Submitted on April 24, 2006
Accepted on June 13, 2007
The Mechanical Behavior of Activated Skeletal Muscle During Stretch: Effects of Muscle Unloading and MyHC Isoform Shifts
Vincent J. Caiozzo1*, Heather Richmond2, Serge Kaska1, and Dahlia Valeroso2
1 Department of Orthopaedics, University of California - Irvine College of Med., Irvine, California, United States
2 Orthopaedics, University of California, Irvine, California, United States
* To whom correspondence should be addressed. E-mail: vjcaiozz{at}uci.edu.
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 to 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 pre-yield modulus (Epre). Similarly, a measure of modulus also can be used to characterize the post yield response (Epost). This study examined the effects of muscle atrophy and altered MyHC isoform composition on both Epre and Epost. Female Sprague-Dawley rats were assigned to: i) control group (CON); ii) a hypothyroid group (-T3); iii) a hyperthyroid group (+T3); iv) a hindlimb suspension group (HS); and v) an hindlimb suspension + hyperthyroid group (HS+T3). These interventions were used to either alter the MHC isoform composition of the muscle or to induce atrophy. Soleus muscles were stretched at strain rates of ~0.15 to 1.25 ML/s. The findings of this study demonstrate that 4 wks of HS can produce a large (i.e., 40-60%) reduction in Epre. HS did not produce a proportional change in Epost. Analyses of the Epre-strain rate relationship demonstrated that there was little dependence on MyHC isoform composition. In summary, the disproportionate decrease in Epre 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.
