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This Article Full Text Full Text (PDF) All Versions of this Article: 585/2/607    most recent jphysiol.2007.141440v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Colombini, B. Articles by Bagni, M. A. Search for Related Content PubMed PubMed Citation Articles by Colombini, B. Articles by Bagni, M. A. Related Collections Skeletal Muscle and Exercise

¹ Dipartimento di Scienze Fisiologiche and Istituto Interuniversitario di Miologia, Università degli Studi di Firenze, Viale G.B. Morgagni 63, I-50134 Firenze, Italy

The mechanism of force enhancement during lengthening was investigated on single frog muscle fibres by using fast stretches to measure the rupture tension of the crossbridge ensemble. Fast stretches were applied to one end of the activated fibre and force responses were measured at the other. Sarcomere length was measured by a striation follower device. Fast stretching induced a linear increase of tension that reached a peak and fell before the end of the stretch indicating that a sudden increase of fibre compliance occurred due to forced crossbridge detachment induced by the fast loading. The peak tension (critical tension, P_c) and the sarcomere length needed to reach P_c (critical length, L_c) were measured at various tensions during the isometric tetanus rise and during force enhancement by slow lengthening. The data showed that P_c was proportional to the tension generated by the fibre under both isometric and slow lengthening conditions. However, for a given tension increase, P_c was 6.5 times greater during isometric than during lengthening conditions. Isometric critical length was 13.04 ± 0.17 nm per half-sarcomere (nm hs^–1) independently of tension. During slow lengthening critical length fell as the force enhancement increased. For 90% enhancement, L_c reduced to 8.19 ± 0.039 nm hs^–1. Assuming that the rupture force of the individual crossbridge is constant, these data indicate that the increase of crossbridge number during lengthening accounts for only 15.4% of the total force enhancement. The remaining 84.6% is accounted for by the increased mean strain of the crossbridges.

(Received 26 July 2007; accepted after revision 9 October 2007; first published online 11 October 2007)
Corresponding author G. Cecchi: Dipartimento di Scienze Fisiologiche, Viale G.B. Morgagni 63, I-50134. Email: giovanni.cecchi{at}unifi.it