Animals and Experimental Groups
Eighteen male Wistar rats (80 days old, 250–300 g) were obtained from the Multidisciplinary Center for Biological Investigation (CEMIB, UNICAMP, Campinas, São Paulo, Brazil). The rats were housed in collective polypropylene cages (4 animals per cage) covered with metallic grids, kept in a temperature-controlled room (22–24° C) under an artificial 12:12-hour light-dark cycle and provided with unlimited access to standard rat chow (4.644 kJ·g−1 at 26% protein, 3% lipid, 54% carbohydrate, and 17% other; Labina, Purina, Paulínia, São Paulo, Brazil) and water. This standard diet follows the recommendations of the Nutrient Requirements of Laboratory Animals (4) and ensures both the welfare of the animals and the reliability of the experimental results. With these controlled variables, rats were randomly assigned to either a trained (TR, N = 9) or sedentary (SE, N = 9) group. All procedures were approved by the Biosciences Institute Ethics Committee, UNESP, Botucatu, SP, Brazil (Protocol No. 018/08-CEEA) and were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals as revised 1996 (15). Throughout the study, every effort was made to minimize the number of animals used, as well as their suffering.
The TR group was subjected to a 12-week resistance training program with excessive training load and insufficient recovery time between bouts using a water jump–exercise model (Figure 1), as described in detail elsewhere (9). Briefly, rats underwent consecutive training sessions (5 d·wk−1) that consisted of jumps (repetitions) to the water surface (38 cm deep; approximately 150% of rat body length), carrying an overload strapped to a vest on the animal's chest. Initially, all animals completed a 1-week pretraining (once daily) that consisted of a progressive number of sets (2–4) and repetitions (5–12) with a 30-second rest between each set, and carrying an overload of 50% body weight (BW). Subsequently, the rats began the 12-week training program, which consisted of progressive overload corresponding to 60% (first to third week), 65% (fourth to sixth week), 70% (seventh to eighth week), 80% (ninth to 10th week), and 85% (11th to 12th week) of BW (Table 1). We have previously demonstrated that this training protocol is effective in promoting type IIA and IID fiber atrophy in rat plantaris muscle (9). The SE group was not exposed to any training stimuli throughout the study. The total time of 1 training session for each animal was approximately 4 minutes, in which each animal performed 10 jumps in approximately 20 seconds. This time remained the same throughout the period of training. Sessions were performed between 2 and 4 PM.
Throughout the experiment, the BW and food intake of the animals were monitored every 2 days. Two days after the last training session, the animals (fed state) were anesthetized with pentobarbital sodium (40 mg·kg−1 IP) and killed by decapitation. Immediately afterward, the middle third (muscle belly) of the plantaris muscle was collected and frozen in liquid nitrogen (−156° C) for morphometric (measurement of muscle fiber CSA) and Western blot (protein expression) analyses. Samples were kept at −80° C until use.
Plantaris muscle histological sections (12 μm thick) were obtained in a cryostat (JUNG CM1800; Leica, Wetzlar, Germany) at −24° C and stained with hematoxylin and eosin (HE) (Figure 2A). The stained sections were used for the photographic documentation of 3 random histological fields (20× lens) from each animal. The images were obtained using a microscope connected to a computer. The muscle fiber CSA (500 fibers/animal) was measured using an image analysis system (software QWin Plus; Leica).
