Abstract Trial design Older adults experience chronic dysregulation of leukocytes and inflammatory cytokines, both at rest and in response to resistance training. Systemic hypoxia modulates leukocytes and cytokines, therefore this study characterized the effects of normobaric hypoxia on the leukocyte and cytokine responses of older adults to resistance training. Methods 20 adults aged 60–70 years performed eight weeks of moderate-intensity resistance training in either normoxia or normobaric hypoxia (14.4% O2), consisting of two lower body and two upper body exercises. Venous blood was drawn before and after the training intervention and flow cytometry was used to quantify resting neutrophils, lymphocytes, monocytes, eosinophils and basophils, in addition to the subsets of lymphocytes (T, B and natural killer (NK) cells). Inflammatory cytokines were also quantified; interleukin 1 beta (IL-1β), IL-4, IL-6, IL-8, IL-10 and tumor necrosis factor alpha (TNF-α). Acute changes in leukocytes and cytokines were also measured in the 24 h following the last training session. Results After the intervention there was a greater concentration of resting white blood cells (p = 0.03; 20.3% higher) T cells (p = 0.008; 25.4% higher), B cells (p = 0.004; 32.6% higher), NK cells (p = 0.012; 43.9% higher) and eosinophils (p = 0.025; 30.8% higher) in hypoxia compared to normoxia, though the cytokines were unchanged. No acute effect of hypoxia was detected in the 24 h following the last training session for any leukocyte population or inflammatory cytokine (p < 0.05). Conclusions Hypoxic training caused higher concentrations of resting lymphocytes and eosinophils, when compared to normoxic training. Hypoxia may have an additional beneficial effect on the immunological status of older adults. Trial registration Australian New Zealand Clinical Trials Registry (ANZCTR). Trial number: ACTRN12623001046695. Registered 27/9/2023. Retrospectively registered. All protocols adhere to the COSORT guidelines.

ABSTRACT:Allsopp, GL, Britto, FA, Wright, CR, and Deldicque, L. The effects of normobaric hypoxia on the acute physiological responses to resistance training: a narrative review. J Strength Cond Res 38(11): 2001-2011, 2024-Athletes have used altitude training for many years as a strategy to improve endurance performance. The use of resistance training in simulated altitude (normobaric hypoxia) is a growing strategy that aims to improve the hypertrophy and strength adaptations to training. An increasing breadth of research has characterized the acute physiological responses to resistance training in hypoxia, often with the goal to elucidate the mechanisms by which hypoxia may improve the training adaptations. There is currently no consensus on the overall effectiveness of hypoxic resistance training for strength and hypertrophy adaptations, nor the underlying biochemical pathways involved. There are, however, numerous interesting physiological responses that are amplified by performing resistance training in hypoxia. These include potential changes to the energy system contribution to exercise and alterations to the level of metabolic stress, hormone and cytokine production, autonomic regulation, and other hypoxia-induced cellular pathways. This review describes the foundational exercise physiology underpinning the acute responses to resistance training in normobaric hypoxia, potential applications to clinical populations, including training considerations for athletic populations. The review also presents a summary of the ideal training parameters to promote metabolic stress and associated training adaptations. There are currently many gaps in our understanding of the physiological responses to hypoxic resistance training, partly caused by the infancy of the research field and diversity of hypoxic and training parameters.

There is growing interest in the use of systemic hypoxia to improve the training adaptations to resistance exercise. Hypoxia is a well-known stimulator of the immune system, yet the leukocyte responses to this training modality remain uncharacterised. The current study characterised the acute leukocyte responses to resistance exercise in normobaric hypoxia. The single-blinded, randomised trial recruited 13 healthy males aged 18-35 years to perform a bout of resistance exercise in normobaric hypoxia (14.4% O2; n = 7) or normoxia (20.9% O2; n = 6). Participants completed 4 × 10 repetitions of lower and upper body exercises at 70% 1-repetition maximum. Oxygen saturation, rating of perceived exertion and heart rate were measured during the session. Venous blood was sampled before and up to 24 hours post-exercise to quantify blood lactate, glucose and leukocytes including neutrophils, lymphocytes, monocytes, eosinophils and basophils. Neutrophils were higher at 120 and 180 minutes post-exercise in hypoxia compared to normoxia (p<0.01), however lymphocytes, monocytes, eosinophils and basophils were unaffected by hypoxia. Oxygen saturation was significantly lower during the four exercises in hypoxia compared to normoxia (p < 0.001). However, there were no differences in blood lactate, heart rate, perceived exertion or blood glucose between groups. Hypoxia amplified neutrophils following resistance exercise, though all other leukocyte subsets were unaffected. Therefore, hypoxia does not appear to detrimentally affect the lymphocyte, monocyte, eosinophil or basophil responses to exercise.

