Melinda S. Hollander

Injury and adaptive mechanisms in skeletal muscle.

Work-related musculoskeletal disorders (MSD) are a major concern in the United States. Overexertion and repetitive motion injuries dominate reporting of lost-time MSD incidents. Over the past three decades, there has been much study on contraction-induced skeletal muscle injury. The effect of the biomechanical loading signature that includes velocity, range of motion, the number of repetitions, force, work-rest cycle, and exposure duration has been studied. More recently, the effect of aging on muscle injury susceptibility and regeneration has been studied. This review will focus on contraction-induced skeletal muscle injury, the effects of repetitions, range of motion, work-rest cycles, and aging on injury susceptibility and regenerative and adaptive pathways. The different physiological phenomena responsive to overt muscle injury versus adaptation will be distinguished. The inherent capability of skeletal muscle to adapt to mechanical loading, given the appropriate exposure signature will also be discussed. Finally, we will submit that repeated high-intensity mechanical loading is a desirable means to attenuate the effects of sarcopenia, and may be the most effective and appealing mode of physical activity to counteract the effects often observed with musculo-skeletal dysfunction in the workplace.

Adaptive stretch-shortening contractions: diminished regenerative capacity with aging

This study determined the age-related changes in acute events responsible for initiating skeletal muscle remodeling and (or) regeneration in the tibialis anterior muscle following a bout of stretch-shortening contractions (SSCs). Changes in muscle performance and morphology were quantified in young and old rats, following an acute exposure to adaptive SSCs at 6, 24, 48, 72, and 120 h postexposure (n = 6 for each age at each recovery period). Following SSC exposure, all performance measures were decreased in old rats throughout the 120 h acute phase. Estimates of edema were increased in the old vs. young exposed muscle at 120 h recovery. Both young and old rats displayed an increase in developmental myosin heavy chain (MHCdev+) labeling in the exposed muscle, indicating muscle regeneration. However, old rats displayed diminished MHCdev+ labeling, compared with young rats, suggesting limited remodeling and (or) regenerative capacity. Based on these data, diminished local muscle remodeling and (or) regeneration with aging may limit skeletal muscle adaptation following mechanical loading.

Magnetic Resonance Imaging of Graded Skeletal Muscle Injury in Live Rats

Introduction Increasing number of stretch-shortening contractions (SSCs) results in increased muscle injury. Methods Fischer Hybrid rats were acutely exposed to an increasing number of SSCs in vivo using a custom-designed dynamometer. Magnetic resonance imaging (MRI) imaging was conducted 72 hours after exposure when rats were infused with Prohance and imaged using a 7T rodent MRI system (GE Epic 12.0). Images were acquired in the transverse plane with typically 60 total slices acquired covering the entire length of the hind legs. Rats were euthanized after MRI, the lower limbs removed, and tibialis anterior muscles were prepared for histology and quantified stereology. Results Stereological analyses showed myofiber degeneration, and cellular infiltrates significantly increased following 70 and 150 SSC exposure compared to controls. MRI images revealed that the percent affected area significantly increased with exposure in all SSC groups in a graded fashion. Signal intensity also significantly increased with increasing SSC repetitions. Discussion These results suggest that contrast-enhanced MRI has the sensitivity to differentiate specific degrees of skeletal muscle strain injury, and imaging data are specifically representative of cellular histopathology quantified via stereological analyses.

Response of tibialis anterior tendon to a chronic exposure of stretch-shortening cycles: age effects

BackgroundThe purpose of the current study was to investigate the effects of aging on tendon response to repetitive exposures of stretch-shortening cycles (SSCs).MethodsThe left hind limb from young (3 mo, N = 4) and old (30 mo, N = 9) male Fisher 344 × Brown Norway rats were exposed to 80 maximal SSCs (60 deg/s, 50 deg range of motion) 3x/week for 4.5 weeks in vivo. After the last exposure, tendons from the tibialis anterior muscle were isolated, stored at -80°C, and then tested using a micro-mechanical testing machine. Deformation of each tendon was evaluated using both relative grip-to-grip displacements and reference marks via a video system.ResultsAt failure, the young control tendons had higher strain magnitude than the young exposed (p < 0.01) and the old control tendons (p < .0001). Total load at inflection was affected by age only (p < 0.01). Old exposed and control tendons exhibited significantly higher loads at the inflection point than their young counterparts (p < 0.05 for both comparisons). At failure, the old exposed tendons carried higher loads than the young exposed tendons (p < 0.05). Stiffness was affected by age only at failure where the old tendons exhibited higher stiffness in both exposed and control tendons than their young counterparts (p < 0.05 and p < 0.01, respectively).ConclusionThe chronic protocol enhanced the elastic stiffness of young tendon and the loads in both the young and old tendons. The old exposed tendons were found to exhibit higher load capacity than their younger counterparts, which differed from our initial hypothesis.