Muscle tone, or tonus, is the continuous, passive, and partial contraction of muscles that exists even when a person is at rest. This tension is the muscle’s resistance to being passively stretched or moved by an external force. The nervous system, primarily through a feedback system involving the spinal cord and muscle spindles, constantly regulates this state of readiness. Maintaining an appropriate level of tone is fundamental for keeping the body upright against gravity, stabilizing joints, and ensuring muscles can react quickly to facilitate movement and maintain balance.
Understanding Muscle Tone vs. Muscle Strength
The terms muscle tone and muscle strength are often confused, but they describe two different aspects of muscle function. Muscle tone is an involuntary, passive state of readiness controlled by the nervous system, determining the firmness within the muscle when it is not actively contracting. In contrast, muscle strength is the conscious ability of a muscle group to generate force against resistance, used to perform physical activities like lifting a weight or running. While tone is neurological and cannot be easily changed through exercise, strength can be significantly improved through physical training.
When and Why Muscle Tone Is Assessed
Assessment of muscle tone is a routine part of a neurological examination, providing insight into the integrity of the central and peripheral nervous systems. A change in tone can be an early indication of various medical conditions, and is particularly important when evaluating infants and children who are not meeting developmental motor milestones. Professionals assess tone following injuries or illnesses that affect the brain or spinal cord, such as stroke or traumatic brain injury. Changes in tone signal damage to motor nerve pathways, helping clinicians localize the injury, monitor the progression of neurological disorders, and gauge the effectiveness of rehabilitation treatments.
Professional Assessment Techniques
Professionals assess muscle tone through a combination of observation, palpation, and passive movement testing. Observation involves noting the patient’s posture and the resting position of their limbs for signs of stiffness or abnormal positioning. Palpation involves gently feeling the muscle belly to assess its firmness or flaccidity while the patient relaxes completely.
The most informative technique is the passive range of motion examination, where the clinician gently moves the patient’s joints through their full range. Normal tone is characterized by smooth, minimal, and consistent resistance felt throughout the movement. To differentiate types of hypertonia, the speed of movement is varied, as resistance that increases with faster movement is a specific sign of spasticity.
Specific biomechanical tests, such as the Wartenberg pendulum test, are also employed to objectively evaluate tone, particularly in the lower limbs. In this test, the patient’s knee is extended and then released to swing freely.
Interpreting Abnormal Findings
Abnormal muscle tone presents in two main forms: hypertonia and hypotonia. Hypertonia is high muscle tone, resulting in increased resistance to passive movement and muscle stiffness. Two common types are spasticity and rigidity.
Spasticity is a velocity-dependent increase in resistance, meaning the faster the limb is moved, the greater the opposition felt, often including an exaggerated “catch.” Rigidity is a uniform increase in resistance present throughout the entire range of motion, independent of movement speed. This constant opposition can be felt as “lead pipe” resistance or, if accompanied by a tremor, a “cogwheel” effect. Hypertonia is often associated with damage to the central nervous system, such as a stroke or cerebral palsy.
Hypotonia, or low muscle tone, is characterized by decreased resistance to passive stretch. Muscles with hypotonia feel soft or “floppy,” making it difficult to maintain posture and potentially delaying the development of coordinated fine and gross motor skills. Hypotonia can be caused by various factors, including lower motor neuron issues, genetic conditions, or cerebellar disorders.