Abstract
Age-related differences in contraction-induced adaptation have been well characterized especially for young and old rodent models but much less so at intermediate ages. Therefore, additional research is warranted to determine to what extent alterations in adaptation are due to maturation versus aging per se. The purpose of our study was to evaluate muscles of Fisher 344XBrown Norway rats of various ages following one month of exposure to stretch-shortening contractions (SSCs). With exposure, muscles mass increased by ~10% for 27 and 30 month old rats vs. ~20% for 3 and 6 month old rats (P < 0.05). For 3 month old rats, maximum isometric force and dynamic peak force increased by 22 ± 8% and 27 ± 10%, respectively (P < 0.05). For 6 month old rats, these forces were unaltered by exposure and positive work capacity diminished by 27 ± 2% (P = 0.006). By 30 months of age, age-related deficits in maximum isometric force, peak force, negative work, and positive work were apparent and SSC exposure was ineffective at counteracting such deficits. Recovery from fatigue was also tested and exposure-induced improvements in fatigue recovery were indicated for 6 month old rats and to a lesser extent for 3 month old rats whereas no such effect was observed for older rats. Alterations in fatigue recovery were accompanied by evidence of substantial type IIb to IIx fiber type shifting. These results highlight the exceptional adaptive capacity for strength at a young age, the inclination for adaptation in fatigue recovery at early adulthood, and diminished adaptation for muscle performance in general beginning at late adulthood. Such findings motivate careful investigation to determine appropriate SSC exposures at all stages of life.
Highlights
With aging, muscle mass and strength decreases of ~30% are sustained by 80 years of age [1, 2]
To investigate the effects of chronic exposure to Stretchshortening contractions (SSCs) at a fundamental level, we developed an in vivo experimental rat model in which the dorsiflexor muscles of the hind limb were exposed to SSCs three times per week for one month [8]
While isometric and peak force increased by 20% to 30% for young rats (3 months old), these forces for young adult rats (6 months old) were unaltered
Summary
Muscle mass and strength decreases of ~30% are sustained by 80 years of age [1, 2]. To investigate the effects of chronic exposure to SSCs at a fundamental level, we developed an in vivo experimental rat model in which the dorsiflexor muscles of the hind limb were exposed to SSCs three times per week for one month [8] For young rats, such exposure induces a ~20% gain in muscle mass and muscle performance [8,9,10]. Physical capacity including balance and strength appears to be sensitive to aging rate throughout adulthood [11, 12] These findings provide the motivation to investigate intermediate ages between youth and old age, especially in regards to muscle performance. The findings are valuable for establishing how early in adulthood adaptive responses are altered and the nature of that alteration – important information when considering the appropriate exposure to contractions for distinct age groups
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