Muscle Tone

Muscle tone refers to the resistance encountered when a relaxed skeletal muscle is passively stretched. Normal tone reflects both the intrinsic mechanical properties of muscle and connective tissue and the activity of neural circuits that regulate muscle contraction.

Muscle tone is important for posture, joint stability, balance, and preparation for movement. Even when a person appears relaxed, the neuromuscular system remains capable of continuously adjusting muscle activity in response to changes in body position and sensory input.

Tone is not equivalent to voluntary muscle contraction or muscle strength. A muscle can have normal strength but abnormal tone, or abnormal strength while resting tone remains relatively preserved.

Components of Muscle Tone

Component	Contribution
Intrinsic muscle properties	Passive elasticity and viscoelastic resistance
Connective tissue	Mechanical resistance from muscle-associated fascia and other tissues
Muscle spindles	Detect muscle length and changes in length
Spinal reflex circuits	Generate responses to muscle stretch
Descending pathways	Modulate spinal motor neuron and interneuron activity

Resting Muscle

A resting skeletal muscle is not mechanically inert. Muscle fibers, tendons, connective tissue, and surrounding fascial structures possess elastic and viscoelastic properties that resist deformation.

Neural mechanisms add a dynamic component that allows resistance to change according to posture, movement, neurological state, and sensory input.

Muscle Tone and Posture

Upright posture requires continuous regulation of muscle activity to oppose gravity and stabilize body segments. This background postural activity is sometimes described as postural tone.

Postural tone is particularly important in axial and proximal muscles and in lower limb muscle groups that contribute to standing.

Antigravity Muscles

Muscles that help maintain the body against gravity are often called antigravity muscles. These include important extensor groups of the lower limbs and muscles that stabilize the trunk and neck.

The exact pattern of activity varies with posture rather than being a fixed contraction of one set of muscles.

Muscle Spindles

Muscle spindles are specialized sensory receptors located within skeletal muscles. They detect muscle length and changes in muscle length and are central to the neural regulation of tone and stretch reflexes.

Spindles are arranged approximately parallel to the ordinary force-producing skeletal muscle fibers.

Intrafusal and Extrafusal Fibers

Fiber Type	Function
Intrafusal fibers	Specialized fibers within muscle spindle that participate in sensory detection
Extrafusal fibers	Ordinary skeletal muscle fibers that generate most muscular force

Intrafusal Muscle Fibers

Intrafusal fibers lie within the connective tissue capsule of the muscle spindle. Their central regions are associated with sensory endings, while their contractile polar regions receive gamma motor innervation.

Nuclear Bag and Nuclear Chain Fibers

Muscle spindles contain different types of intrafusal fibers, including nuclear bag fibers and nuclear chain fibers. Their sensory properties contribute to detection of static muscle length and dynamic changes in length.

Primary Sensory Endings

Primary spindle afferents are carried by group Ia fibers. They are particularly sensitive to muscle stretch and to the rate at which muscle length changes.

Secondary Sensory Endings

Secondary spindle afferents are primarily carried by group II fibers. They provide information related particularly to sustained muscle length.

Muscle Spindle Afferents

Afferent	General Sensory Role
Group Ia	Strongly responsive to stretch and velocity of length change
Group II	Important for information about maintained muscle length

Stretch Reflex

The stretch reflex is a neural response in which stretching a muscle increases activation of that muscle. It contributes to resistance against unexpected lengthening and helps regulate posture.

Muscle spindle afferents detect the stretch and transmit signals into the spinal cord, where they influence motor neurons supplying the stretched muscle.

Monosynaptic Component

Group Ia afferents make direct excitatory connections with alpha motor neurons supplying the same muscle and closely related synergists. This direct connection forms the classic monosynaptic component of the stretch reflex.

Reciprocal Inhibition

Stretch reflex circuitry can also inhibit antagonist muscles through spinal interneurons. This process, known as reciprocal inhibition, helps coordinate opposing muscle groups.

