Muscle Plasticity and Repeatability to Alterations in Mechanical Loading Conditions
MetadataShow full item record
The breadth and depth of muscle biology research has consistently demonstrated the ability for skeletal muscle to adapt quickly and specifically to stimuli placed upon it, often regarded as plasticity. Changes in muscle mass are often tied to losses, recoveries, and gains in muscle function during mechanical unloading, reloading, or overloading (respectively). While a cadre of research has studied the impacts of disuse or exercise training on muscle, few efforts have detailed the capacity for muscle to fully recover after chronic disuse, which we define as an example of biological resilience. Furthermore, there are no known works that have characterized the effects of multiple bouts of long-duration unloading and recovery, which we propose as biological repeatability. To determine if resilience and repeatability should be considered facets of muscle plasticity, a cross-sectional study design was created to assess muscle protein turnover throughout several changes in mechanoloading. Male Sprague-Dawley rats (n = 108) were placed in one of the following groups: 28 d unloading (HU), 28 d unloading with 56 d recovery with either passive ambulation reloading (HU+REC) or chronic resistance training (HU+EX), groups exposed to 28 d unloading, one of the recovery strategies, and then a second 28 d unweighting period (HU+REC+HU & HU+EX+HU), animals undergoing two unloading and two passive recovery cycles (HU+REC+HU+REC), or control (CON). At the end of the study, muscle mass, rates of synthesis, and markers of anabolism/catabolism were assessed in the lower limbs. Results included muscle mass being highly coupled to anabolic signaling and subsequent protein synthesis. Muscle mass was significantly and consistently lower during unloading, but fully recovered with each reloading period. Our moderate-intensity, moderate-volume resistance exercise was insufficient to further improve recovery. And finally, the anabolic regulator DEPTOR was proven to correlate inversely with muscle mass. This study provides the first known evidence that repeatability is a component of muscle plasticity. The value in this work adds immediate benefits to manned space travel, clinical populations affected by serial unloading, as well as expands our general understanding of muscle biology.
Shimkus, Kevin Lee (2016). Muscle Plasticity and Repeatability to Alterations in Mechanical Loading Conditions. Doctoral dissertation, Texas A&M University. Available electronically from