the stored energy is released and helps propel the leg and body forward, thereby assisting the muscles that normally pull the leg forward. Lastly, because muscles act like springs, they help dampen jerky movements and allow for more “fluidlike” movements of most muscles (Ghez, 1991).
Susan A. Darby, Robert J. Frysztak, in Clinical Anatomy of the Spine, Spinal Cord, and Ans (Third Edition), 2014
Hypertonicity, an increase in muscle contractility, includes spasticity (velocity-dependent increase in resistance to passive stretch) and rigidity (increased uniform resistance that is present throughout the whole range of motion and is independent of velocity) secondary to a neurologic lesion of the CNS or upper motor neuron system.
Hypotonicity, a decrease in muscle contractility, includes flaccidity (diminished resistance to passive stretching and tendon reflexes) from a neurologic lesion of the lower motor neuron system (or as in the early stage of spinal cord injury [SCI] known as spinal shock).
Dystonia, a hyperkinetic movement disorder, is characterized by disordered tone and involuntary movements involving large portions of the body resulting from a lesion in the basal ganglia.
( Hillary A. Reinhold, Michele P. West, in Acute Care Handbook for Physical Therapists (Fourth Edition), 2014