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The purpose of this study was to find the composition shift of myosin heavy chain (MyHC) isoforms in overtraining in fast- and slow-twitch skeletal muscles and different changes in MyHC isofom composition, synthesis and turnover rate between 4-week and 6-week overtraining. Male Wistar rats were randomly assigned to 4-week and 6-week endurance training, 4-week and 6-week overtraining groups. Plantaris (Pla), extensor digitorum longus (EDL), and soleus (Sol) muscles were studied. Daily excretion of 3-methylhistidine (3-MeHis) pool as an indicator for protein degradation increased in the 4-week and 6-week overtraining group to 4.04 +/- 0.21 and 4.32 +/- 0.23 %/day subsequently in comparison with the control group (2.16 +/- 14 %/day, p < 0.001). In Pla muscle MyHC I isoform synthesis rate was 33 200 +/- 2150 (after 6-week overtraining 27 100 +/- 1800, p < 0.05), IIa 32 600 +/- 2100; IId 27 300 +/- 1890 and IIb isoform 20 100 +/- 1600 (after 6-week overtraining 15 500 +/- 1400, p < 0.05) dpm/M leucine/min. Actin synthesis rate increased in fast-twitch muscles during 4- and 6-week overtraining, and in soleus muscle during 6-week overtraining. In EDL and Sol muscle MyHC isoform composition during 6-week overtraining did not change significantly. During the 6-week overtraining the relative content of MyHC I and IIb isoforms decreased and IIa and IId isoforms increased in Pla muscle. The initial increase of MyHC IIb isoform after 4-week overtraining shows the higher stability of this isoform in comparison with MyHC I isoform in fast-twitch muscles during high volume exercise.

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Aging is a multifactorial process that results in changes in the skeletal muscle then leads to decreased levels of muscle strength. Knee osteoarthritis (OA) is one of the most predominant causes of pain, functional decline and disability in the elderly population worldwide. In China the prevalence of radiographic knee OA was 42.8% in older women and 21.5% in older men. It has been shown to affect a multitude of Isokinetic muscle strength parameters. This study sought to investigate the different impacts of knee osteoarthritis and aging on isokinetic muscle strength of the lower limbs in women. We found that knee osteoarthritis might have a further bad influence on knee and hip joint of elderly women and they also had a significantly higher flexor-to-extensor moment ratios (f/e) than older controls. Reduced muscular strength emerge as a result of aging, isokinetic muscle strength decrease of lower limbs in KOA patients are more pronounced compared to older controls, especially in knee.

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