Force: velocity pproperties of kitten muscles.

1. The characteristics of isometric contractions and force:velocity properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles of neonatal kittens were determined in situ.2. The mean contraction time is 51 msec for EDL and 70 msec for SOL and the half-relaxation time is 51 msec for EDL and 109 msec for SOL.3. The average maximum isometric tetanic tension per unit cross-sectional area of muscle is 1.27 kg/cm(2) for EDL and 1.17 kg/cm(2) for SOL.4. The average twitch:tetanus ratio is 0.28 for EDL and 0.119 for SOL; the low value for SOL was found for both indirect and direct stimulation.5. The average maximum speed of shortening of a sarcomere is 22.8 mu/sec for EDL and 12.7 mu/sec for SOL.6. These properties of neonatal muscles are compared with those of adult cat muscles and discussed in connexion with differentiation of mammalian muscles into fast and slow types.