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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. Gandevia3,*

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


Figure 1
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  		Fig. 1. Testing equipment and experiment set up. (A) Testing equipment. (B) Schematic diagram of the testing equipment and experimental set up. The subject lies prone, strapped to the lying board, with the assumed axes of rotation of the knee and ankle aligned with potentiometers by laser pointers. The right foot, strapped to the footplate, is manually rotated through the ankle's range of movement. The footplate is controlled by a `floating' balance mechanism to accommodate slight changes in the location of the ankle axis during rotation as well as the subjects' differing foot shapes. The lying board can be moved up and down by a motor to change the knee angle without moving the lower leg. An ultrasound transducer was stabilized over the midbelly of the medial gastrocnemius to generate images of muscle fascicles during ankle rotation. Passive ankle torque, ankle and knee angles and ultrasound images were recorded simultaneously.

 

Figure 2
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  		Fig. 2. (A) Schematic diagram of single joint structures crossing plantar and dorsal aspects of the ankle and the gastrocnemius crossing both ankle and knee. Passive ankle torque is assumed due to resistive forces from these structures. (B) Raw data of passive ankle torques when the ankle was rotated from full plantarflexion to full dorsiflexion from one subject measured at four of the eight different knee angles (0°, 20°, 70° and 100°). Differences in torque-angle relations are assumed to reflect changes in the length of the gastrocnemius (see Materials and methods).

 

Figure 3
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  		Fig. 3. Single frame ultrasound image of the medial gastrocnemius from one subject. Three sets of three points (white crosses) were marked on the first frame to identify the lines of the superficial aponeurosis (top white line), the deep aponeurosis (bottom white line) and a muscle fascicle (diagonal white line). The length of the fascicles (lf) is defined by the intersections of the three lines (white circles). The pennation angle of the fascicles ({alpha}) is the acute angle between the fascicle line and the deep aponeurosis line. The longitudinal displacement of the fascicle (d) is used to calculate the length of the tendon (length of the muscle–tendon unit – d).

 

Figure 4
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  		Fig. 4. Relationship between changes in the length of muscle fascicles and the whole muscle–tendon unit during muscle lengthening in one subject. (A–C) The relationship between changes in the lengths of muscle fascicles and the whole muscle–tendon unit when the knee was at 20°, 60° and 90°, respectively. The relationship at the three different knee angles are very similar. (D) The three relationships superimposed, and the fitted line (in red) using Eqn 3 (see Materials and methods). The vertical red arrow shows slack length of the whole muscle–tendon unit, and the horizontal red arrow indicates the slack length of the muscle fascicles.

 

Figure 5
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  		Fig. 5. (A) An example of passive length–tension relationships of the muscle–tendon unit, muscle fascicles and tendon of the gastrocnemius of one subject. (B) Length–tension curves of muscle fascicles and tendons of six subjects.

 

Figure 6
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  		Fig. 6. Changes in length–tension curves from one subject associated with errors in estimation of model parameters, experimentally recorded ankle passive torques and other torque calculations. (A–C) Changes in the passive length tension curve due to 5% errors of each parameter. (D) Changes in the passive length–tension curve after scaling up or down by 2% and 5% the initial experimentally recorded passive ankle torque. (E) Changes in the length–tension curve after adding biases to the footplate torques (of 5%) and the foot torque (of 25%). See List of symbols for definitions of parameters.

 




© The Company of Biologists Ltd 2007