dc.description.abstract | 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. | en |