In young adults, the hormonal responses to resistance exercise are amplified by normobaric hypoxia. Hormone concentrations and metabolism are typically dysregulated with age, yet the impact of hypoxia on these responses to resistance exercise are uncharacterised. Therefore, this study aimed to characterise the acute and chronic hormonal and metabolic responses of older adults to resistance training in normobaric hypoxia. Adults aged 60–75 years completed 8 weeks of resistance training in either normoxia (20.9% O2; n = 10) or normobaric hypoxia (14.4% O2, n = 10) twice weekly at 70% of their predicted 1-repetition maximum. Growth hormone, glucose, lactate, insulin, homeostatic model assessment of insulin resistance (HOMA-IR), cortisol, total testosterone, adrenaline, noradrenaline and dopamine were quantified at pre- and post-training, and in the 60 min following the first training session (untrained state) and the last training session (trained state). Eight weeks of training in hypoxia did not affect the resting levels of the hormones or physiological factors measured. However, hypoxia significantly blunted the acute growth hormone response in the 15 min following the last training session at week eight (43.87% lower in the hypoxic group; p = 0.017). This novel and unexpected finding requires further investigation. All other hormones were unaffected acutely by hypoxia in the 60 min following the first and the last training session. Chronic resistance training in normobaric hypoxia supresses the growth hormone response to exercise in older adults. All other hormones and metabolic markers were unaffected both acutely and chronically by hypoxia.

Ageing causes a decline in leukocyte function and blunted leukocyte responses to resistance exercise. Systemic hypoxia exposure augments the leukocyte response to resistance exercise in young adults, yet this response remains uncharacterised in older adults. This study characterised the effects of normobaric hypoxia on the acute leukocyte and inflammatory cytokine responses to resistance exercise in older adults. We recruited 20 adults aged 60-70 years to perform an acute bout of resistance exercise in normobaric hypoxia (FiO2 14.4%; n = 10) or normoxia (FiO2 20.93%; n = 10). Participants completed 4 × 10 repetitions of lower and upper body exercises at 70% of their predicted 1-repetition maximum. Venous blood was sampled before and up to 24 hours post-exercise to quantify neutrophils, lymphocytes, monocytes, eosinophils, basophils and cytokines (IL-1β, IL-4, IL-6, IL-8, IL-10, TNFα). Flow cytometry was used to classify lymphocytes as T (CD4+ helper and CD8+ cytotoxic), B and NK cells, in addition to the expression of the senescence marker CD45RA on T cells. The hypoxic group showed a larger lymphocyte response over the 24 hours post-exercise compared to the normoxic group (p = 0.035). Specifically, there were greater concentrations of CD4+ T helper cells following hypoxic exercise compared to normoxia (p = 0.046). There was also a greater proportion of CD45RA+ CD4+ T helper cells, suggesting that the cells were more senescent (p = 0.044). Hypoxia did not impact any other leukocyte population or cytokine following exercise. Normobaric hypoxia increases the lymphocyte response to an acute bout of resistance exercise in older adults.