Stretch Reflex Circuit

Step	Event
1	Muscle is lengthened
2	Muscle spindle is stretched
3	Ia afferent firing increases
4	Spinal alpha motor neurons are excited
5	Extrafusal muscle fibers contract and oppose stretch

Alpha Motor Neurons

Alpha motor neurons innervate extrafusal skeletal muscle fibers. Their activation produces the force-generating contractions used for movement and postural responses.

Alpha motor neuron excitability is influenced by sensory afferents, spinal interneurons, and descending motor pathways.

Gamma Motor Neurons

Gamma motor neurons innervate the contractile portions of intrafusal muscle fibers. Their activity changes tension within the spindle and therefore regulates spindle sensitivity.

Gamma Motor System

When gamma motor neurons activate intrafusal fibers, the polar regions contract and help maintain tension in the sensory portion of the spindle. This allows the spindle to remain responsive across different muscle lengths.

Alpha-Gamma Coactivation

During many voluntary movements, alpha and gamma motor neurons are activated together. Alpha-gamma coactivation allows extrafusal fibers to shorten while maintaining appropriate sensitivity of the muscle spindle.

Importance of Alpha-Gamma Coactivation

Motor System	Target	Role
Alpha motor neurons	Extrafusal fibers	Generate muscle force
Gamma motor neurons	Intrafusal fibers	Adjust spindle sensitivity
Coactivation	Both systems	Maintains spindle responsiveness during contraction

Gamma Loop

The term gamma loop describes a functional sequence in which gamma motor neuron activity changes intrafusal fiber tension, alters spindle afferent activity, and consequently influences alpha motor neuron activity.

This mechanism contributes to regulation of muscle responsiveness and postural activity.

Golgi Tendon Organs

Golgi tendon organs are sensory receptors located near musculotendinous junctions. They respond primarily to tension generated within the muscle-tendon unit.

Their group Ib afferents enter spinal circuits and contribute to regulation of muscular force.

Muscle Spindles Versus Golgi Tendon Organs

Feature	Muscle Spindle	Golgi Tendon Organ
Primary stimulus	Muscle length and stretch	Muscle-tendon tension
Arrangement	Functionally parallel with muscle fibers	Functionally in series with muscle fibers
Main afferents	Ia and II	Ib
Major role	Length sensing and stretch reflexes	Force sensing and force regulation

Spinal Cord Regulation

Muscle tone is influenced by networks of spinal interneurons that receive sensory information and descending commands. These circuits determine how strongly motor neurons respond to peripheral input.

Descending Control of Tone

Supraspinal pathways continuously influence spinal motor circuits. Important contributions arise from the cerebral cortex, brainstem reticular formation, vestibular nuclei, cerebellum, and associated motor systems.

Reticulospinal Pathways

Reticulospinal pathways descend from the brainstem reticular formation and have important roles in posture, locomotion, automatic movement, and regulation of spinal motor neuron excitability.

Vestibulospinal Pathways

Vestibulospinal pathways originate from vestibular nuclei and contribute to postural responses associated with head position, balance, and gravity.

The lateral vestibulospinal tract has particularly important influences on extensor activity involved in maintaining upright posture.

Corticospinal Influence

The corticospinal system is primarily associated with voluntary movement, particularly skilled movement, but cortical motor control also interacts with spinal and brainstem systems that regulate tone.

Cerebellar Influence

The cerebellum influences muscle tone indirectly through its connections with vestibular nuclei, reticular formation, motor cortex, and other motor systems.

It contributes to coordination, adjustment of motor output, and adaptation of postural responses.

Major Neural Influences on Tone

System	Contribution
Spinal reflex circuits	Respond to muscle stretch and sensory input
Reticulospinal system	Regulates posture and motor neuron excitability
Vestibulospinal system	Influences antigravity and balance-related activity
Corticospinal system	Interacts with spinal circuits during voluntary motor control
Cerebellum	Coordinates and modulates motor output

Postural Tone

Postural tone represents the background muscular activity that contributes to maintaining body segments in appropriate positions against gravity.

It is continuously modified by changes in support surface, body orientation, voluntary movement, vestibular signals, proprioception, and visual information.