Western Blot Analysis
MAFbx, MyoD, myogenin, and IGF-I protein levels were measured by Western blot assays of protein extracts from the plantaris muscle and normalized to actin. Muscle samples were homogenized in lysis buffer (1% Triton X-100, 10 mM sodium pyrophosphate, 100 mM NaF, 10 μg·ml−1 aprotinin, 1 mM phenylmethylsulfonylfluoride, 0.25 mM Na3VO4, 150 mM NaCl, and 50 mM Tris-HCl pH 7.5). The samples were centrifuged at 10,000g for 20 minutes, and 50 μl of the homogenate fraction was resuspended in 25 μl of Laemmli loading buffer (2% SDS, 20% glycerol, 0.04 mg·ml−1 bromophenol blue, 0.12 M Tris-HCl pH 6.8, and 0.28 M β-mercaptoethanol). Fifty micrograms of total protein was separated by 1-dimensional SDS-PAGE and stained with Coomassie blue to confirm equal loading of each sample. Proteins were transferred from the gel to a nitrocellulose membrane (Bio-Rad Laboratories, Hercules, CA, USA). Using a vacuum-enhanced detection system (SNAP i.d.; Millipore, Billerica, MA, USA), nonspecific binding sites were blocked with a 3% bovine serum albumin (BSA) solution in phosphate-buffered saline (PBS-T: 0.1 M NaH2PO4·H2O, 0.1 M Na2HPO4·7H2O, 0.15 M NaCl, 0.1% Tween-20, pH 7.4) for 10 minutes, followed by specific primary antibody incubation for 10 minutes (Table 2). After 4 wash steps with PBS-T, membranes were incubated for 10 minutes with specific horseradish peroxidase–conjugated secondary antibodies corresponding to the primary antibodies used (Table 2). Finally, immunoreactive protein signals were detected using the SuperSignal West Pico Chemiluminescent Substrate Kit (Thermo Fisher Scientific, Rockford, IL, USA), according to the manufacturer's instructions. Signals were quantified using densitometry analysis software (Image J, version 1.71; NIH, Hagenberg, Austria).
Data are expressed as mean ± SD. Shapiro-Wilk normality test was used to verify data normal distribution. To ensure data reliability, the statistical procedure was performed after the preliminary study of the variable related to normality and equality of variance between the groups, with the statistical power of 80% for the comparisons assessed. Differences in BW, muscle fiber CSA, MyoD, myogenin, IGF-I, and MAFbx protein expression between SE and TR groups were determined using a 2-tailed unpaired t-test. Statistical significance was set at p ≤ 0.05. Statistical analyses were performed using a software package (SPSS for Windows, version 13.0; SPSS, Inc., Chicago, IL, USA).
The groups began the experiment with similar BWs (SE: 286.7 ± 18.9 vs. TR: 276.4 ± 15.5 g, p > 0.05), indicating similar health and physical activity levels. After the 12-week training program, the TR group displayed a lower BW compared with the SE group (SE: 498.6 ± 30.5 vs. TR: 440.9 ± 30.3 g, p ≤ 0.05). Furthermore, no difference in weekly food intake was found between the groups (SE: 187.4 ± 41.6 vs. TR: 167.7 ± 41.2 g, p > 0.05). These results indicated that the loss of BW in the TR group was caused by physiological changes (e.g., protein breakdown and/or fat loss) but not behavioral changes (e.g., decrease in food intake).
Cross-sectional Area of Muscle Fibers
Plantaris muscle fiber CSA was measured using HE-stained tissue sections; a representative image is shown in Figure 2A, and the corresponding data are presented in Figure 2B. An atrophic effect occurred in the plantaris muscle fibers CSA of the TR group compared with the SE group (SE: 3033 ± 402 vs. TR: 2530 ± 201 μm2, p ≤ 0.05) after the 12-week training program. The muscle atrophy was corroborated by decreases in the muscle weight-to-BW ratio (data not shown).
Muscle Protein Expression
Figure 3 shows the levels of MAFbx, MyoD, myogenin, and IGF-I protein expression in the plantaris muscles from the TR and SE groups. After 12 weeks, the expression of MyoD, myogenin, and IGF-I protein decreased by 27% (SE: 1.19 ± 0.02 vs. TR: 0.86 ± 0.08, p ≤ 0.05; Figure 3C), 29% (SE: 0.77 ± 0.15 vs. TR: 0.55 ± 0.06, p ≤ 0.05; Figure 3D), and 43% (SE: 1.34 ± 0.04 vs. TR: 0.76 ± 0.17, p ≤ 0.05; Figure 3B), respectively. However, MAFbx protein expression was 20% greater in the TR group compared with that in the SE group (SE: 0.82 ± 0.08 vs. TR: 0.99 ± 0.12, p ≤ 0.05; Figure 3A).
