Short Range Stiffness

A direct measure of muscle stiffness can be obtained by characterizing short-range stiffness (SRS). SRS describes the elastic properties of muscle in response to small, rapid changes in length. Measurements of SRS can be made in an in situ muscle preparation that allows for direct measures of muscle force and length. The SRS of individual muscles scales linearly with actively generated force and is thought to be directly related to the number of attached cross bridges (16, 17, 19). We measured SRS with a fast length perturbation before the end of tetanic stimulation (27). The perturbation had a displacement of 2 mm and a speed of 2 m/s. For the cat soleus, this corresponded to a displacement of ∼2.7% of the soleus fascicle length at L_o (x_fo) and a nominal speed of 54 x_fo/s. For cat MG, the displacement was 5.4% of fascicle length with a speed of 108 x_fo/s. SRS is nearly invariant to the speed of stretch within this range (29). We also measured the SRS of passive muscles using the same methods. Passive fascicle SRS shows a linear increase with stress. The mechanism is less clear but is likely to result from stretch of titin and the extracellular matrix (20, 30–32).