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First published online August 25, 2003

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Mechanical properties of rat soleus aponeurosis and tendon during variable recruitment in situ

Ryan J. Monti¹, Roland R. Roy^2,*, Hui Zhong² and V. R. Edgerton^1,2

¹ Department of Physiological Science, University of California Los Angeles, Los Angeles, CA 90095-1761, USA
² Brain Research Institute, University of California Los Angeles, Los Angeles, CA 90095-1761, USA

View larger version (60K): [in a new window]   Fig. 1. (A) Captured video frame showing the typical spacing of the metal particles along the length of the tendon and aponeurosis. The circled particle is implanted in the tibia for use as a reference point to compensate for any limb movement. (B) Representation of the extent of the aponeurosis as a reference for the actual placement of the particles.

View larger version (99K): [in a new window]   Fig. 2. (A) The same captured video frame shown in Fig. 1, indicating the level of the histological section shown in B. The section is stained with hemotoxylin and eosin and illustrates the placement and encapsulation of the metal particle. Note that it is completely surrounded by connective tissue that is physically continuous with the aponeurosis.

View larger version (20K): [in a new window]   Fig. 3. Representative force (A) and strain (B) patterns for a single particle pair during a tetanic contraction. The mean force and strain from a series of video frames (shaded region; 100 ms) were used to construct the force-strain curves for every particle pair studied.

View larger version (11K): [in a new window]   Fig. 4. A representative force-strain curve for a single pair of particles (middle aponeurosis; Lo). Each point (x) is from a maximal tetanic contraction via a different combination of ventral root filament bundles. At the lowest recruitment levels used in this study (15-20% of Po), the strains were typically more than 50% of the strains attained at Po. This implies the existence of a highly compliant toe region. The straight line is a tangent to the force-strain curve at 75% of Po, and its slope is the stiffness of the tissue at that point. Po for this muscle was 1.58 N. Thus, it can be seen from the relationship between this tangent line and the force-strain curve that the stiffness of the tissue did not vary much above 30% of Po (0.5 N for this muscle).

View larger version (20K): [in a new window]   Fig. 5. The relationship between peak strain (strain at Po) and muscle-tendon unit (MTU) length for each of the five segments defined in this study: DA, dorsal aponeurosis; MA, middle aponeurosis; PA, proximal aponeurosis; PT/DA, proximal tendon/distal aponeurosis; TEND, tendon. Strains in the tendon were significantly (P<0.05) less than those in all regions of the aponeurosis and did not vary significantly with increasing MTU length. By contrast, peak strains in the aponeurosis tended to increase with lengthening of the MTU. Values are means ± 1 S.D. (N=4).

View larger version (22K): [in a new window]   Fig. 6. The relationship between stiffness (N/% strain) and muscle-tendon unit (MTU) length for each of the five segments defined in this study; for definitions of abbreviations, see Fig. 5. Tendon stiffness was greater than that of all regions of the aponeurosis and did not vary significantly with increasing MTU length. The stiffness of the aponeurosis was relatively constant across lengths but was significantly (P<0.05) higher at Lo+2 mm than at any other length. Values are means ± 1 S.D. (N=4).

View larger version (14K): [in a new window]   Fig. 7. Variations in stiffness among the five segments studied at Po or 0.25 of Po. Stiffness declines from the tendon to the aponeurosis, and the tendon was significantly (P<0.05; indicated on graph) stiffer than all three regions that did not include any overlap between tendon and aponeurosis. In addition, while the general pattern holds for contractions at 0.25 of Po, the tissue is much more compliant. TEND, tendon; PTDA, proximal tendon/distal aponeurosis; DA, dorsal aponeurosis; MA, middle aponeurosis; PA, proximal aponeurosis.

View larger version (24K): [in a new window]   Fig. 8. Hypothetical relationships between muscle fiber and connective tissue recruitment. (A) All connective tissue is loaded at every recruitment level, leading to a linear increase in connective tissue stress with an increasing number of muscle fibers recruited. (B) The amount of connective tissue is a constant function of recruitment, leading to a constant stress in the connective tissue. (C) The amount of connective tissue recruited is disproportionately high at low forces, with an increase as force rises. This leads to a non-linear increase in connective tissue stress with recruitment. For details, see text.