Our laboratory recently reported that a resistance training program with excessive training load and inappropriate recovery between bouts induces atrophy and phenotypic alterations in rat skeletal muscle fibers (9), indicating an increase in the protein catabolism/anabolism ratio. To expand on these previous findings, we hypothesized that skeletal muscle atrophy induced by excessive training load and insufficient recovery would be associated with upregulation of catabolic protein expression (MAFbx) and downregulation of anabolic protein expression (MyoD, myogenin, and IGF-I). Our results confirmed this hypothesis by showing, for the first time, that muscle atrophy induced by an imbalance in intensity and recovery of training was associated with increased MAFbx protein expression and decreased MyoD, myogenin, and IGF-I protein expression.
The upregulation of MAFbx protein expression observed in our study extends the findings from a previous study involving different atrophy conditions (6,13,34). Sandri et al. (34), using a model of in vitro atrophy (e.g., differentiated myotubes that were deprived of nutrients for 6 hours), found a 2.5-fold increase in MAFbx mRNA expression in conjunction with a 60% reduction in myotube diameter. These results are in agreement with the in vivo response to other treatments (6,13,19). For example, mice muscles that were subjected to immobilization or treatment with a glucocorticoid showed a marked increase in MAFbx gene expression (6). Additionally, mice that are null for MAFbx (MAFbx −/−) seem to be more resistant to denervation-induced muscle loss than a control group of littermates (6). Considering these previous studies, it seems reasonable to assume that MAFbx plays a key role in regulating muscle catabolism in several atrophy models (e.g., disuse, hormonal intervention, and gene knockout); this fact could explain the 20% increase in MAFbx protein expression that was associated with muscle fiber atrophy and weight loss in the TR group, compared with the SE group, in this study. Despite strong evidence showing that increased MAFbx expression was associated with muscle atrophy, it is unclear why the 12-week resistance training program promoted a persistent increase in MAFbx protein expression. It is important to note that the common process of muscle regeneration in response to acute resistance training is associated with reduced MAFbx expression (analyses at 8, 12, and 24 hours after exercise) (23,26,28,43); this downregulation in MAFbx expression appeared to be necessary for promoting muscle regeneration (29). Therefore, it is likely that increased MAFbx protein expression and consequent muscle fiber atrophy during the 12-week training program was caused by the accumulation of functional and metabolic stress, which was induced by excessive training overload and insufficient recovery time between training sessions. Our results reveal new insight into the molecular mechanisms that govern muscle atrophy by showing for the first time that MAFbx protein expression is persistently increased when muscles are subjected to an excessive accumulation of training stress that results in the skeletal muscle atrophy.
Interestingly, increased MAFbx protein expression was associated with the downregulation of MyoD (−27%) and myogenin (−29%) protein expression after the 12-week training program. MyoD and myogenin are members of the family of muscle-specific bHLH (basic helix-loop-helix) transcription factors that are involved in the proliferation and differentiation of SC in response to load-induced activation (5,14). It has been demonstrated that increased MyoD and myogenin expression is essential for successful skeletal muscle hypertrophy (16,24), and the contrary is true for skeletal muscle atrophy (21,42). The theoretical basis of these studies of muscle atrophy suggests that increased MAFbx expression can regulate the decrease in MyoD and myogenin expression and, consequently, induce muscle atrophy. Indeed, Lagirand-Cantaloube et al. (21) observed that increased MAFbx expression occurred concurrently with the selective suppression of MyoD in cultured myotubes undergoing atrophy. The authors also showed that shRNA-mediated silencing of the MAFbx gene in myotubes inhibited the MyoD proteolysis that was linked to atrophy. Furthermore, the overexpression of a mutant MyoDK133R (which is insensitive to MAFbx-mediated polyubiquitination) prevented atrophy of mouse primary myotubes and skeletal muscle fibers in vivo (21). In addition to MyoD, it has been shown that MAFbx can also regulate the degradation of myogenin in cell culture and in fasting mice (18). The authors found an increase in the polyubiquitination of myogenin in MAFbx-overexpressing cells and a stabilization of myogenin in MAFbx-knockdown cells, which indicated that myogenin can be a target of the MAFbx protein. Collectively, these results revealed that MAFbx was responsible for MyoD and myogenin degradation and that this process appeared to be required for muscle atrophy promotion (18,21,42).