Allsopp, GL, Hoffmann, SM, Feros, SA, Pasco, JA, Russell, AP, and Wright, CR. The effect of normobaric hypoxia on resistance training adaptations in older adults. J Strength Cond Res 36(8): 2306-2312, 2022-The effect of normobaric hypoxia on strength, body composition, and cardiovascular fitness was investigated after a resistance training intervention in older adults. A single-blinded, randomized control trial recruited 20 healthy adults aged 60-75 years for an 8-week resistance training intervention in normoxia (n = 10) or normobaric hypoxia (14.4% O-2; n = 10). Subjects performed 2 sessions per week of upper-body and lower-body exercises at 70% of 1 repetition maximum (1RM). Pretraining and post-training, maximal oxygen uptake (V?O(2)max), muscular endurance (30 maximal knee flexions/extensions), and 5RM were assessed, with 5RM used to calculate 1RM. Subjects underwent whole-body dual-energy x-ray absorptiometry (DXA) at pretraining and post-training for fat and lean mass quantification. Significance was set at p < 0.05. Subjects in both groups substantially improved their calculated 1RM strength for leg extension, pectoral fly, row, and squat (normoxia; 30, 38, 27, and 29%, hypoxia; 43, 50, 28, and 64%, respectively); however, hypoxia did not augment this response. Hypoxia did not enhance V?O(2)max or muscular endurance responses after the training intervention, with no improvements seen in either group. Fat mass and lean mass remained unchanged in both groups after the intervention. In summary, 8 weeks of resistance training in hypoxia was well tolerated in healthy older adults and increased upper-body and lower-body strength. However, the magnitude of strength and lean muscle improvements in hypoxia was no greater than normoxia; therefore, there is currently no evidence to support the use of hypoxic resistance training in older adults.

Selenoprotein S (Seps1) can be protective against oxidative, endoplasmic reticulum (ER), and inflammatory stress. Seps1 global knockout mice are less active, possess compromised fast muscle ex vivo strength, and, depending on context, heightened inflammation. Oxidative. ER, and inflammatory stress modulates contractile function; hence, our aim was to investigate the effects of Seps1 gene dose on exercise performance. Seps1(-/-) knockout, Seps1(-/+) heterozygous, and wild-type mice were randomized to 3 days of incremental, high-intensity treadmill running or a sedentary control group. On day 4, the in situ contractile function of fast tibialis anterior (TA) muscles was determined. Seps1 reduction or deletion compromised exercise capacity. decreasing distance run. TA strength was also reduced. In sedentary Seps1(-/-) knockout mice, TA fatigability was greater than wild-type mice, and this was ameliorated with exercise. Whereas, in Seps1(-/)(+) heterozygous mice, exercise compromised TA endurance. These impairments in exercise capacity and TA contractile function were not associated with increased inflammation or a dysregulated redox state. Seps1 is highly expressed in muscle fibers and blood vessels. Interestingly, Nos1 and Vegfa mRNA transcripts were decreased in TA muscles from Seps1(-/-) knockout and Seps1(-/+) heterozygous mice. Impaired exercise performance with Seps1 reduction or deletion cannot be attributed to heightened cellular stress, but it may potentially be mediated, in part. by the effects of Seps1 on the microvas-culature.

Introduction: As we age, complex mechanisms cause skeletal muscle atrophy. Older adults often experience low muscle mass and strength, that are associated with increased morbidity and mortality. Resistance training is an effective tool to prevent age-related muscle atrophy and declining strength, and novel training parameters are valuable for improving program efficacy. The use of hypoxia (low O2) during resistance training elicits superior muscle hypertrophy and strength gains in young men. This study therefore aimed to determine the responses of older adults to hypoxic resistance training, and hypothesised that muscle hypertrophy and strength gains would be greater in hypoxia compared to normoxia.

Chronic metabolic stress leads to cellular dysfunction, characterized by excessive reactive oxygen species, endoplasmic reticulum (ER) stress and inflammation, which has been implicated in the pathogenesis of obesity, type 2 diabetes and cardiovascular disease. The ER is gaining recognition as a key organelle in integrating cellular stress responses. ER homeostasis is tightly regulated by a complex antioxidant system, which includes the seven ER-resident selenoproteins - 15 kDa selenoprotein, type 2 iodothyronine deiodinase and selenoproteins S, N, K, M and T. Here, the findings from biochemical, cell-based and mouse studies investigating the function of ER-resident selenoproteins are reviewed. Human experimental and genetic studies are drawn upon to highlight the relevance of these selenoproteins to the pathogenesis of metabolic disease. ER-resident selenoproteins have discrete roles in the regulation of oxidative, ER and inflammatory stress responses, as well as intracellular calcium homeostasis. To date, only two of these ER-resident selenoproteins, selenoproteins S and N have been implicated in human disease. Nonetheless, the potential of all seven ER-resident selenoproteins to ameliorate metabolic dysfunction warrants further investigation.

