Impact of repetition number on muscle performance and histological response.

UNLABELLED: Skeletal muscle injury is major concern in sport- and occupation-related fields. PURPOSE: We investigated the effects of increasing stretch-shortening contraction (SSC) repetition number in vivo and the resulting changes in functional performance and quantitative morphometry in rat skeletal muscle. METHODS: Functional testing was performed on the ankle dorsiflexor muscles of Sprague-Dawley rats, which were randomly exposed to 30 SSC, 70 SSC, 150 SSC, or 15 isometric contractions of equal duration. Changes in functional performance and muscle morphometry were assessed at 48 h after exposure. Stereology was used to quantify the volume density of degenerative myofibers and normal myofibers in the tibialis anterior muscle from each group, as well as measures of inflammation and swelling and changes in the interstitial space. RESULTS: At 48 h there was a significant decline in isometric force for the 70- and 150-SSC groups (P < 0.05 and P < 0.05, respectively). Stereological measures indicated significant decreases in the percentage of volume density of normal myofibers in the 70- and 150-SSC groups (P < 0.05). Measures for percentage of volume density of degenerative myofibers and inflammation were increased (P < 0.0001 and P < 0.05, respectively) in the 70- and 150-SSC groups. Moreover, a significant increase in the percentage of volume density of degenerative myofibers in the 150-SSC group compared with the 70-SSC group was observed (P < 0.05). CONCLUSION: These data strongly suggest that exposure to increasing SSC repetitions results in increased functional decrements and morphometric indices of myofiber degeneration and inflammation, and that there is an apparent threshold (repetition number) at which this occurs.

The influence of velocity of stretch-shortening contractions on muscle performance during chronic exposure: age effects.

Aging increases injury susceptibility and impairs the ability to adapt to repetitive exposures of mechanical loading. The objective of this research was to investigate if movement velocity affects muscle response to a chronic administration of stretch-shortening cycles (SSCs) differently in young vs. old rats. Dorsiflexor muscles of old (30 months, n=5) and young rats (12 weeks, n=6) were exposed 3 times/week for 4.5 weeks to a protocol of 80 maximal SSCs per exposure in vivo. Skeletal muscle response was characterized by high- (500 degrees/s) and low- (60 degrees/s) velocity dynamic performance, which was evaluated using peak eccentric force, isometric pre-stretch force, eccentric force enhancement above the isometric pre-stretch force, negative work, and positive work. The performance of the young and old groups was not statistically different at the start of the exposure. By the end of the exposure, however, a statistical difference was noted-performance increased significantly in the young animals and decreased significantly in the old animals. The SSC velocity had a profound effect on muscle response. The young animals' high- and low-velocity performances increased during the chronic exposure period, whereas the old animals' performances declined. High-velocity performance increased more than low-velocity performance in young animals. In contrast, old animals suffered the most loss in high-velocity performance over the chronic exposure period. A chronic exposure of SSCs results in a significant performance increase in young animals, and a significant performance decrease in old animals. These differences are more profound during high-velocity movements. These findings suggest that age may impair the ability of skeletal muscle to adapt to repetitive mechanical loading, particularly during high-velocity movements.

Stereological analysis of muscle morphology following exposure to repetitive stretch-shortening cycles in a rat model.