Axial Muscle Tone

Muscles of the neck and trunk contribute to stabilization of the head and vertebral column. Their activity helps maintain alignment while allowing voluntary movement of the limbs.

Proximal Muscle Tone

Muscles around the shoulder and pelvic girdles provide proximal stability from which distal limb movements can be performed efficiently.

Lower Limb Tone

During standing, lower limb muscles continuously regulate joint position and body sway. Plantarflexors are particularly important in controlling the tendency of the body to rotate forward around the ankle.

Tone and Joint Stability

Background muscular activity contributes to dynamic joint stability. Muscles can respond to changes in load and joint position before excessive displacement occurs.

Passive structures such as ligaments, capsules, and joint geometry also contribute to stability.

Tone and Movement Readiness

A regulated level of motor neuron excitability allows muscles to respond rapidly when movement is initiated or when an unexpected perturbation occurs.

Excessively low or high tone can interfere with efficient movement.

Tone During Voluntary Movement

Muscle tone is dynamically adjusted during movement. The nervous system changes the excitability of agonists, antagonists, stabilizers, and postural muscles according to the task.

Agonist and Antagonist Coordination

Efficient movement requires appropriate activation and relaxation of opposing muscle groups. Excessive simultaneous activation can increase stiffness, while insufficient stabilization can reduce movement control.

Co-Contraction

Co-contraction occurs when agonist and antagonist muscles are activated simultaneously. Controlled co-contraction can increase joint stiffness and stability, particularly during demanding or uncertain tasks.

Co-contraction is not identical to resting muscle tone, although both influence resistance around a joint.

Muscle Tone and Sleep

Muscle tone changes with behavioral state. Skeletal muscle activity generally decreases during sleep, with particularly marked suppression of most skeletal muscle tone during rapid eye movement sleep.

Clinical Examination of Muscle Tone

Muscle tone is commonly assessed by asking the patient to relax while the examiner passively moves a limb through its range of motion.

The examiner evaluates the amount, pattern, and velocity dependence of resistance.

Clinical Features Assessed

Feature	Observation
Resistance	Amount of opposition to passive movement
Velocity dependence	Whether faster movement changes resistance
Distribution	Which muscle groups and limbs are affected
Symmetry	Comparison between sides
Range dependence	Whether resistance changes at different joint positions

Normal Muscle Tone

Normal tone allows passive movement with mild resistance appropriate to the muscle group, joint, movement speed, and individual. It permits both stability and flexibility.

Hypotonia

Hypotonia is decreased resistance to passive movement. Limbs may feel unusually loose or offer less resistance than expected when moved by an examiner.

Hypotonia can result from disorders affecting different levels of the motor system and therefore does not identify a single anatomical lesion by itself.

Causes of Hypotonia

Reduced tone may occur with lesions affecting peripheral nerves, anterior horn cells, muscles, cerebellar systems, or during certain phases of central nervous system injury.

The associated pattern of weakness, reflex changes, coordination, sensation, and other neurological findings helps localize the cause.

Flaccidity

Flaccidity describes marked reduction in muscle tone, often accompanied by weakness. It is classically associated with disruption of lower motor neuron pathways but can also occur transiently after acute central nervous system injury.

Hypertonia

Hypertonia is abnormally increased resistance to passive movement. Different neurological disorders produce different patterns of increased tone.

Two important patterns are spasticity and rigidity.

Spasticity

Spasticity is a form of hypertonia associated with lesions of upper motor neuron pathways. A characteristic feature is increased resistance to passive stretch that is related to the velocity of movement.

Faster passive movement generally produces greater resistance.

Velocity Dependence

The velocity-dependent nature of spasticity reflects abnormal regulation of stretch-sensitive spinal reflex circuits following disruption of descending motor control.

Clasp-Knife Phenomenon

In some individuals with spasticity, substantial initial resistance during passive movement may suddenly decrease as movement continues. This pattern is commonly described as a clasp-knife response.

Clonus

Clonus consists of rhythmic, involuntary contractions produced by sustained stretch in a hyperexcitable reflex system. It is often assessed at the ankle or patella.

