The mechanical and thermal properties of frog slow muscle fibres.

1. A study has been made of the mechanical behaviour and the heat production of frog slow muscle fibres in iliofibularis nerve-muscle preparations at 20 degrees C.2. The slow fibre isometric tension and its rate of development increase with stimulation frequency, the increases beyond 30 Hz being relatively small. Relaxation rate also increases with stimulation frequency. The tension-length curve and maximum isometric tension (250 mN.mm(-2)) are similar to those of twitch fibres. The maximum shortening velocity is estimated to be 0.11 tonus bundle lengths per second.3. For contractions up to 60 sec at 30-50 Hz the slow fibre heat rate is steady at 6 mJ.g(-1).sec(-1). Slow fibres produce aerobic recovery heat with a time course similar to that of twitch fibres.4. The accuracy of the results is discussed, and a comparison is made with the properties of twitch fibres. It is concluded that the tension-producing reactions are thirty times slower in the slow fibres.