Metabolic stress is a physiological process that occurs during exercise in response to low energy that leads to metabolite accumulation [lactate, phosphate inorganic (Pi) and ions of hydrogen (H⁺)] in muscle cells. Traditional exercise protocol (i.e., Resistance training) has an important impact on the increase of metabolite accumulation, which influences hormonal release, hypoxia, reactive oxygen species (ROS) production and cell swelling. Changes in acute exercise routines, such as intensity, volume and rest between sets, are determinants for the magnitude of metabolic stress, furthermore, different types of training, such as low-intensity resistance training plus blood flow restriction and high intensity interval training, could be used to maximize metabolic stress during exercise. Thus, the objective of this review is to describe practical applications that induce metabolic stress and the potential effects of metabolic stress to increase systemic hormonal release, hypoxia, ROS production, cell swelling and muscle adaptations.

It has been reported that chronic exercise can promote changes in many organs because of cellular adaptations. Skeletal muscle is extremely adjustable in response to contractile activity[1,2], therefore, repeated muscle contractions during exercise can lead to numerous metabolic modifications[3,4]. Overtime, these adaptive responses have shown beneficial effects on health, body composition and performance[5-7].

During acute exercise, the energy used by skeletal muscle contractions are essential in transforming organelles, enzymatic activity, intracellular signaling and transcriptional responses[8-10]. Metabolic stress is a physiological process that occurs during exercise in response to low energy which leads to metabolite accumulation [lactate, phosphate inorganic (Pi) and ions of hydrogen (H⁺)] in muscle cells[11,12]. Researchers have suggested that metabolic stress has an important impact on hormonal release, hypoxia, cell swelling and production of reactive oxygen species (ROS)[13-15]. All of these components can initiate anabolic signaling for muscle growth and adaptations on energy metabolism[16].

In situations with elevated ATP hydrolysis and glycolytic flux in muscle cells, there is a great accumulation of adenosine monophosphate (AMP) and metabolites[12,17,18]. Furthermore, the reduction of intracellular oxygen levels can also lead to hypoxia[19]. All these metabolic parameters are a powerful stimulus to activate AMP-activated protein kinase (AMPK) and hypoxia-inducible factor (HIF-1α) pathway, the main regulators of mitochondrial biogenesis and angiogenesis[20,21].

Moreover, metabolite accumulation and hypoxia that is produced during exercise may increase ROS production through mitochondrial electron transport chain[22,23]. It is well established that ROS production by endurance exercise has positive effects on mitochondrial adaptations because it stimulates peroxisome proliferator-activated receptor gamma coactivator (PGC-1α) and p38 mitogen-activated protein kinase (p38MAPK) pathways[24]. Scientific evidence shows that suppression of ROS production through the use of the antioxidants impairs some adaptive responses to endurance exercise[25,26], and these results suggest that ROS production has positive effects on mitochondrial adaptations.

Nevertheless, besides stimulating mitochondrial biogenesis and angiogenesis, the metabolic stress also has positive effects on muscle hypertrophy. Resistance training (RT) has great impact on increasing metabolite accumulation, which influences hormonal release, hypoxia, ROS production and cell swelling. All these processes can mediate anabolic signaling that stimulates muscle protein synthesis and activation of satellite cells[13-15].

In this context, changes in acute exercise routines (intensity, volume and rest between sets) are the main factors in determining the magnitude of metabolic stress[27-29]. Furthermore, blood flow restriction training has been considered a tool to maximize metabolic stress[30,31]. Studies have reported great effects of this training method on aerobic adaptations and muscle hypertrophy[32,33].

Therefore, the purpose of this paper is to describe practical applications that cause metabolic stress. In addition, we will discuss the potential effects of metabolic stress on the increase of systemic hormonal release, hypoxia, ROS production, and cell swelling for enhancing muscle adaptations.

Skeletal muscle hypertrophy depends on positive muscle protein balance (protein synthesis exceeds breakdown)[34]. Thus, RT is excellent for the stimulation of anabolic signaling and the promotion of muscle hypertrophy[35]. Metabolic stress is one of the primary mechanisms that makes RT increase muscle mass, mainly due to the rise of anabolic hormonal release, hypoxia, ROS production and cell swelling[13]. However, studies have shown that the magnitude of metabolic stress depends on the changes of acute RT program variables[14,15].

