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This Article Full Text Free Full Text (PDF) Free All Versions of this Article: 103/4/1150    most recent 00469.2006v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Caiozzo, V. J. Articles by Valeroso, D. Search for Related Content PubMed PubMed Citation Articles by Caiozzo, V. J. Articles by Valeroso, D.

The mechanical behavior of activated skeletal muscle during stretch: effects of muscle unloading and MyHC isoform shifts

Departments of ¹Orthopedics and ²Physiology and Biophysics, College of Medicine, University of California, Irvine, California

Submitted 24 April 2006 ; accepted in final form 13 June 2007

The response of activated skeletal muscle to a ramp stretch is complex. Force rises rapidly above the isometric plateau during the initial phase of stretch. However, after a strain of 1–2%, force yields and continues to rise but with a slower slope. The resistance to stretch during the initial phase can be characterized by the stiffness of the muscle and/or the preyield modulus (E_pre). Similarly, a measure of modulus also can be used to characterize the postyield modulus response (E_post). This study examined the effects of muscle atrophy and altered myosin heavy chain (MyHC) isoform composition on both E_pre and E_post. Female Sprague-Dawley rats were assigned to 1) control group, 2) a hypothyroid group, 3) a hyperthyroid group, 4) a hindlimb suspension group, and 5) a hindlimb suspension + hyperthyroid group. These interventions were used either to alter the MyHC isoform composition of the muscle or to induce atrophy. Soleus muscles were stretched at strain rates that ranged from 0.15 to 1.25 muscle length/s. The findings of this study demonstrate that 4 wk of hindlimb suspension can produce a large (i.e., 40–60%) reduction in E_pre. Hindlimb suspension did not produce a proportional change in E_post. Analyses of the E_pre-strain rate relationship demonstrated that there was little dependence on MyHC isoform composition. In summary, the disproportionate decrease in E_pre of atrophied muscle has important implications with respect to issues related to joint stability, especially under dynamic conditions and conditions where the static joint stabilizers (i.e., ligaments) have been compromised by injury.

short-range stiffness; muscle atrophy; rehabilitation; ramp stretch; myosin heavy chain isoform; elastic modulus; single fiber mechanics