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, P0, was 0.16–0.2MNm−2 and the sarcomere length of the muscle at the optimum length, L0, was 10.8+1.0 μm. Passive tension was high at L0. Correlated measurements of the operating length of the muscle and L0 indicate that the operating length is at a point on the ascending limb of the length-tension curve approximately 0.77 L0. 3. The relationship between activation level of the muscle and the length-tension 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/P0 = 0.11 + 0.02 and b = 1.07 = 0.24 mm s−1 were obtained. The maximum velocity of shortening per half sarcomere was approximately 4.2μms−1. 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.