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Human Physiology · Nervous system and exercise

Spinal Reflexes and Muscle Tone

The spinal cord contains circuits that respond automatically to stretch, tension and pain. This page explains the stretch reflex and its control, muscle tone, what changes after upper motor neuron lesions, and how to measure and manage spasticity.

10Sections
5Figures
2Tables
10Questions

Part 9 · Nervous System and Exercise · Chapter 42 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the components of a reflex arc.
  • Explain the stretch reflex, reciprocal inhibition and gamma control.
  • Describe tendon organ, withdrawal and crossed extensor reflexes.
  • Define muscle tone and spasticity.
  • Distinguish upper and lower motor neuron lesions.
  • Measure spasticity and interpret evidence on stretching.

Why it matters

Reflex testing is part of every neurological examination. Abnormal tone after stroke, spinal cord injury, brain injury and cerebral palsy shapes much neurological physiotherapy.

Understanding the physiology prevents ineffective treatment and helps you explain findings.

The reflex arc

A reflex arc has a receptor, an afferent neuron, a central synapse or interneurons, an efferent neuron and an effector. Reflexes are fast and automatic but are constantly adjusted by the brain.

Figure 1 · The stretch reflex

Monosynaptic stretch reflex with reciprocal inhibition and gamma motor control Stretching the agonist excites its muscle spindle. Ia afferents synapse directly on alpha motor neurons, which contract the same muscle, and via an inhibitory interneuron relax the antagonist. Gamma motor neurons set spindle sensitivity, and descending pathways adjust both. Spinal cordαINγAgonist (e.g. quadriceps)muscle spindleAntagonist (e.g. hamstrings)Ia afferentα motor neuron: contractinhibit antagonistγ motor neuron setsspindle sensitivityDescending pathwaysadjust α and γexcitability
One synapse, under constant supervision. The knee jerk tests this loop; descending control normally restrains it.
ReflexReceptorResponse
Stretch (myotatic)Muscle spindleSame muscle contracts; antagonist relaxes
Inverse stretch (autogenic inhibition)Golgi tendon organHigh tension reduces drive to same muscle
Withdrawal (flexor)NociceptorsLimb flexes away from the stimulus
Crossed extensorNociceptorsOpposite limb extends to bear weight

For muscle spindle structure, see the Human Anatomy muscle pages.

Muscle tone

Tone is the resistance felt when a relaxed limb is moved passively. It comes from tissue elasticity and a small reflex contribution.

Descending pathways from the cortex and brainstem normally keep reflex excitability in check.

Figure 2 · Upper versus lower motor neuron lesions

Comparison of upper and lower motor neuron lesion signs Upper motor neuron lesions give increased tone, brisk reflexes with clonus, an extensor plantar response, little early wasting and weakness of muscle groups. Lower motor neuron lesions give reduced tone and reflexes, a flexor or absent plantar response, early marked wasting, fasciculations and weakness of individual muscles. Upper motor neuronLower motor neuronToneincreased (spastic)reduced (flaccid)Reflexesbrisk; clonusreduced or absentPlantar responseextensor (Babinski)flexor or absentWastingmild, late (disuse)marked, earlyFasciculationsabsentmay be presentWeakness patterngroups of musclesindividual muscles
Location predicts the picture. Spasticity appears after the acute phase of a central lesion, as reflex circuits lose descending control.

Spasticity

Spasticity is a velocity-dependent increase in resistance to passive stretch, caused by hyperexcitable stretch reflexes after an upper motor neuron lesion. Over time muscles also shorten and stiffen.

Figure 3 · Spasticity depends on speed

Resistance to passive stretch at different speeds in normal muscle, spasticity and contracture In normal muscle, resistance rises little with speed. In spasticity, resistance rises sharply at faster stretch speeds because of the hyperactive stretch reflex. In contracture or tissue stiffness, resistance is high even at slow speeds. Speed of passive stretch (slow to fast)Resistance feltNormalSpasticity: rises with speedContracture or stiffness:high even when slow
Test at two speeds. A slow stretch shows tissue length; a fast stretch reveals the reflex component. Schematic.

