Force-velocity properties of fast-twitch and slow-twitch muscles of the kitten

The characteristics of the isometric and force-velocity properties of the fast-twitch flexor digitorum longus (f.d.l.) and the slow-twitch soleus muscles of the developing kitten were determined at post-natal ages of 2 days and 1,2,4 and 6 weeks. The contractile properties of the isometric twitch and tetanus were similar to those previously reported by other authors. The minimum stimulation frequency required for isotonic contractions to achieve the maximum speed of shortening at any given load was 200 Hz for f.d.l. and 150 Hz for soleus at all ages. The extrapolated maximum speed of sarcomere (sarc.) shortening (Vmax) was significantly different between the f.d.l. (22.3 microns s-1 sarc.-1) and soleus (13.1 microns s-1 sarc.-1) at 2 days of age. The f.d.l. Vmax achieved adult values by 4 weeks of age, while there was little or no change in the soleus Vmax. The force-velocity properties are discussed and compared to the changes which have been observed in other contractile properties of the cat f.d.l. and soleus muscles.