The mechanical properties and heat production of chicken latissimus dorsi muscles during tetanic contractions.

1. A study has been made of the mechanical properties and heat production of chicken anterior (ALD) and posterior (PLD) latissimus dorsi muscles during contractions at 20 degrees C.2. There is a difference between these two muscles in the time course of the isometric response. The PLD reaches maximum tetanic tension 10 times faster and relaxes 8 times faster than ALD. The ratio of heat rate to isometric tension (heat rate/tension x length) for PLD is 7-8 times larger than for ALD.3. ALD maintains substantial isometric tension for more than 2 min of stimulation. In PLD tetanic tension begins to fall after only 1 sec.4. The ALD muscle does not show the ;activation' heat seen at the start of contraction with frog and toad muscle but this may be present in PLD.5. There is a range of stimulation frequencies for both muscles over which the fused tetanic tension increases with stimulation frequency.6. The tension-length curve of ALD has a pronounced plateau and is broader than that of PLD.7. The normalized force-velocity relations for the two muscles are similar and may be fitted by Hill's equation with a value of a/P(0) = 0.15-0.16. The maximum velocity of unloaded shortening of PLD is 4-5 times that of ALD.8. Preliminary experiments indicate that the resting heat rate of both muscles is 4 times greater than that of frog muscle at the same temperature. The recovery heat rate of ALD is similar to that of frog.