Structural, mechanical and myothermic properties of rabbit rectococcygeus muscle.

1. The fine structure of rabbit rectococcygeus muscle has been studied with the electron microscope. 2. The mechanical performance and the heat production of this muscle has been investigated during tetanic contractions at 27 degrees C. 3. In isometric contractions a force of 164 +/- 27 mN/mm2 (mean +/- S.D., n = 17) is developed and the heat production is linearly related to the force. 4. There is a relationship between the duration of stimulation (t) and the total heat production (H) of the type H = A plus bt, where A and b are constants. 5. After-loaded isotonic experiments show that the relationship between force and velocity can be fitted by the 'characteristic equation' of Hill (1938). 6. The value of a/P0 (0-302 +/- 0-093, mean +/- S.D.) is slightly higher than in frog skeletal muscle but the constant b is about 50 times smaller. 7. The ratio of work/total energy production, for the stimulus conditions employed, was maximally 0-185. 8. The ratio of total enthalpy to initial enthalpy is difficult to measure accurately but is probably about 2.