Relationship between muscle fiber conduction velocity and muscle fiber composition was studied in vastus lateralis of well-trained athlete students, who differed considerably regarding their fiber type distribution with averaged 69.2% fast twitch (FT) muscle fibers for sprinters (12 male subjects) and 39.7% FT muscle fibers for distance runners (7 male subjects) . Measurement of muscle fiber conduction velocity during brief and maintained isometric contraction was made directly, using the zero-crossing time delay method between two surface myoelectric signals. Muscle fiber conduction velocities increased 4.40 to 4.84 m/sec for sprinters and 3.91 to 4.31 m/sec for distance runners as developing force increased from 25 to 100% maximum voluntary contraction (MVC) . The correlation coefficients between conduction velocity and % FT fibers were 0.59, 0.63, 0.64 and 0.84 for 25, 50, 75 and 100% MVC, respectively. During maintained at 50% of MVC conduction velocities in sprinters decreased linealy from 4.77 to 4.38 m/sec and in distance runners decreased from 4.11 to 3.96 m/sec at only last stage. It was suggested that the individual difference in conduction velocity was caused by different muscle fiber composition and that the changes of conduction velocity depended on the recruitment of muscle fibers.

We investigated the effects of strength training a muscle fiber conduction velocity in biceps brachii of 7 male students. The subjects were trained to exhaustion by 60% of maximum isotonic voluntary contraction with 3 sets/day, 3 days/week for 16 weeks. The muscle fiber conduction velocity was measured with a surface electorode array placed along the muscle fibers, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. Upper arm girth significantly increased (p<0.01), from 29.2±1.4 cm (means±S. D.) to 30.6±1.5 cm. On the other hand, training induced no significant changes in upper arm skinfold. A significant difference between pre- and post-training was found in maximum isotonic strength (p<0.01) . Although maximum isometric strength showed no significant changes with training, there was a tendency for an increase in maximum isometric strength. Muscle fiber conduction velocity increased by 3.5% during training period, but this was not significant. These results suggest no effects of strength training on muscle fiber conduction velocity.

To investigate whether muscle fiber conduction velocity reflects muscle fiber type, we studied the extensor digitorum longus (EDL) and soleus (SOL) muscle in 7 male rats aged 12 weeks. Muscle fiber conduction velocity was measured with a surface electrode array during stimulated contraction and calculated from the delay between two action potentials along muscle fibers for a given inter-electrode distance. Conduction velocity in the EDL (2.71±0.50 m/s) was significantly higher (p<0.05) than that in the SOL (2.14±0.34 m/s) . Fiber type, fiber area and fiber diameter were determinated by myosin ATPase staining and NADH-tetrazolium reductase staining. Muscle fiber composition of the EDL and SOL was 94.6±1.8 and 14.8±4.3% FT fibers, respectively, and mean muscle fiber diameter was 62.7±6.2 pm and 79.2±7.8pm, respectively, that of the EDL being significantly smaller (p<0.01) than that of the SOL. It was suggested that individual differences in conduction velocity were caused by differences in muscle fiber composition rather than differences in muscle diameter.