Selenoprotein S (Seps1) is an endoplasmic reticulum (ER) resident antioxidant implicated in ER stress and inflammation. In human vastus lateralis and mouse hindlimb muscles, Seps1 localization and expression were fiber-type specific. In male Seps1(+/-) heterozygous mice, spontaneous physical activity was reduced compared with wild-type littermates (d = 1.10, P = 0.029). A similar trend was also observed in Seps1(-/-) knockout mice (d = 1.12, P = 0.051). Whole body metabolism, body composition, extensor digitorum longus (EDL), and soleus mass and myofiber diameter were unaffected by genotype. However, in isolated fast EDL muscles from Seps1(-/-) knockout mice, the force frequency curve (FFC; 1-120 Hz) was shifted downward versus EDL muscles from wild-type littermates (d = 0.55, P = 0.002), suggestive of reduced strength. During 4 min of intermittent, submaximal (60 Hz) stimulation, the genetic deletion or reduction of Seps1 decreased EDL force production (d = 0.52, P < 0.001). Furthermore, at the start of the intermittent stimulation protocol, when compared with the 60-Hz stimulation of the FFC, EDL muscles from Seps1(-/-) knockout or Seps1(+/-) heterozygous mice produced 10% less force than those from wild-type littermates (d = 0.31, P < 0.001 and d = 0.39, P = 0.015). This functional impairment was associated with reduced mRNA transcript abundance of thioredoxin-1 (Trx1), thioredoxin interacting protein (Txnip), and the ER stress markers Chop and Grp94, whereas, in slow soleus muscles, Seps1 deletion did not compromise contractile function and Trx1 (d = 1.38, P = 0.012) and Txnip (d = 1.27, P = 0.025) gene expression was increased. Seps1 is a novel regulator of contractile function and cellular stress responses in fast-twitch muscles.

The transcriptional coactivators peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and PGC-1β are positive regulators of skeletal muscle mass and energy metabolism; however, whether they influence muscle growth and metabolic adaptations via increased protein synthesis is not clear. This study revealed PGC-1α or PGC-1β overexpression in C2C12 myotubes increased protein synthesis and myotube diameter under basal conditions and attenuated the loss in protein synthesis following the treatment with the catabolic agent, dexamethasone. To investigate whether PGC-1α or PGC-1β signal through the Akt/mTOR pathway to increase protein synthesis, treatment with the PI3K and mTOR inhibitors, LY294002 and rapamycin, respectively, was undertaken but found unable to block PGC-1α or PGC-1β’s promotion of protein synthesis. Furthermore, PGC-1α and PGC-1β decreased phosphorylation of Akt and the Akt/mTOR substrate, p70S6K. In contrast to Akt/mTOR inhibition, the suppression of ERRα, a major effector of PGC-1α and PGC-1β activity, attenuated the increase in protein synthesis and myotube diameter in the presence of PGC-1α or PGC-1β overexpression. To characterize further the biological processes occurring, gene set enrichment analysis of genes commonly regulated by both PGC-1α and PGC-1β was performed following a microarray screen. Genes were found enriched in metabolic and mitochondrial oxidative processes, in addition to protein translation and muscle development categories. This suggests concurrent responses involving both increased metabolism and myotube protein synthesis. Finally, based on their known function or unbiased identification through statistical selection, two sets of genes were investigated in a human exercise model of stimulated protein synthesis to characterize further the genes influenced by PGC-1α and PGC-1β during physiological adaptive changes in skeletal muscle.

