We measured the dynamic stiffness of passive single extensor cells from the abdomen of the shrimp Pandalus danae. These stiffness measurements, consisting of both elastic and viscous moduli, were carried out by imposing sinusoidal length changes on single cells and recording, simultaneously, the resultant force. Our measurements were made over a broad range of loading frequencies (1–120 Hz) and sarcomere lengths (1.9-3.2μm), encompassing the physiological range of these parameters.
We found that: (1) there is an exponential increase in both elastic and viscous moduli with increases in sarcomere length; (2) there is a weak increase in these moduli with increasing frequency; (3) the relative energy dissipated by the viscous behavior of cells is largest in the relevant physiological range of loading frequencies and sarcomere lengths, with the magnitude of the viscous modulus nearly equal to the elastic modulus; and (4) the energy dissipated by the viscous behavior of these cells alone constitutes about 10 % of the total energy required to accelerate the animal in its aquatic environment. We propose that such large values for viscous energy dissipation in passive cells may be necessary to prevent unstable responses of muscle to the rapid dynamic loads imposed by the escape event.