Therefore, the muscle atrophy observed in our study may be explained by the MAFbx-dependent decrease in MyoD and myogenin protein expression. Our results extend the findings of previous in vitro and in vivo studies involving several atrophy conditions (8,21,42) by showing for the first time that excessive training load with inappropriate recovery can reverse the anabolic effects of myogenic pathways that govern muscle hypertrophy (e.g., reduce the MyoD and myogenin levels) and thus induce muscle atrophy. Moreover, our results show that MyoD and myogenin are degraded by a common mechanism associated with increased MAFbx protein expression. From a practical perspective, the quantitative analysis of these proteins (as expressed by increased MAFbx protein content and decreased MyoD and myogenin protein content) could be important and complementary with other biochemical markers (e.g., elevated creatine kinase and cortisol levels, reduced testosterone levels, and decline in performance) to confirm a possible overtraining diagnosis during intensified training.
Despite the important role of MAFbx in muscle atrophy, the exact molecular mechanisms that control its increase during excessive training load remain unknown. It is known that the IGF-I signaling pathway downregulates the transcription of MAFbx by activating PI3K and Akt (34). Okada et al. (29) found that an increase in IGF-I protein expression occurred concurrently with reduced MAFbx expression during the time course of muscle regeneration (3 and 5 days after exercising) in mice subjected to forced eccentric contraction by electrical stimulation. Although it is not possible to establish a direct causal relationship in this study, the authors have suggested that the increase in IGF-I expression may have contributed to a concomitant reduction in the MAFbx expression required for muscle regeneration. Additionally, IGF-I signaling was shown to suppress protein breakdown (33); transgenic mice that overexpressed IGF-I were more resistant to skeletal muscle atrophy that was induced by chronic left-ventricular dysfunction (35), and local IGF-I injection was sufficient to block disuse atrophy (41). These studies also showed that IGF-I/Akt signaling completely suppressed MAFbx expression. If the increased IGF-I levels are capable of suppressing MAFbx expression and preventing muscle atrophy, it seems logical to think that the downregulation of IGF-I is required to increase the MAFbx protein level and promote muscle atrophy. In fact, our results showed that the TR group undergoing muscle atrophy showed a 43% decrease in IGF-I protein expression concomitant with increased MAFbx protein expression (compared with the SE group). These data suggest that the development of therapies to stimulate the IGF-I signaling pathway could also contribute to the treatment of muscle wasting in conditions of excessive training load and possibly other metabolic and neuromuscular diseases. However, there are some limitations that must be acknowledged and addressed regarding this study because (a) we did not measure the other proteins involved with muscle atrophy (e.g., MuRF-1) and hypertrophy (e.g., Akt, mTOR, p70S6k) processes and (b) we did not examine the serum levels of stress hormones (e.g., cortisol) and muscle injury-related proteins (e.g., creatine kinase activity). Future studies are required to address these issues during conditions of excessive training load with inappropriate recovery between bouts in athletes and amateurs.
In conclusion, our data showed for the first time that muscle atrophy induced by resistance training with excessive training load and inappropriate recovery time between bouts was associated with upregulation of the MAFbx catabolic protein and downregulation of the MyoD, myogenin, and IGF-I anabolic proteins. This result indicates that both the catabolic and anabolic pathways are affected at the translational level in overworked muscles undergoing atrophy.