Excessive inflammation is a hallmark of muscle myopathies, including Duchenne muscular dystrophy (DMD). There is interest in characterising novel genes that regulate inflammation due to their potential to modify disease progression. Gene polymorphisms in Selenoprotein S ( Seps1 ) are associated with elevated proinflammatory cytokines, and in vitro SEPS1 is protective against inflammatory stress. Given that SEPS1 is highly expressed in skeletal muscle, we investigated whether the genetic reduction of Seps1 exacerbated inflammation in the mdx mouse. F1 male mdx mice with a heterozygous Seps1 deletion ( mdx : Seps1 −/+ ) were generated. The mdx:Seps1 −/+ mice had a 50% reduction in SEPS1 protein expression in hindlimb muscles. In the extensor digitorum longus (EDL) muscles, mRNA expression of monocyte chemoattractant protein 1 ( Mcp-1 ) (P=0.034), macrophage marker F4/80 (P=0.030), and transforming growth factor-β1 ( Tgf-β1 ) (P=0.056) were increased in mdx:Seps1 −/+ mice. This was associated with a reduction in muscle fibre size; however, ex vivo EDL muscle strength and endurance were unaltered. In dystrophic slow twitch soleus muscles, SEPS1 reduction had no effect on the inflammatory profile nor function. In conclusion, the genetic reduction of Seps1 appears to specifically exacerbate the inflammatory profile of fast-twitch muscle fibres, which are typically more vulnerable to degeneration in dystrophy.

[This corrects the article on p. 170 in vol. 5, PMID: 24822049.].

Granulocyte colony-stimulating factor (G-CSF) was originally discovered in the context of hematopoiesis. However, the identification of the G-CSF receptor (G-CSFR) being expressed outside the hematopoietic system has revealed wider roles for G-CSF, particularly in tissue repair and regeneration. Skeletal muscle damage, including that following strenuous exercise, induces an elevation in plasma G-CSF, implicating it as a potential mediator of skeletal muscle repair. This has been supported by preclinical studies and clinical trials investigating G-CSF as a potential therapeutic agent in relevant disease states. This review focuses on the growing literature associated with G-CSF and G-CSFR in skeletal muscle under healthy and disease conditions and highlights the current controversies.

Hair analysis has been increasingly used to establish long-term biomarkers of exposure to both endogenous and exogenous substances, with a special emphasis on steroidal hormones. Hair cortisol and cortisone have been associated to physiological and psychological strains, anxiety and depression. Hair is a very complex matrix, which might jeopardize analyte detection at low concentrations. A new, highly selective and sensitive method based on fragments of second order, MS3 (MS/MS/MS), was developed and validated for the analysis of hair cortisol and cortisone. An online solid phase extraction was performed on a C8 restricted access material (RAM) phase following by separation on a reversed-phase C18 column using methanol and 0.02% ammonium hydroxide as mobile phase. The developed method required minimal sample preparation and the injection of only 50 µL of sample leading to a LOQ of 2 pg mg−1. Good linear responses were observed in the range 2–200 pg mg−1 (R2>0.99) and extraction recoveries ranged between 77–125% and 70–123% for cortisol and cortisone, respectively. Intra- and inter-assay coefficients of variation were between 1.4 and 14%. In order to evaluate the applicability of the method, preliminary tests (N=33) were conducted in 3 cm hair samples (close to scalp) of healthy volunteers with an age range of 4–63. Average concentrations in hair were 12.7±14 pg mg−1 and 41.6±42 pg mg−1 for cortisol and cortisone, respectively. Further investigations on cortisol and cortisone as biomarkers for chronic psychological strain will be assessed as a next step.

Disadvantaged communities provide adverse psychosocial exposures that have been linked to high levels of stress, and this may provide one explanatory pathway linking socioeconomic disadvantage to obesity. This study used hair cortisol analysis to quantify associations between stress and body mass index (BMI), and between hair cortisol and perceived psychological stress levels, in women and children living in socioeconomically disadvantaged neighborhoods. Participants were a volunteer sample of 70 women from the Resilience for Eating and Activity Despite Inequality study, including 30 maternal-child pairs. Women self-reported body weight, height and perceived psychological stress using the Perceived Stress Scale (PSS), and provided hair samples for themselves and their child. Children's body weight and height were measured. Following extraction, hair cortisol levels were measured using enzyme-linked immunosorbent assay. Multiple linear regression models examined associations between stress and BMI, and between hair cortisol and perceived stress levels in women and children. Women's hair cortisol levels were not associated with their BMI or PSS scores. Women's PSS scores were positively associated with their BMI (p=0.015). Within maternal-child pairs, mothers and children's hair cortisol levels were strongly positively associated (p=0.006). Maternal hair cortisol levels and PSS scores were unrelated to their child's zBMI. Children's hair cortisol levels were not associated with their zBMI or with their mother's PSS score. Findings suggest that cortisol-based and perceived psychological measures of stress may be distinct among women and children living in disadvantaged neighborhoods. Perceived psychological measures may be more important predictors of weight-related risk.

