The postnatal development of some twitch and fatigue properties of the ankle flexor and extensor muscles of the cat.

The isometric responses of the medial gastrocnemium (MG), soleus (SOL) and anterior tibial (TA) muscles to single shocks and different modes of repetitive stimulation were studied in kittens of varying postnatal ages and in adult cats. The postnatal decrease in time-to-peak and half-relaxation time of the twitch contractions were similar for the MG and TA muscles and adult values were attained at around 6-7 weeks of age. The SOL muslce displayed a transient decrease in contraction time during the first postnatal weeks, followed later by a slowing towards adult values. The susceptibility to fatigue during iterative stimulation was smallest in the SOL at all ages studied, and usually largest in TA. It changed only little for the MG and SOL postnatally while increasing markedly for the TA up until 6-7 weeks of age. Tetanic contraction resulted in similar depressions in contractile tension of all three muscles in the youngest kittens, but the SOL displayed a greater ability to recover from this depression than the MG and, in particular, the TA muscles. Tetanus resistance increased postnatally and adult responses were attained at 6-7 weeks of age.