Repetitive motion is one risk factor associated with contraction-induced muscle injury, which leads to skeletal muscle degeneration, inflammation, and dysfunction. Since current methods are unable to quantify the acute degenerative and inflammatory responses of muscle tissue concurrently, the purpose of this study was to quantify the temporal myofiber response after exposure to injurious stretch-shortening cycles (SSCs) using a standardized stereological technique. Functional testing was performed on the ankle dorsiflexor muscles of Sprague-Dawley rats in vivo. Rats were anesthetized and exposed to 15 sets of 10 SSCs. Control rats were exposed to 15 sets of single isometric contractions of the same stimulation duration. Changes in muscle morphometry were assessed at 0.5, 24, 48, 72, and 240 h post-exposure to quantify the degree of myofiber degeneration and inflammation in the tibialis anterior muscle from each group. There was an increase in the volume density and average thickness of degenerating myofibers over time in the muscle collected from rats exposed to SSCs (p < 0.0001) that was significantly greater than in muscle exposed to isometric contractions at 24, 48, and 72 h post-exposure (p = 0.003). The volume density of degenerative myofibers was associated with functional deficits at 48 h. Stereological quantification of degenerative myofibers and interstitial space changes were associated with functional defects 48-72 h after SSC-induced injury, thus demonstrating stereology is an accurate measure of SSC-induced skeletal muscle injury.

Quantitative histology and MGF gene expression in rats following SSC exercise in vivo.

PURPOSE: We investigated the effects of muscle length during stretch-shortening cycles (SSC) in vivo on changes in MGF gene expression and quantitative morphometry in rat skeletal muscle. METHODS: Dorsiflexor muscles of male Sprague-Dawley rats were exposed to seven sets of 10 SSC at 500 degrees .s(-1). Animals were randomly assigned to a long muscle length injury group (L-inj), short muscle length injury group (S-inj), or isometric group (Iso), with recoveries examined at 6 or 48 h post-injury for each group. Following exposure, animals were euthanized, and the tissue was prepared for either histology (quantitative morphometry) or RNA isolation, followed by quantitative real-time reverse transcriptase polymerase chain reaction. mRNA levels were measured for mechano-growth factor (MGF), while 18S ribosomal RNA served as the internal reference sample. RESULTS: Stereological measures indicative of edema and myofiber degeneration were significantly increased in the L-inj SSC group at 48 h when compared with the S-inj or Iso group. MGF mRNA was increased transiently at 6 h in the isometric group. In contrast, MGF mRNA was increased at 48 h in the S-inj, but was not increased at either time point in the L-inj group. CONCLUSION: These data strongly indicate that exposure to SSC at longer muscle lengths result in greater morphometric indices of inflammation and degeneration than SSC conducted at a shorter muscle lengths or isometric contractions, at the same time that the adaptation to SSC was prolonged and, apparently, not resolved in the L-inj group that was manifested by the lack of up-regulation in MGF mRNA.

Impact of stretch-shortening cycle rest interval on in vivo muscle performance.

INTRODUCTION: Overuse and overtraining models have implicated both metabolic and mechanical disturbances as contributors to muscle damage and performance decrement but have produced equivocal results. The purpose of the present study was to investigate the impact of rest interval between sets of stretch-shortening cycles (SSC) on static and dynamic muscle performance METHODS: Animals were randomly assigned to groups (N = 8 per group) of 10-s, 1-min, or 5-min rest between sets of isometric contractions (10-s, 1-min, or 5-min CON), or SSC (10-s, 1-min, or 5-min INJ). The dorsiflexor muscles were exposed in vivo to either seven sets of 10 SSC (500 degrees . s) or seven sets of isometric contractions. Performance was characterized by isometric exertions and positive, negative, and net work, at pretest, during the sets of SSC, and 48 h postexposure RESULTS: The isometric force at 48 h after the 10-s and 5-min INJ groups were statistically different from the 1-min group (P < 0.05), whereas there was no difference in the CON groups. Negative work of the INJ groups were statistically lower at 48 h than pretest values (P < 0.05), whereas there was no change in positive work. Of the real-time parameters, there was a difference in minimum force and positive work (P < 0.05) with treatment with the 10-s INJ group being most affected. CONCLUSION: SSC conducted at shorter work-rest cycles resulted in a more profound isometric force decrement 48 h postexposure, and in real-time changes in isometric prestretch force and positive work. These results indicate that short rest intervals between athletic or vocational tasks of heightened physical exertion (i.e., high intensity) may adversely affect performance and increase injury susceptibility.