Bohannon and Smith tested the Modified Ashworth Scale in 30 patients with intracranial lesions [1]. Raters agreed on 86.7% of elbow flexor ratings [1].

Figure 4 · Modified Ashworth Scale

Grades of the Modified Ashworth Scale from 0 to 4 Six grades: 0 no increase in tone; 1 slight catch or minimal end-range resistance; 1 plus slight catch then minimal resistance through less than half the range; 2 more marked increase but easily moved; 3 considerable increase, passive movement difficult; 4 rigid. 0No increase in tone1Slight catch and release, or minimal resistance at end of range1+Slight catch, then minimal resistance through less than half the range2More marked increase through most of range; part still easily moved3Considerable increase; passive movement difficult4Affected part rigid in flexion or extensionElbow flexors, 30 patients: 86.7% agreement between raters [1]
Simple and reliable at the elbow. The scale grades resistance to passive movement; it does not separate reflex from tissue stiffness.

Practical skill · testing tone

Position the patient relaxed and supported. Move the limb through full range first slowly, then quickly. Note a catch, clonus or resistance throughout. Compare sides.

Contracture and stretching

Contractures reduce joint mobility after neurological and non-neurological conditions. Stretch is widely used to prevent and treat them [2].

Harvey and colleagues reviewed 49 studies with 2,135 participants [2]. High-quality evidence showed stretch did not produce clinically important improvements in joint mobility [2].

Figure 5 · Stretch alone does not fix contractures

Summary of a systematic review of stretch for contractures Forty-nine studies with 2,135 participants. Stretch improved joint mobility by only about 2 degrees in neurological conditions and by a trivial standardised amount in other conditions, with no important effect on pain. No study lasted more than seven months. Studies / participants49 / 2,135Joint mobility, neurological+2° (0 to 3°), not importantJoint mobility, other conditionsSMD 0.2 (0 to 0.3), not importantPain, other conditionsno important effectLongest stretch programme7 monthsHigh-quality evidence: regular stretch did not produce clinically important change [2]
Rethink passive stretching. Effects were consistent across conditions; longer programmes have not been tested [2].

These results were consistent across conditions [2]. Positioning, active training, splinting and medical treatment of spasticity may still have roles, but brief passive stretch alone is unlikely to help.

Clinical relevance for the physiotherapist

  • Examine reflexes systematically. Grade and compare sides.
  • Separate the components. Test slow and fast to distinguish reflex from stiffness.
  • Measure. The Modified Ashworth Scale is quick and reliable at the elbow [1].
  • Question routine stretching. It does not meaningfully change joint mobility [2].
  • Train function. Active, task-specific practice addresses weakness, often the bigger problem.

Assessment and application

FindingInterpretationAction
Brisk reflexes, clonus, extensor plantarUpper motor neuron lesionNeurological rehabilitation; monitor tone
Absent ankle reflex with wasting and fasciculationLower motor neuron lesionInvestigate cause; strengthen
Resistance high at slow and fast speedContracture or stiffnessPositioning, active range, splint review
Catch only on fast stretchSpasticityGrade with MAS [1]; treat if limiting function

Common mistakes

  • Calling all stiffness spasticity.
  • Testing tone only at one speed.
  • Relying on short daily passive stretching to prevent contracture [2].
  • Ignoring weakness while treating tone.
  • Testing reflexes with a tense patient.

Exam preparation

Long answer

Describe the stretch reflex. Add a note on muscle tone and its abnormalities.

  • Reflex arc components.
  • Stretch reflex, reciprocal inhibition, gamma loop (Figure 1).
  • Other spinal reflexes (table).
  • Upper versus lower motor neuron lesions (Figure 2).
  • Spasticity and its measurement (Figures 3, 4) [1].

Short notes

  • Golgi tendon organ.
  • Clonus.
  • Babinski sign.
  • Modified Ashworth Scale [1].

Viva and practical

  • “How many synapses in the knee jerk?” — one.
  • “What sets spindle sensitivity?” — gamma motor neurons.
  • “Does stretching prevent contracture?” — not to a clinically important degree [2].
  • OSCE: examine lower-limb reflexes and grade tone.

