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This Article Full Text Full Text (PDF) All Versions of this Article: 580/3/1007    most recent jphysiol.2006.119982v1 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 HighWire Citing Articles via Google Scholar Google Scholar Articles by Edman, K. A. P. Articles by Josephson, R. K. Search for Related Content PubMed PubMed Citation Articles by Edman, K. A. P. Articles by Josephson, R. K. Related Collections Skeletal Muscle and Exercise

¹ Department of Experimental Medical Science, Biomedical Centre, F11, University of Lund, S-221 84 Lund, Sweden

Force–velocity (F–V) relationships were determined for single frog muscle fibres during the rise of tetanic contraction. F–V curves obtained using isotonic shortening early in a tetanic contraction were different from those obtained at equivalent times with isovelocity shortening, apparently because changing activation early in the contraction leads, in isovelocity experiments, to changing force and changing series elastic extension. F–V curves obtained with isotonic and with isovelocity shortening are similar if the shortening velocity in the isovelocity trials is corrected for series elastic extension. There is a progressive shift in the scaling of force–velocity curves along the force axis during the course of the tetanic rise, reflecting increasing fibre activation. The time taken for F–V curves to reach the steady-state position was quite variable, ranging from about 50 ms after the onset of contraction (1–3°C) to well over 100 ms in different fibres. The muscle force at a fixed, moderately high shortening velocity relative to the force at this velocity during the tetanic plateau was taken as a measure of muscle activation. The reference velocity used was 60% of the maximum shortening velocity (V_max) at the tetanic plateau. The estimated value of the fractional activation at 40 ms after the onset of contraction was used as a measure of the rate of activation. The rate of rise of isometric tension in different fibres was correlated with the rate of fibre activation and with V_max during the plateau of the tetanus. Together differences in rate of activation and in V_max accounted for 60–80% of the fibre-to-fibre variability in the rate of rise of isometric tension, depending on the measure of the force rise time used. There was not a significant correlation between the rate of fibre activation and V_max. The steady-state F–V characteristics and the rate at which these characteristics are achieved early in contraction are seemingly independent. A simulation study based on F–V properties and series compliance in frog muscle fibres indicates that if muscle activation were instantaneous, the time taken for force to rise to 50% of the plateau value would be about 60% shorter than that actually measured from living fibres. Thus about 60% of the force rise time is a consequence of the time course of activation processes and about 40% represents time taken to stretch series compliance by activated contractile material.

(Received 25 August 2006; accepted after revision 12 February 2007; first published online 15 February 2007)
Corresponding author R. K. Josephson: Department of Experimental Medical Science, Biomedical Centre, F11, University of Lund, S-221 84 Lund, Sweden.  Email: rkjoseph{at}uci.edu

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