Propagation Velocity in Electrically Activated Muscle Fibres in Man

Summary. The arrival time of muscle action potentials evoked by direct local stimulation was determined in 3 to 5 points of the brachial biceps. The stimulus artifact was reduced by stimulating with a bipolar electrode via a doubly screened transformer with the screen around the secondary coil connected to the muscle. It was ascertained that potentials from the same few fibres were picked up at the different electrodes as indicated by an identical threshold for the steep potentials of short duration. The propagation velocity was constant over 10–11 cm muscle length investigated and, therefore, the muscle fibres must pass uninterrupted over this distance. In ten subjects the propagation velocity varied between 3.3 and 5.2 m/sec. and averaged 4.02 ± 0.13 m/sec. (36.5°C.). In different regions of the same muscle it varied up to 15 per cent. There was no significant difference in the velocity over muscles from subjects of different age (3–74) years). The small variation in propagation velocity in spite of large differences in fibre circumference (2.5–3) times, 19 biopsies) suggest the presence of a mechanism which synchronizes the propagation of the impulses from a small group of simultaneously activated fibres. Differences in propagation velocity of the order of magnitude found with electrical stimulation can only account for a minor part of the variation in muscle action potential duration obtained with random leads at voluntary effort.