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First published online November 19, 2007
Journal of Experimental Biology 210, 4159-4168 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.002204

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Passive mechanical properties of human gastrocnemius muscle–tendon units, muscle fascicles and tendons in vivo

P. D. Hoang¹, R. D. Herbert¹, G. Todd², R. B. Gorman³ and S. C. Gandevia^3,*

¹ School of Physiotherapy, Faculty of Health Sciences, University of Sydney, 2141, Australia
² School of Molecular and Biomedical Science, University of Adelaide, 5005, Australia
³ Prince of Wales Medical Research Institute University of New South Wales, Cnr Barker Street and Easy Street, Randwick, NSW 2031, Australia

^* Author for correspondence (e-mail: s.gandevia{at}unsw.edu.au)

Accepted 21 August 2007

This study provides the first in vivo measures of the passive length–tension properties of relaxed human muscle fascicles and their tendons. A new method was used to derive passive length–tension properties of human gastrocnemius muscle–tendon units from measures of ankle stiffness obtained at a range of knee angles. Passive length–tension curves of the muscle–tendon unit were then combined with ultrasonographic measures of muscle fascicle length and pennation to determine passive length–tension curves of the muscle fascicles and tendons. Mean slack lengths of the fascicles, tendons and whole muscle–tendon units were 3.3±0.5 cm, 39.5±1.6 cm and 42.3±1.5 cm, respectively (means ± s.d., N=6). On average, the muscle–tendon units were slack (i.e. their passive tension was zero) over the shortest 2.3±1.2 cm of their range. With combined changes of knee and ankle angles, the maximal increase in length of the gastrocnemius muscle–tendon unit above slack length was 6.7±1.9 cm, of which 52.4±11.7% was due to elongation of the tendon. Muscle fascicles and tendons underwent strains of 86.4±26.8% and 9.2±4.1%, respectively, across the physiological range of lengths. We conclude that the relaxed human gastrocnemius muscle–tendon unit falls slack over about one-quarter of its in vivo length and that muscle fascicle strains are much greater than tendon strains. Nonetheless, because the tendons are much longer than the muscle fascicles, tendons contribute more than half of the total compliance of the muscle–tendon unit.

Key words: gastrocnemius, muscle, strain, tendon

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