Mechanical Responses of a Crustacean Slow Muscle

1. Mechanical properties of the abdominal ventral superficial muscle of the hermit crab, Pagurus pollicarus, were examined under isometric and iso-velocity conditions. The muscle was activated by stimulating its motor nerve at different frequencies. 2. Length-isometric tension relations were measured. Peak tension, Po, was 0'16-0'2MNm" and the sarcomere length of the muscle at the optimum length, Lo, was 10-8+ 1*0 fim. Passive tension was high at Lo. Correlated measurements of the operating length of the muscle and Lo indicate that the operating length is at a point on the ascending limb of the length-tension curve approximately 0-77 Lo. 3. The relationship between activation level of the muscle and the lengthtension relation indicates that the curve is not substantially displaced along the length axis by increasing activation level; increased force is primarily due to an increase in the slope of the ascending limb of the curve. 4. The force-velocity relation was obtained by measuring the force at a reference length during iso-velocity shortening of an active muscle. Hill constants of a/Po = 0-11 + 002and b = 1-07 + 0*24 mm s" were obtained. The maximum velocity of shortening per half sarcomere was approximately 4-2/ims". 5. Stretch of an active muscle did not produce an abrupt short range yield but a gradual transition between short range and terminal stiffness. This behaviour is shown to be due not to differences in cross bridge stiffness between VSM and other muscle but to a non cross bridge stiffness with a value that is one-fifth that of vertebrate muscle. 6. Such a low stiffness may provide an intrinsic mechanism for simplifying load compensation in the absence of rapid proprioceptive reflexes for the control of muscle stiffness.