[The Hoffmann reflex in the normal new-borns. Post-natal evolution of the conduction velocity of the alpha motor and the Ia sensitive fibres of the ulnar nerve (author's transl)].

The conduction velocities of the alpha motor fibres and the IA sensitive fibres of the ulnar nerve have been studied in a group of full term newborn babies and during the neonatal period. At birth the average conduction velocity of the alpha motor fibres is 25,7 m/sec (sigma = to 2,4 m/sec) that of the IA afferent fibres is 35 m/sec (sigma = to 3,2 m/sec). The post natal evolution of the conduction velocity as a function of age has been established for both types of fibres. The time required for nervous conduction between the olecranon process and the wrist diminishes clearly in the course of the first two years because of a discordance between the augmentations of the length and of the diameter of the nerve fibers. The evolution of this conduction time as a function of the age and of the length of the forearm, has been established for the period between birth and the age of six years. The Hoffmann reflex in the inferior limb is present in all infants at birth; its latency is 14,2 msec; the ratio R = to Hmax/Mmax x 100 = to 55,75 p. 100 is identical to that found in the adult. A Hoffmann reflex exists in 66 p. 100 of the population studied in the ulnar nerve. Its latency is 11,1 msec; R = to Hmax/Mmax is equal to 30 p. 100. This reflex disappears progressively during the development.