Current evidence

Exam and traditional teaching

Daily passive stretching prevents contractures in neurological patients.

What the evidence adds

The Modified Ashworth Scale is reliable for elbow flexors [1].

Stretch does not produce clinically important gains in joint mobility [2].

What the student should understand

Reflexes reveal where the nervous system is damaged. Treat function, not only tone.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. In the original reliability study of the Modified Ashworth Scale, raters agreed on about:
  1. (A) 50% of ratings
  2. (B) 86.7% of ratings
  3. (C) 100% of ratings
  4. (D) 20% of ratings

Answer: (B) The two raters agreed on 86.7% of ratings [1].

Q2. Which muscle group was tested in that study?
  1. (A) Plantarflexors
  2. (B) Elbow flexors
  3. (C) Hip adductors
  4. (D) Neck extensors

Answer: (B) Elbow flexor spasticity was graded [1].

Q3. The Kendall tau correlation between raters was:
  1. (A) 0.15
  2. (B) 0.50
  3. (C) 0.847
  4. (D) 1.0

Answer: (C) Tau was 0.847 [1].

Q4. The study population was:
  1. (A) 30 patients with intracranial lesions
  2. (B) 300 healthy students
  3. (C) Children with cerebral palsy only
  4. (D) Athletes

Answer: (A) Thirty patients with intracranial lesions [1].

Q5. Regular stretch improved joint mobility in neurological conditions by about:
  1. (A) 2 degrees
  2. (B) 20 degrees
  3. (C) 45 degrees
  4. (D) 90 degrees

Answer: (A) Mean difference 2 degrees, not clinically important [2].

Q6. How many studies were included in the stretch review?
  1. (A) 9
  2. (B) 49
  3. (C) 490
  4. (D) 4

Answer: (B) Forty-nine studies with 2,135 participants [2].

Q7. The quality of evidence that stretch does not produce clinically important change in joint mobility was:
  1. (A) Very low
  2. (B) Low
  3. (C) High
  4. (D) Not graded

Answer: (C) High-quality evidence [2].

Q8. The longest stretch programme studied lasted:
  1. (A) 1 week
  2. (B) 7 months
  3. (C) 5 years
  4. (D) 10 years

Answer: (B) No study performed stretch for more than seven months [2].

Q9. Reported adverse events with stretch in neurological conditions included:
  1. (A) Skin breakdown, bruising, blisters and pain
  2. (B) Fractures only
  3. (C) None
  4. (D) Heart attack

Answer: (A) Eight adverse events including skin breakdown and blisters [2].

Q10. The Modified Ashworth Scale grades:
  1. (A) Strength
  2. (B) Resistance to passive movement
  3. (C) Sensation
  4. (D) Coordination

Answer: (B) It grades spasticity by manual resistance testing [1].

Quick review

Key takeaways

  • Stretch reflex: spindle, Ia, alpha motor neuron, same muscle; antagonist inhibited.
  • Gamma motor neurons set spindle sensitivity.
  • UMN lesions: spasticity, brisk reflexes, Babinski; LMN: flaccid, wasting.
  • Spasticity is velocity dependent; contracture is not.
  • MAS raters agreed on 86.7% at the elbow [1].
  • Stretch did not meaningfully improve joint mobility [2].

Further reading

SourceWhy it earns its place
Bohannon RW, Smith MB — Phys Ther, 1987Modified Ashworth Scale [1]
Harvey LA et al. — Cochrane, 2017Stretch for contractures [2]
Kandel, Principles of Neural ScienceSpinal reflexes

References

  1. Bohannon RW, Smith MB. Interrater reliability of a modified Ashworth scale of muscle spasticity. Physical Therapy. 1987;67(2):206–207. doi:10.1093/ptj/67.2.206 PMID 3809245 Reliability study
  2. Harvey LA, Katalinic OM, Herbert RD, et al. Stretch for the treatment and prevention of contractures. Cochrane Database of Systematic Reviews. 2017;1(1):CD007455. doi:10.1002/14651858.CD007455.pub3 PMID 28146605 Systematic review and meta-analysis

Related topics

Reviewed by the Physiotherapist India Team. · Human Physiology contents