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This Article Full Text Full Text (PDF) All Versions of this Article: 567/3/1047    most recent jphysiol.2005.091124v1 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 Sasaki, K. Articles by Ishii, N. Search for Related Content PubMed PubMed Citation Articles by Sasaki, K. Articles by Ishii, N.

¹ Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan

Unloaded shortening velocity (V₀) of human triceps surae muscle was measured in vivo by applying the ‘slack test’, originally developed for determining V₀ of single muscle fibres, to voluntary contractions at varied activation levels (ALs). V₀ was measured from 10 subjects at five different ALs defined as a fraction (5, 10, 20, 40 and 60%) of the maximum voluntary contraction (MVC) torque. Although individual variability was apparent, V₀ tended to increase with AL (R² = 0.089; P = 0.035) up to 60%MVC (8.6 ± 2.6 rad s^–1). This value of V₀ at 60%MVC was comparable to the maximum shortening velocity of plantar flexors reported in the previous studies. Electromyographic analysis showed that the activities of soleus, medial gastrocnemius and lateral gastrocnemius muscles increased with AL during isometric contraction and after the application of quick release in a similar manner. Also, it showed that the activity of an antagonist, tibialis anterior muscle, was negligible, even though a slight increase took place after the quick release of agonist. Correlation analysis showed that there were no significant correlations between V₀ and MVC torque normalized with respect to body mass, although the correlation coefficient was relatively high at low ALs. The results suggest that in human muscle, V₀ represents the unloaded velocity of the fastest muscle fibres recruited, and increases with AL possibly because of progressive recruitment of faster fibres. Individual variability may be explained, at least partially, by the difference in fibre-type composition.

(Received 23 May 2005; accepted after revision 7 July 2005; first published online 7 July 2005)
Corresponding author K. Sasaki: Laboratory of Sports Sciences, Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan. Email: cc47710{at}mail.ecc.u-tokyo.ac.jp