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Spring-like leg behaviour, musculoskeletal mechanics and control in maximum and submaximum height human hopping

Maarten F. Bobbert and L. J. Richard Casius
Philosophical Transactions: Biological Sciences
Vol. 366, No. 1570, Integration of muscle function for producing and controlling movement (27 May 2011), pp. 1516-1529
Published by: Royal Society
Stable URL: http://www.jstor.org/stable/23035557
Page Count: 14
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Spring-like leg behaviour, musculoskeletal mechanics and control in maximum and submaximum height human hopping
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Abstract

The purpose of this study was to understand how humans regulate their 'leg stiffness' in hopping, and to determine whether this regulation is intended to minimize energy expenditure. 'Leg stiffness' is the slope of the relationship between ground reaction force and displacement of the centre of mass (CM). Variations in leg stiffness were achieved in six subjects by having them hop at maximum and submaximum heights at a frequency of 1.7 Hz. Kinematics, ground reaction forces and electromyograms were measured. Leg stiffness decreased with hopping height, from 350 N m -1 kg -1 at 26 cm to 150 N m -1 kg -1 at 14 cm. Subjects reduced hopping height primarily by reducing the amplitude of muscle activation. Experimental results were reproduced with a model of the musculoskeletal system comprising four body segments and nine Hill-type muscles, with muscle stimulation STIM(t) as only input. Correspondence between simulated hops and experimental hops was poor when STIM(t) was optimized to minimize mechanical energy expenditure, but good when an objective function was used that penalized jerk of CM motion, suggesting that hopping subjects are not minimizing energy expenditure. Instead, we speculated, subjects are using a simple control strategy that results in smooth movements and a decrease in leg stiffness with hopping height.

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