The benefits of resistance training for skeletal muscle mass and strength are well characterised. Early adaptations to unaccustomed resistance exercise include increased muscle strength, whereas later adaptations include muscle hypertrophy and further improvements in muscle strength (Schoenfeld, 2010). Although these strength and hypertrophy gains following resistance training are well documented in healthy and clinical populations, the mechanisms surrounding these phenomena are less implicit. The skeletal muscle microenvironment is tightly regulated and responds rapidly to resistance exercise. A single bout of unaccustomed resistance exercise can cause damage to skeletal muscle, where the structural integrity of the myofibres is compromised. This was theorised to be a significant event which stimulates hypertrophic responses within the muscle microenvironment (Schoenfeld, 2010). However, progression through a resistance training programme is marked by an attenuation in muscle damage which is termed the ‘repeated bout effect’. These findings have lead researchers to question the role of muscle damage in muscle hypertrophy following resistance training. One key factor which has an important yet poorly understood association with muscle damage and hypertrophy following resistance exercise is muscle protein synthesis. Previous work by Damas and colleagues has demonstrated that resistance exercise is a potent stimulator of skeletal muscle protein synthesis (Damas et al. 2015). In the hours following an unaccustomed bout of resistance exercise, an increase in myofibrillar muscle protein synthesis (MyoPS) can be detected. Repeated bouts of resistance exercise cause cumulative periods of increased MyoPS where net protein synthesis is greater than protein degradation, thus favouring muscle hypertrophy (Damas et al. 2015). However, the MyoPS response to resistance exercise is not equivocal as the resistance training programme progresses. Attenuation of the MyoPS response to resistance exercise can be observed as early as 3 weeks into a training programme (Brook et al. 2015). Interestingly, the MyoPS response to initial resistance exercise bouts is not correlated with muscle hypertrophy that occurs later in the training programme (Damas et al. 2015). However, the MyoPS response to later bouts of the resistance training programme correlates strongly with muscle hypertrophy (Brook et al. 2015). The authors noted that initial bouts of unaccustomed resistance exercise cause pronounced muscle damage, which stimulates growth mechanisms and increases protein synthesis to support tissue repair. Damas therefore proposed that the lack of correlation between the MyoPS response to initial exercise bouts and subsequent muscle hypertrophy could be due to exercise-induced muscle damage, and that the early MyoPS response is focused on repairing damaged muscle. However, following resistance training the MyoPS response is a more dedicated driver of muscle hypertrophy. Continuing on from previous research, Damas et al. therefore investigated the modulation of muscle hypertrophy during resistance training by measuring MyoPS and muscle damage throughout a resistance training intervention. The study is described in a recent article published in The Journal of Physiology (Damas et al. 2016). The authors hypothesised that the MyoPS response to the initial bout of resistance exercise would not be related to muscle hypertrophy at the end of the training programme, but that MyoPS responses at weeks 3 and 10 would be related to hypertrophy. This hypothesis would imply that the acute MyoPS response to initial bouts of resistance training cannot be used to predict subsequent muscle hypertrophy. Using a robust study design, Damas et al. (2016a) conducted a 10 week resistance training intervention in healthy young men to assess skeletal muscle damage, MyoPS and hypertrophic responses. Ten participants were recruited to perform high intensity lower body resistance training twice weekly. Participants were assessed at three key time points throughout the study: the initial training session, in week 3 and in the final training session of week 10. At each time point, muscle biopsy samples of the vastus lateralis were collected for analysis prior to, and 24 and 48 h following, the exercise bout. Investigators examined integrated muscle protein fractional synthesis rates