Figure 6. Underdamped restretch or restretch + step release.

Shortening ramps were terminated by step stretches that were underdamped to various extents, causing ‘ringing’ of the servomotor. Force records with ringing amplitude of 0%, 2.8% and 6.2%L_o are shown, with the lowest T_min resulting from the largest ringing. The full amplitude of ringing was not sampled in the computer data, but the actual time courses were recorded separately on an oscilloscope (inset to fibre length graph). Records were in PM control, as underdamping tended to destabilize the feedback loop during the large restretches. Single- and double-exponential functions are shown for the records corresponding to 0% and 6.2% ringing. For the critically damped restretch, where the double-exponential fit is only slightly better than the single exponential (see residuals and double–single difference traces), k_r1 = 6.3 s⁻¹, k_rs = 4.0 s⁻¹, k_rf = 11.2 s⁻¹ and A′_f = 0.62, and for the underdamped restretch, k_r1 = 4.5 s⁻¹, k_rs = 2.4 s⁻¹, k_rf = 10.8 s⁻¹ and A′_f = 0.56. CS = 4.6 × 10³μm², length = 2.45 mm.