Rigidity

Rigidity is increased resistance to passive movement that is classically less dependent on movement velocity than spasticity.

It is commonly associated with disorders involving basal ganglia motor circuits.

Lead-Pipe Rigidity

Lead-pipe rigidity describes relatively uniform resistance throughout passive movement.

Cogwheel Rigidity

Cogwheel rigidity describes a ratchety or intermittent quality of resistance during passive movement. It can occur when rigidity is combined with tremor.

Spasticity Versus Rigidity

Feature	Spasticity	Rigidity
Velocity dependence	Prominent	Usually less velocity dependent
Typical association	Upper motor neuron pathway lesion	Basal ganglia dysfunction
Distribution	Often affects particular muscle groups preferentially	Can involve agonists and antagonists
Characteristic patterns	Clasp-knife response, clonus may occur	Lead-pipe or cogwheel quality

Upper Motor Neuron Lesions

Damage to descending motor pathways can alter the balance of excitatory and inhibitory influences on spinal circuits. Over time, this can produce hyperreflexia, spasticity, abnormal synergies, and impaired selective movement.

Lower Motor Neuron Lesions

Damage to alpha motor neurons or their peripheral axons interrupts the final neural pathway to extrafusal skeletal muscle. Findings can include weakness, reduced tone, reduced reflexes, muscle atrophy, and fasciculations depending on the lesion.

Cerebellar Disorders

Cerebellar lesions can be associated with hypotonia, particularly in affected limbs, together with impaired coordination and other cerebellar signs.

Basal Ganglia Disorders

Disorders of basal ganglia circuits can alter muscle tone and automatic movement. Rigidity is a characteristic feature of parkinsonian syndromes.

Muscle Tone and Deep Tendon Reflexes

Deep tendon reflexes and muscle tone both depend partly on stretch-reflex circuitry, but they are not identical measurements. Reflexes test a brief phasic response, while tone is assessed during passive movement over a larger range.

Deep Tendon Reflex

A tendon tap briefly stretches the associated muscle spindle, activating Ia afferents and producing contraction through spinal reflex pathways.

Hyperreflexia

Hyperreflexia refers to exaggerated deep tendon reflexes and commonly accompanies upper motor neuron dysfunction and spasticity.

Hyporeflexia

Hyporeflexia refers to reduced reflex responses and commonly occurs with disruption of peripheral reflex pathways.

Tone, Strength, and Reflexes

Feature	What It Assesses
Muscle tone	Resistance to passive movement
Muscle strength	Ability to generate voluntary force
Deep tendon reflex	Phasic response of stretch-reflex circuitry

Passive Mechanical Resistance

Not all resistance during passive movement is neural in origin. Muscles, tendons, fascia, joint capsules, ligaments, skin, and other connective tissues contribute mechanical resistance.

This distinction becomes important when evaluating contracture or structural stiffness.

Contracture

A contracture is a structural shortening or loss of extensibility of muscles, tendons, capsules, or other soft tissues that limits passive range of motion.

Contracture can coexist with abnormal neural tone but is mechanically distinct from it.

Tone Versus Contracture

Feature	Abnormal Neural Tone	Contracture
Primary mechanism	Altered neural control	Structural tissue shortening or stiffness
Velocity effect	May be prominent, especially in spasticity	Mechanical restriction persists
Passive range	May remain available under some conditions	Often structurally limited

Muscle Tone and Balance

Appropriate tone helps maintain readiness for rapid postural corrections. Too little tone can reduce stability, while excessive tone can restrict adaptive movement and make balance responses less efficient.

Muscle Tone and Gait

Walking requires continuous modulation of muscle activity. Muscles must alternate between activation and relative relaxation while providing sufficient stiffness for weight acceptance, propulsion, and limb control.

Abnormal tone can therefore substantially alter gait patterns.

Spasticity and Gait

Spasticity can interfere with selective joint movement and alter the timing of muscle activation. Depending on the muscles affected, it can contribute to abnormal limb positioning and inefficient walking.