through administration of deuterated water (D2O). Muscle damage was measured directly and indirectly by Z-band streaming and systemic creatine kinase, respectively. Muscle hypertrophy was quantified through fibre cross sectional area analysis of frozen vastus lateralis sections. Key statistical analyses included multiple regression analysis and Pearson's correlation coefficient. The 10 week resistance training intervention successfully increased knee extension maximum voluntary contraction (MVC), and increased vastus lateralis muscle mass by 14%. As expected, 24 h after the initial resistance training bout, direct and indirect measures of muscle damage were markedly elevated. Large elevations in MyoPS were observed 24 h after the initial exercise bout, followed by a significant diminution at 48 h. In the 48 h following training in week 3, both muscle damage and MyoPS responses were attenuated compared to the initial exercise bout. No marker of muscle damage was elevated above baseline in the 48 h following the final training session in week 10; however, the MyoPS response was not further attenuated. Interestingly, when the MyoPS response to training was normalised to the area of Z-banding (the direct measure of muscle damage), there was no difference in MyoPS response at any time point. Together, these findings support the authors’ hypothesis that the large MyoPS response to the initial resistance exercise bout is partly due to exercise-induced muscle damage. Due to the attenuation of muscle damage that occurs following repeated bouts of exercise, the authors also hypothesised that only during the later stages of the training intervention would MyoPS be correlated with muscle hypertrophy. As expected, MyoPS following the initial resistance exercise bout was not correlated with subsequent muscle fibre hypertrophy at week 10. However, both the week 3 and week 10 MyoPS responses were significantly correlated with the subsequent change in vastus lateralis cross sectional area, as measured by ultrasound. More convincingly, the MyoPS response in week 10 was strongly correlated with muscle fibre hypertrophy, which was quantified from laminin-stained sections. Together, these findings lead the authors to conclude that the larger MyoPS response following the initial unaccustomed exercise bout was directed toward the repair of damaged muscle, rather than muscle hypertrophy. The authors suggest that the attenuation of muscle damage (repeated bout effect) can potentially explain the attenuated MyoPS response, and that the MyoPS at week 10 was directed toward muscle hypertrophy and not the repair of muscle damage. In summary, the MyoPS response to resistance exercise was largest following the initial exercise bout, but was not correlated with subsequent muscle hypertrophy. The large MyoPS response to the initial exercise bout was associated with the largest muscle damage. When normalising MyoPS to muscle damage, there were no differences in MyoPS between the three exercise bouts, suggesting that the increased MyoPS response to the initial exercise bout was potentially directed toward the repair of damaged muscle. Only after the attenuation of muscle damage at week 10 of training was MyoPS strongly correlated with vastus lateralis muscle fibre hypertrophy. These findings suggest that the muscle damage following the initial exercise bout was partly responsible for the large MyoPS response. These data further confirm that the MyoPS response to initial resistance exercise bouts cannot be used as a predictor of subsequent muscle hypertrophy. The findings from this study could also be valuable for understanding muscle growth responses in populations with excessive muscle damage responses to physical activity and resistance training. To further disseminate the regulation of muscle hypertrophy following resistance training, populations such as the elderly, who display impaired MyoPS responses, could also be considered. Overall, the authors should be commended for the use of robust study parameters to investigate the relationship between protein synthesis, muscle damage and muscle hypertrophy following resistance training. The findings of Damas et al. (2016) highlight the fact that only after the attenuation of muscle damage does the protein synthesis response to resistance training correlate with skeletal muscle hypertrophy. None declared.