This study has examined the chronic response of the MAFbx, MyoD, myogenin, and IGF-I proteins to an excessive accumulation of training stress, which resulted in the skeletal muscle atrophy. The increased MAFbx levels and concomitant decreased MyoD, myogenin, and IGF-I content confirm that these proteins are key elements to regulate muscle proteolysis in overworked muscles. From a practical perspective, the results indicate that quantitative analysis of these proteins (as expressed by increased MAFbx protein content and decreased MyoD, myogenin, and IGF-I protein content) can be important and complementary with other biochemical markers (e.g., elevated creatine kinase and cortisol levels, reduced testosterone levels, and decline in performance) to confirm a possible overtraining diagnosis during intensified training. Additionally, these proteins may be the targets of new therapeutic interventions to treat muscle wasting in overworked muscles undergoing atrophy, and possibly other neuromuscular diseases.
This study was supported by São Paulo Research Foundation (FAPESP), Proc. 08/52641-1, by the National Council for Scientific and Technological Development (CNPq), Proc. 130628/2008-5, and by CAPES. This work is part of the MSc thesis that was presented by RWAS to the University of Campinas, UNICAMP, 2010.
1. Adams GR, McCue SA. Localized infusion of IGF-I
results in skeletal muscle hypertrophy in rats. J Appl Physiol (1985) 84: 1716–1722, 1998.
2. Aguiar AF, de Souza RW, Aguiar DH, Aguiar RC, Vechetti IJ Jr, Dal-Pai-Silva M. Creatine does not promote hypertrophy in skeletal muscle in supplemented compared with nonsupplemented rats subjected to a similar workload. Nutr Res 31: 652–657, 2011.
3. Armstrong LE, VanHeest LJ. The unknown mechanism of the overtraining syndrome: Clues from depression and psychoneuroimmunology. Sports Med 32: 185–209, 2002.
4. Benevenga NJ, Calvert C, Eckhert CD, Fahey GC, Greger JL, Keen CL, et al.. Nutrient Requirements of the Laboratory Rat. Nutrient Requirements of Laboratory Animals. (4th Revised ed.). Washington, DC: National Academy Press, 1995. pp. 11–79.
5. Bickel CS, Slade J, Mahoney ED, Haddad D, Dudley A, Adams GR. Time course of molecular responses of human skeletal muscle to acute bouts of resistance exercise. J Appl Physiol (1985) 98: 482–488, 2005.
6. Bodine SC, Latres E, Baumhueter S, Lai VK, Nunez L, Clarke BA, Poueymirou WT, Panaro FJ, Na E, Dharmarajan K, Pan ZQ, Valenzuela DM, DeChiara TM, Stitt TN, Yancopoulos GD, Glass DJ. Identification of ubiquitin ligases required for skeletal muscle atrophy
. Science 294: 1704–1708, 2001.
7. Chargé SB, Rudnicki MA. Cellular and molecular regulation of muscle regeneration. Physiol Rev 84: 209–238, 2004.
8. Cong H, Sun L, Liu C, Tien P. Inhibition of atrogin-1/MAFbx
expression by adenovirus-delivered small hairpin RNAs attenuates muscle atrophy
in fasting mice. Hum Gene Ther 22: 313–324, 2011.
9. De Souza RW, Aguiar AF, Carani FR, Campos GE, Padovani CR, Silva MD. High-intensity resistance training with insufficient recovery time between bouts induce atrophy and alterations in myosin heavy chain content in rat skeletal muscle. Anat Rec (Hoboken) 294: 1393–1400, 2011.
10. Fatouros IG, Destouni A, Margonis K, Jamurtas AZ, Vrettou C, Kouretas D, Mastorakos G, Mitrakou A, Taxildaris K, Kanavakis E, Papassotiriou I. Cell-free plasma DNA as a novel marker of aseptic inflammation severity related to exercise overtraining. Clin Chem 52: 1820–1825, 2006.
