High-intensity stretch-shortening contraction training modifies responsivity of skeletal muscle in old male rats

Age-dependent stress response DNA demethylation and gene upregulation accompany nuclear and skeletal muscle remodeling following acute resistance-type exercise in rats

“…The mid-belly of each TA muscle was covered with tissue freezing media and immersed in isopentane (−160 °C). This tissue was then cryosectioned at 12 μm thickness, hematoxylin and eosin stained, and analyzed by a standard stereological method ( Baker et al 2006 ; Rader et al 2017b , 2018 ). At 2-mm offset from either side of the midpoint, five equally spaced fields (at 40× magnification) were evaluated.…”

“…The index of circularity was determined by the equation 4 π (area/perimeter 2 ) with a perfect circle as a value of 1 and increasing elongation as the value decreases. Colocalization analysis was performed using the ImageJ plugin Colocalization_class which was utilized for determination of nuclear staining for GADD45α thereby labeling colocalized regions as light yellow ( Rader et al 2018 ).…”

“…Each point was determined to be overlaying a nondegenerative muscle fiber, degenerative muscle fiber, cellular interstitium, or noncellular interstitium. Degenerative muscle fibers were defined as having (i) loss of contact with surrounding fibers, (ii) interdigitation of the sarcolemma by cellular infiltrates, and (iii) internalization of cellular infiltrates (Baker et al 2006;Rader et al 2018). Points overlaying interstitial nuclei were considered cellular interstitium, whereas points overlaying interstitial regions without nuclei were coded as noncellular interstitium.…”

Efficacy of Age-Specific High-Intensity Stretch-Shortening Contractions in Reversing Dynapenia, Sarcopenia, and Loss of Skeletal Muscle Quality

“…In a subsequent investigation from this same group, both the mode (i.e., isometric contractions alone, utilized as a minimalistic RTET approach) as well as repetition number was studied. With regards to both the utilization of isometric contractions in age-dependent RTET and/or reducing the number of SSC repetitions (i.e., compared with the previous stated study), neither the isometric contraction RTET nor the reduced SSC repetition protocols were successful in restoring skeletal muscle performance, skeletal muscle mass, and skeletal muscle quality [86]. In these fundamental in vivo studies, it has been established that skeletal muscle quality is modifiable with increasing age; yet, these data suggest that the RTET prescription or mechanical loading paradigm must be precisely prescribed in order to achieve these benefits (i.e., increased function, skeletal muscle mass, skeletal muscle quality, etc.)…”

“…For these reasons and with these limitations in mind, fundamental, in vivo animal models that are valid and representative of the human exposure-response relationship with respect to aging and exercise are an essential tool for investigating and quantifying these phenomena due to the exquisite control they allow over the biomechanical loading exposure as well as providing a means to evaluate, characterize, and quantify the integrated skeletal muscle response systematically. Interestingly, only recently have investigations begun to question and quantify the impact age-specific RTET imparts on skeletal muscle quality and whether this specific and sensitive outcome metric (i.e., "biomarker") is modifiable with aging [13,38,66,86]. Initial studies by Cutlip and colleagues provided a basis for investigating non-injurious SSC RTET exposure in a validated, fundamental rodent model of aging, which quantified the adaptive and maladaptive response in young and old skeletal muscle, respectively [13].…”