The benefits of resistance training for skeletal muscle mass and strength are well characterised. Early adaptations to unaccustomed resistance exercise include increased muscle strength, whereas later adaptations include muscle hypertrophy and further improvements in muscle strength (Schoenfeld, 2010). Although these strength and hypertrophy gains following resistance training are well documented in healthy and clinical populations, the mechanisms surrounding these phenomena are less implicit. The skeletal muscle microenvironment is tightly regulated and responds rapidly to resistance exercise. A single bout of unaccustomed resistance exercise can cause damage to skeletal muscle, where the structural integrity of the myofibres is compromised. This was theorised to be a significant event which stimulates hypertrophic responses within the muscle microenvironment (Schoenfeld, 2010). However, progression through a resistance training programme is marked by an attenuation in muscle damage which is termed the ‘repeated bout effect’. These findings have lead researchers to question the role of muscle damage in muscle hypertrophy following resistance training. One key factor which has an important yet poorly understood association with muscle damage and hypertrophy following resistance exercise is muscle protein synthesis. Previous work by Damas and colleagues has demonstrated that resistance exercise is a potent stimulator of skeletal muscle protein synthesis (Damas et al. 2015). In the hours following an unaccustomed bout of resistance exercise, an increase in myofibrillar muscle protein synthesis (MyoPS) can be detected. Repeated bouts of resistance exercise cause cumulative periods of increased MyoPS where net protein synthesis is greater than protein degradation, thus favouring muscle hypertrophy (Damas et al. 2015). However, the MyoPS response to resistance exercise is not equivocal as the resistance training programme progresses. Attenuation of the MyoPS response to resistance exercise can be observed as early as 3 weeks into a training programme (Brook et al. 2015). Interestingly, the MyoPS response to initial resistance exercise bouts is not correlated with muscle hypertrophy that occurs later in the training programme (Damas et al. 2015). However, the MyoPS response to later bouts of the resistance training programme correlates strongly with muscle hypertrophy (Brook et al. 2015). The authors noted that initial bouts of unaccustomed resistance exercise cause pronounced muscle damage, which stimulates growth mechanisms and increases protein synthesis to support tissue repair. Damas therefore proposed that the lack of correlation between the MyoPS response to initial exercise bouts and subsequent muscle hypertrophy could be due to exercise-induced muscle damage, and that the early MyoPS response is focused on repairing damaged muscle. However, following resistance training the MyoPS response is a more dedicated driver of muscle hypertrophy. Continuing on from previous research, Damas et al. therefore investigated the modulation of muscle hypertrophy during resistance training by measuring MyoPS and muscle damage throughout a resistance training intervention. The study is described in a recent article published in The Journal of Physiology (Damas et al. 2016). The authors hypothesised that the MyoPS response to the initial bout of resistance exercise would not be related to muscle hypertrophy at the end of the training programme, but that MyoPS responses at weeks 3 and 10 would be related to hypertrophy. This hypothesis would imply that the acute MyoPS response to initial bouts of resistance training cannot be used to predict subsequent muscle hypertrophy. Using a robust study design, Damas et al. (2016a) conducted a 10 week resistance training intervention in healthy young men to assess skeletal muscle damage, MyoPS and hypertrophic responses. Ten participants were recruited to perform high intensity lower body resistance training twice weekly. Participants were assessed at three key time points throughout the study: the initial training session, in week 3 and in the final training session of week 10. At each time point, muscle biopsy samples of the vastus lateralis were collected for analysis prior to, and 24 and 48 h following, the exercise bout. Investigators examined integrated muscle protein fractional synthesis rates through administration of deuterated water (D2O). Muscle damage was measured directly and indirectly by Z-band streaming and systemic creatine kinase, respectively. Muscle hypertrophy was quantified through fibre cross sectional area analysis of frozen vastus lateralis sections. Key statistical analyses included multiple regression analysis and Pearson’s correlation coefficient. The 10 week resistance training intervention successfully increased knee extension maximum voluntary contraction (MVC), and increased vastus lateralis muscle mass by 14%. As expected, 24 h after the initial resistance training bout, direct and indirect measures of muscle damage were markedly elevated. Large elevations in MyoPS were observed 24 h after the initial exercise bout, followed by a significant diminution at 48 h. In the 48 h following training in week 3, both muscle damage and MyoPS responses were attenuated compared to the initial exercise bout. No marker of muscle damage was elevated above baseline in the 48 h following the final training session in week 10; however, the MyoPS response was not further attenuated. Interestingly, when the MyoPS response to training was normalised to the area of Z-banding (the direct measure of muscle damage), there was no difference in MyoPS response at any time point. Together, these findings support the authors’ hypothesis that the large MyoPS response to the initial resistance exercise bout is partly due to exercise-induced muscle damage. Due to the attenuation of muscle damage that occurs following repeated bouts of exercise, the authors also hypothesised that only during the later stages of the training intervention would MyoPS be correlated with muscle hypertrophy. As expected, MyoPS following the initial resistance exercise bout was not correlated with subsequent muscle fibre hypertrophy