Nerve conduction · Core

F waves and H reflexes

F waves & H reflex

Late responses assess long pathways but rarely localize a single lesion alone.

3 learning sectionsCompiled 2026-09-08English edition · Traditional Chinese available

01F waves

  • After supramaximal stimulation, backfiring of a small subset of motor neurons produces a late response; this is not a sensory reflex.
  • Minimum latency, persistence and morphology describe different properties. Height and distal CMAP affect interpretation.

02H reflexes

  • The pathway includes Ia afferents and motor efferents. The soleus H reflex commonly assesses an S1-related circuit.
  • Compare sides and consider age, posture and activation. Several peripheral lesions can cause abnormalities.

03Use them appropriately

  • An abnormal F response suggests a long motor-pathway problem but cannot specify one root by itself.
  • An H reflex does not replace appropriate S1 muscle sampling and other studies.

Related nerves

Tibial nerve

Pause and explain

Explain in your own words: “F waves assess motor round-trip conduction; H reflexes include sensory input.” Then think of a situation in which it could be misinterpreted.

Show a review prompt

With a low CMAP, absent F waves may reflect too few available motor units.

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References and further reading

Educational synthesis; independent neuromuscular specialist review is pending. Publication dates and compilation dates are distinct. This is an original teaching synthesis; consult the source for complete methods and criteria.