Rigidity and Gait

Rigidity can reduce normal flexibility of the trunk and limbs and contribute to decreased movement amplitude and altered postural responses.

Hypotonia and Movement

Reduced tone can decrease joint stability and make it more difficult to maintain posture against gravity. Movement may require greater reliance on compensatory strategies.

Tone in Infancy and Development

Muscle tone changes during development as the nervous and musculoskeletal systems mature. Assessment of tone is an important part of neurological examination in infants and children because abnormal patterns can provide information about motor system development.

Effects of Position on Tone

Body position can influence muscle activity through vestibular, proprioceptive, and mechanical mechanisms. Tone assessed in one posture may therefore differ from resistance observed in another.

Effects of Emotional and Cognitive State

Attention, anxiety, effort, pain, and difficulty relaxing can increase voluntary or involuntary muscle activity during examination. Clinical assessment of tone therefore requires careful attempts to minimize active resistance.

Effects of Pain

Pain can produce protective muscle activation or guarding. This resistance should be distinguished from neurological hypertonia whenever possible.

Muscle Guarding

Muscle guarding is protective activation that limits movement around a painful or threatened region. It is functionally different from the persistent abnormalities of tone associated with neurological lesions.

Assessment Principles

Assessment	Purpose
Slow passive movement	Assesses baseline mechanical resistance and range
Faster passive movement	Helps identify velocity-dependent resistance
Comparison between sides	Identifies asymmetry
Reflex examination	Provides information about related spinal pathways
Strength testing	Distinguishes tone from voluntary force generation
Range-of-motion assessment	Identifies structural restriction or contracture

Modified Ashworth Scale

The Modified Ashworth Scale is commonly used clinically to grade resistance encountered during passive movement in patients with increased tone. It provides an ordinal estimate of resistance but does not isolate all components contributing to hypertonia.

Modified Tardieu Approach

Assessment methods based on the Tardieu concept compare responses at different movement velocities and can help characterize velocity-dependent resistance and available passive range.

Functional Consequences of Abnormal Tone

Abnormal muscle tone can affect posture, joint alignment, movement efficiency, balance, gait, hand function, transfers, and the ability to perform daily activities.

Its functional impact depends on severity, distribution, associated weakness, sensory deficits, contracture, and the demands of the task.

Functions of Normal Muscle Tone

Function	Role
Postural support	Contributes to maintenance of body position against gravity
Joint stability	Provides dynamic support around joints
Movement readiness	Maintains appropriate motor system responsiveness
Balance control	Supports rapid postural adjustments
Movement coordination	Allows controlled interaction between agonists, antagonists, and stabilizers

Key Features of Muscle Tone

Feature	Key Point
Clinical definition	Resistance of relaxed muscle to passive stretch or movement
Major sensory receptor	Muscle spindle
Primary spindle afferent	Group Ia fiber
Secondary spindle afferent	Group II fiber
Extrafusal motor neuron	Alpha motor neuron
Intrafusal motor neuron	Gamma motor neuron
Low tone	Hypotonia
High tone	Hypertonia
Velocity-dependent hypertonia	Spasticity
Less velocity-dependent hypertonia	Rigidity

Anatomical and Functional Importance

Muscle tone represents the interaction between the mechanical properties of the musculoskeletal system and the neural circuits controlling skeletal muscle. Muscle spindles continuously provide information about muscle length, while alpha and gamma motor systems regulate extrafusal force production and spindle sensitivity.

Spinal reflex circuits are modified by descending influences from the brainstem, cerebral cortex, cerebellum, and other motor systems. This allows tone to change according to posture, movement, balance requirements, and behavioral state.

Normal tone supports posture and joint stability without preventing flexible movement. Abnormally reduced tone can impair postural support, while excessive tone can restrict movement and alter coordination. The pattern of abnormal tone can also provide important information about the anatomical level and type of neurological dysfunction.

Muscle tone therefore integrates muscle mechanics, proprioception, stretch reflexes, alpha and gamma motor control, descending pathways, postural stability, joint control, and clinical neurological examination.