Facilitation is an increase in muscle strength following repeated contraction. Clinically, facilitation may be demonstrated by the appearance of tendon-reflexes after prolonged (ca. 30 seconds) forced maximal contractions against resistance, e.g., the biceps jerk after elbow flexion, knee jerk after knee extension; and by Lambert’s sign (increased force grip with sustained contraction).
This phenomenon of post-tetanic potentiation is most commonly seen in the Lambert-Eaton myasthenic syndrome (LEMS), a disorder of neuromuscular junction transmission associated with the presence of autoantibodies directed against presynaptic voltage-gated calcium ion (Ca2+) channels (VGCC). The mechanism is thought to be related to an increased build up of Ca2+ ions within the presynaptic terminal with the repetitive firing of axonal action potentials, partially overcoming the VGCC antibody-mediated ion channel blockade, and leading to release of increasing quanta of acetylcholine.