What causes involuntary toe curling during a foot stretch, potentially triggered by a cramp or spasm of the long toe flexor?

Involuntary Toe Curling During Foot Stretching

Involuntary toe curling during foot stretching is most commonly caused by exercise-associated muscle cramps originating from hyperexcitability of the intramuscular terminal motor axons innervating the long toe flexors, triggered by the mechanical stretch stimulus.

Pathophysiology

The mechanism involves irregular repetitive firing of motor unit action potentials at 200-300 Hz due to hyperexcitability of intramuscular terminal motor axons 1. This is distinct from contractures (which are myogenic and involve inability of muscle to relax) or other movement disorders 2.

• The stretch reflex during foot dorsiflexion can trigger this hyperexcitable state in the flexor digitorum longus and flexor hallucis longus muscles 1
• These are true cramps originating from peripheral nerves, not central nervous system disorders 3

Clinical Context and Differential Diagnosis

Common Benign Causes

• Idiopathic exercise-associated cramps are the most frequent etiology, particularly in young active individuals and those over 65 years (affecting 50% of elderly) 1
• Pregnancy-related cramps are common in young women 1
• Cold exposure can provoke symptoms in susceptible individuals 4

Red Flags Requiring Further Workup

Severe or frequent cramping with abnormal neurological examination warrants EMG and laboratory testing to exclude:

• Motor unit diseases: neuropathy, radiculopathy, plexopathy, neuromyotonia, or cramp-fasciculation syndrome (which can precede ALS) 1
• Metabolic disorders: electrolyte abnormalities, thyroid dysfunction 1
• Drug-induced cramps: review medication list 1
• Paramyotonia congenita: consider if symptoms worsen with cold, positive family history, and symptoms develop after 20 minutes of exercise with progressive toe curling 4

Mimics to Exclude

• Painful legs-moving toes syndrome: characterized by neuropathic pain with writhing toe movements, but movements are continuous and involuntary rather than cramping episodes 5
• Tethered cord syndrome: presents with progressive symptoms, sensory deficits, bladder/bowel dysfunction, and orthopedic deformities—not isolated exercise-induced cramping 6

Diagnostic Approach

History should specifically assess:

• Timing relative to exercise (during vs. after, duration of activity before onset) 4
• Cold sensitivity 4
• Family history of similar symptoms 4
• Medication use 1
• Associated neurological symptoms (weakness, sensory changes, bowel/bladder dysfunction) 1

Physical examination must include:

• Complete neurological examination including strength, sensation, and reflexes 1
• Observation for muscle atrophy or deformities 6
• Provocation testing with stretching 1

Laboratory and electrodiagnostic testing is indicated only if:

• Severe frequency or intensity of cramps 1
• Abnormal neurological examination 1
• Suspicion for underlying neuromuscular disease 1

Management

First-Line Treatment

Immediate stretching of the affected muscle during the cramp is the most effective acute intervention 1, 2. For toe flexor cramps, this involves passive dorsiflexion of the toes and ankle.

Preventive physical measures:

• Regular stretching exercises of calf muscles and toe flexors 3-5 times daily 1
• Adequate warm-up before exercise 4
• Avoidance of cold exposure if this is a trigger 4

Pharmacologic Options (Limited Efficacy)

Pharmacologic treatment should be reserved for severe, disabling symptoms due to modest effectiveness and potential toxicity:

• Quinine sulfate is effective but has a worrisome side-effect profile 1, 3
• Membrane-stabilizing drugs (such as phenytoin for paramyotonia congenita) may be equally effective with better tolerability 1, 4
• Other options with limited evidence: vitamin B complex, naftidrofuryl, calcium channel blockers (diltiazem), gabapentin 1

When to Refer

Referral to neurology is indicated for:

• Abnormal EMG findings 1
• Progressive symptoms or associated weakness 1
• Suspected underlying neuromuscular disease 1
• Failure of conservative measures in athletes requiring return to sport 4

Common Pitfalls

• Do not pursue extensive workup for isolated, infrequent cramps in otherwise healthy individuals with normal examination 1
• Do not overlook medication review—many drugs can precipitate cramps 1
• Do not confuse with contractures (myogenic, inability to relax) or movement disorders like painful legs-moving toes 2, 5
• Do not miss paramyotonia congenita in young athletes with cold-provoked symptoms and positive family history 4