Jendrassik’s maneuver is used to enhance or bring out absent or depressed tendon (phasic stretch) reflexes by isometric contraction of distant muscle groups, e.g., clenching teeth, or making a fist, interlocking fingers and pulling the hands against one another. If previously absent reflexes are then elicited, this may be denoted +/−. Co-contraction increases the gain in the monosynaptic reflex arc, as distinct from facilitation or post-tetanic potentiation which is seen in Lambert-Eaton myasthenic syndrome following tetanic contraction of muscles involved in the reflex.
Jendrassik E. Ueber allgemeine Localisation der Reflexe. DeutscheArchiv fur Klinische Medicin 1894; 52: 569-600
Delwaide P, Toulouse P. Facilitation of monosynaptic reflexes by voluntary contractions of muscle in remote parts of the body. Mechanisms involved in the Jendrassik maneuver. Brain 1981; 104: 701-709