11. Glass DL. Molecular mechanisms modulating muscle mass. Trends Mol Med 9: 344–350, 2003.
12. Glass DJ. Skeletal muscle hypertrophy and atrophy signaling pathways. Int J Biochem Cell Biol 37: 1974–1984, 2005.
13. Gomes MD, Lecker SH, Jagoe RT, Navon A, Goldberg AL. Atrogin-1, a muscle-specific F-box protein highly expressed during muscle atrophy
. Proc Natl Acad Sci USA 98: 14440–14445, 2001.
14. Haddad F, Adams GR. Selected contribution: Acute cellular and molecular responses to resistance exercise. J Appl Physiol (1985) 93: 394–403, 2002.
15. Institute for Laboratory Animal Research, National Research Council. Guide for the Care and Use of Laboratory Animals. Washington, DC: National Academy Press, 2010.
16. Ishido M, Kami K, Masuhara M. Localization of MyoD
and cell cycle regulatory factors in hypertrophying rat skeletal muscles. Acta Physiol Scand 180: 281–289, 2004.
17. Jamurtas AZ, Fatouros IG, Buckenmeyer PJ, Kokkinidis E, Taxildaris K, Kambas A, Kyriazis G. Effects of plyometric exercise on muscle soreness and creatine kinase levels and its comparison to eccentric and concentric exercise. J Strength Cond Res 14: 68–74, 2000.
18. Jogo M, Shiraishi S, Tamura TA. Identification of MAFbx
as a myogenin
-engaged F-box protein in SCF ubiquitin ligase. FEBS Lett 583: 2715–2719, 2009.
19. Jones SW, Hill RJ, Krasney PA, O’Conner B, Peirce N, Greenhaff PL. Disuse atrophy and exercise rehabilitation in humans profoundly affects the expression of genes associated with the regulation of skeletal muscle mass. FASEB J 18: 1025–1027, 2004.
20. Kuipers H, Keizer HA. Overtraining in elite athletics: Review and directions for the future. Sports Med 6: 79–92, 1988.
21. Lagirand-Cantaloube J, Cornille K, Csibi A, Batonnet-Pichon S, Leibovitch MP, Leibovitch SA. Inhibition of atrogin-1/MAFbx
proteolysis prevents skeletal muscle atrophy
in vivo. PLoS One 4: e4973, 2009.
22. Lehmann M, Foster C, Keul J. Overtraining in endurance athletes: A brief review. Med Sci Sports Exerc 25: 854–862, 1993.
23. Louis E, Raue U, Yang Y, Jemiolo B, Trappe S. Time course of proteolytic, cytokine, and myostatin gene expression after acute exercise in human skeletal muscle. J Appl Physiol (1985) 103: 1744–1751, 2007.
24. Lowe DA, Always SE. Stretch-induced myogenin
, and MRF4 expression and acute hypertrophy in quail slow-tonic muscle are not dependent upon satellite cell proliferation. Cell Tissue Res 296: 531–539, 1999.
25. Margonis K, Fatouros IG, Jamurtas AZ, Nikolaidis MG, Douroudos I, Chatzinikolaou A, Mitrakou A, Mastorakos G, Papassotiriou I, Taxildaris K, Kouretas D. Oxidative stress biomarkers responses to physical overtraining: Implications for diagnosis. Free Radic Biol Med 43: 901–910, 2007.
26. Mascher H, Tannerstedt J, Brink-Elfegoun T, Ekblom B, Gustafsson T, Blomstrand E. Repeated resistance exercise training induces different changes in mRNA expression of MAFbx
and MuRF-1 in human skeletal muscle. Am J Physiol Endocrinol Metab 294: E43–E51, 2008.
27. Meeusen R, Duclos M, Gleeson M, Rietjens G, Steinacker J, Urhausen A. Prevention, diagnosis and treatment of the overtraining syndrome. Eur J Sports Sci 6: 1–14, 2006.
28. Nedergaard A, Vissing K, Overgaard K, Kjaer M, Schjerling P. Expression patterns of atrogenic and ubiquitin proteasome component genes with exercise: Effect of different loading patterns and repeated exercise bouts. J Appl Physiol (1985) 103: 1513–1522, 2007.
