DEPRESSION OF TETANIC FORCE INDUCED BY LOADED SHORTENING OF FROG-MUSCLE FIBERS

1. Single fibres isolated from the anterior tibialis muscle of Rana te mporaria were allowed to shorten against a high load during a 2.5-4.0 s fused tetanus (1-3-degrees-C) and the maximum force produced at the short length was compared with that recorded during a fixed-end tetanu s at the same overall fibre length. Changes in length of marked, conse cutive segments (ca 0.5 mm in length) along the fibre were measured th roughout the tetanus using a photoelectric recording system. 2. Loaded shortening (load ca 3/4 of maximum tetanic force) starting from appro ximately 2.55 mum sarcomere length and ending near slack fibre length depressed the tetanic force by 13 +/- 2 % (mean +/- S.E.M., n = 10) an d caused a marked redistribution of sarcomere length along the fibre. Unloaded shortening over the same range caused no force deficit and di d not lead to increased dispersion of sarcomere length. 3. Loaded shor tening below slack length produced less force depression and less non- uniformity of sarcomere length than did a corresponding intervention a bove slack length. 4. The force deficit after loaded shortening, both above and below slack fibre length, was positively correlated (P < 0.0 05) to the coefficient of variation of the sarcomere length along the fibre. 5. The decrease in active force after loaded shortening, and it s relation to increased dispersion of sarcomere length along the fibre , could be simulated closely by a computer model in which the muscle f ibre was assumed to consist of eleven discrete segments acting in seri es with a passive elastic element. 6. Experiments were performed in wh ich the length of an individual segment of the intact muscle fibre was strictly controlled throughout a tetanus. Loaded shortening of such a 'length-clamped' segment caused no force depression during the subseq uent isometric phase either above or below slack fibre length. 7. The results suggest strongly that force depression after loaded shortening of a single muscle fibre is attributable to non-uniform sarcomere beh aviour along the fibre. The experimental evidence supports the view th at: (i) the myosin cross-bridges act as independent force generators; and (ii) their steady-state performance during a tetanus is unaffected by the preceding contractile activity.