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This Article Full Text Full Text (PDF) Supplemental data All Versions of this Article: 584/2/677    most recent jphysiol.2007.140053v1 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 Loram, I. D. Articles by Lakie, M. Search for Related Content PubMed PubMed Citation Articles by Loram, I. D. Articles by Lakie, M. Related Collections Integrative

¹ Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Alsager ST7 2HL, UK
² pplied Physiology Research Group, School of Sport and Exercise Sciences, University of Birmingham, Birmingham B15 2TT, UK

Using short duration perturbations, previous attempts to measure the intrinsic ankle stiffness during human standing have revealed a substantial stabilizing contribution (65–90% normalized to load stiffness ‘mgh’). Others regard this method as unsuitable for the low-frequency conditions of quiet standing and believe the passive contribution to be small (10–15%). This latter view, consistent with a linear Hill-type model, argues that during standing, the contractile portion of the muscle is much less stiff than the tendon. Here, for upright subjects, we settle this issue by measuring the stiffness of the contractile portion of the passive calf muscles using low-frequency ankle rotations. Using ultrasound we tracked the changes in muscle contractile length and partitioned the ankle rotation into contractile and extra-contractile (series elastic) portions. Small ankle rotations of 0.15 and 0.4 deg show a contractile to series elastic stiffness ratio (K_ce/K_se) of 12 ± 9 and 6.3 ± 10, respectively, with both elements displaying predominantly elastic behaviour. Larger, 7 deg rotations reveal the range of this ratio. It declines in a non-linear way from a high value (K_ce/K_se = 18 ± 11) to a low value (K_ce/K_se = 1 ± 0.4) as rotation increases from 0.1 to 7 deg. There is a marked transition at around 0.5 deg. The series elastic stiffness (K_se/mgh) remains largely constant (77 ± 13%) demonstrating the contractile component origin of passive, short range stiffness. The linear Hill-type model does not describe the range-related stiffness relevant to the progression from quiet standing to perturbed balance and movement and can lead to inaccurate predictions regarding human balance.

(Received 3 July 2007; accepted after revision 22 August 2007; first published online 6 September 2007)
Corresponding author I. Loram: Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Alsager ST7 2HL, UK. Email: i.loram{at}mmu.ac.uk

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