29. Okada A, Ono Y, Nagatomi R, Kishimoto KN, Itoi E. Decreased muscle atrophy
) expression in regenerating muscle after muscle-damaging exercise. Muscle Nerve 38: 1246–1253, 2008.
30. Olson EN. Regulation of muscle transcription by the MyoD
family. Circ Res 72: 1–6, 1993.
31. Petibois C, Carzola G, Poortmans JR, Déléris G. Biochemical aspects of overtraining in endurance sports: The metabolism alteration process syndrome. Sports Med 33: 83–94, 2003.
32. Rommel C, Bodine SC, Clarke BA, Rossman R, Nunez L, Stitt TN, Yancopoulos GD, Glass DJ. Mediation of IGF-1-induced skeletal myotube hypertrophy by PI(3)K/Akt/mTOR and PI(3)K/Akt/GSK3 pathways. Nat Cell Biol 3: 1009–1013, 2001.
33. Sacheck JM, Ohtsuka A, McLary SC, Goldberg AL. IGF-I
stimulates muscle growth by suppressing protein breakdown and expression of atrophyrelated ubiquitin ligases, atrogin-1 and MuRF1. Am J Physiol Endocrinol Metab 287: E591–E601, 2004.
34. Sandri M, Sandri C, Gilbert A, Skurk C, Calabria E, Picard A, Walsh K, Schiaffino S, Lecker SH, Goldberg AL. Foxo transcription factors induce the atrophy-related ubiquitin ligase atrogin-1 and cause skeletal muscle atrophy
. Cell 117: 399–412, 2004.
35. Schulze PC, Fang J, Kassik KA, Gannon J, Cupesi M, MacGillivray C, Lee RT, Rosenthal N. Transgenic overexpression of locally acting insulin-like growth factor-1 inhibits ubiquitinmediated muscle atrophy
in chronic left-ventricular dysfunction. Circ Res 97: 418–426, 2005.
36. Seene T, Kaasik P, Alev K, Pehme A, Riso EM. Composition and turnover of contractile proteins in volume-overtrained skeletal muscle. Int J Sports Med 25: 438–445, 2004.
37. Seene T, Umnova M, Kaasik P. The exercise myopathy. In: Overload, Performance Incompetence and Regeneration in Sport. Lehmann M., Foster C., Gastmann U., Keizer H., Steinacker J., eds. New York, Kluwer Academic/Plenum Publishers, pp. 119–130, 1999.
38. Smith LL. Cytokine hypothesis of overtraining: A physiological adaptation to excessive stress? Med Sci Sports Exerc 32: 317–333, 2000.
39. Smith LL. Tissue trauma: The underlying cause of overtraining syndrome? J Strength Cond Res 18: 185–193, 2004.
40. Smith LL, Miles MP. Exercise-induced muscle injury and inflammation. In: Exercise and Sport Science. Garrett W.E., Kirkendall D.T., eds. Philadelphia, PA: Lippincott Williams Wilkins, 2000. 401–411.
41. Stitt TN, Drujan D, Clarke BA, Panaro F, Timofeyva Y, Kline WO, Gonzalez M, Yancopoulos GD, Glass DJ. The IGF1/PI3K/Akt pathway prevents expression of muscle atrophy
-induced ubiquitin ligases by inhibiting FOXO transcription factors. Mol Cell 14: 395–403, 2004.
42. Tintignac LA, Lagirand J, Batonnet S, Sirri V, Leibovitch MP, Leibovitch SA. Degradation of MyoD
mediated by the SCF (MAFbx
) ubiquitin ligase. J Biol Chem 280: 2847–2856, 2005.
43. Yang Y, Jemiolo B, Trappe S. Proteolytic mRNA expression in response to acute resistance exercise in human single skeletal muscle fibers. J Appl Physiol (1985) 101: 1442–1450, 2006.
Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
overworked muscles; muscle atrophy; MAFbx; MyoD; myogenin; IGF-I