Scientific evidence shows that load, number of repetitions, and reset between intervals are important factors to induce metabolite accumulation. Gonzalez et al[29] found that acute RT with moderate repetitions combined with short rest intervals (70% 1RM, 10-12 repetitions and one minute rest interval) shows an increase in blood lactate, serum concentration of lactate dehydrogenase, growth hormone (GH) and cortisol when compared to higher loads, low repetitions combined with longer rest intervals (90% 1RM, 3-5 repetitions and three minute rest intervals). Concerning these findings, duration of rest intervals may reflect directly on the magnitude of metabolic stress. In a review study, researchers demonstrated that short interval sets (less than one minute) are essential in increasing blood lactate and GH production, mainly because of insufficient recovery of phosphocreatine and H⁺ accumulation[36].

Additionally, Nishimura et al[37] demonstrated higher effects of muscle hypertrophy when RT is performed during hypoxia, possibly because of the strong influence of hormonal release, the recruitment of fast-twitch muscle fibers, ROS production and cell swelling[38]. During RT, muscle contractions compress blood vessels in active muscles, and this occlusion can lead to a reduction of oxygen levels and, consequently, resulting in a hypoxic environment[39]. Intramuscular hypoxia during exercise can increase the necessity of anaerobic latic metabolism by activation of HIF-1α that regulates the expression of glycolytic enzymes[40]. Thus, exercise that produces high levels of lactate can be associated with hypoxia. One study showed that performing hypertrophy-type RT (70% 1RM, 10 repetitions and 90 s rest intervals) induces higher production of lactate and reduction in pH than performing a strength-type RT (85% 1RM, 4-6 repetitions with five minute rest intervals)[41]. In this context, it can be hypothesized that RT can generate hypoxia when performed at moderate/high repetitions combined with short rest intervals, possibly due to a high demand on anaerobic metabolism.

Furthermore, another study found that knee extension RT at low intensity (50% 1RM) generates a significant decrease in muscle oxygenation when compared to high-intensity (80% 1RM) exercise performed with one-second rest between repetitions[42]. These findings suggest, keeping continuous tension on muscles without relaxation can be essential to reducing oxygen levels and maximizing the levels of hypoxia in the skeletal muscle.

Research suggests that ROS production also has important implications on muscle hypertrophy[43,44]. In addition, studies have shown that utilization of antioxidants can modify protein signaling after a RT session and impairs muscle mass gains[45,46]. Muscle contractions during exercise produces ROS at low physiological levels and plays an important role in cell signaling to promote beneficial adaptations[47]. Researchers have found that the production of ROS has an influence in stimulating anabolic signaling, because ROS can act with a signaling molecule to activate the mammalian target of rapamycin (mTOR) through IGF-1 and MAPK pathways[48,49].

Although it is becoming clear that ROS has a profound impact on muscle hypertrophy, the limits of these adaptations are not clear. Hornberger et al[50] observed that selenium-deficient transgenic mice (animals with decreased protein expression of antioxidant enzymes containing selenium) exhibited an increased muscle hypertrophy when stimulated by synergist ablation (a muscle overload model), compared to other animals. In this study, rapamycin treatment (a pharmacological inhibitor of mTOR) completely abolished the hypertrophy effect, thus proving that mTOR is necessary for hypertrophy. It is interesting to note that, contrary to this study (where muscle antioxidant defense was decreased and muscle hypertrophy was optimized), other studies evaluating the impact of antioxidants in humans (through vitamin E and C supplementation) were shown to impair muscle hypertrophy response and cell signaling leading to muscle hypertrophy[45,46]. Several studies have observed that RT increases hypoxia, metabolite accumulation and ROS production, which seems to be strictly related[22,23,51,52]. In this context, we can hypothesize that RT with moderate/high repetitions and short rest intervals can be a stimulus to produce ROS.

Another potent anabolic signaling event produced by RT is cell swelling. Studies have demonstrated that cell swelling mediated by hydration can lead to an increase in protein synthesis and a decrease in proteolysis mainly through the activation of MAPK pathway[53-55]. During intense muscle contractions, veins are obstructed but the arterial system keeps the delivery of blood active[13]. This process can increase intracellular swelling, which leads to an increased pressure against the cytoskeleton. Thus, the cell perceives a threat and initiates an anabolic signaling response to promote reinforcement of its ultrastructure[56]. Studies indicate that cell swelling occurs during metabolite accumulation (lactate, H⁺ and Pi) which leads to additional intracellular fluid[57,58]. Therefore, it seems reasonable to conclude that RT during hypertrophy causes high metabolite accumulation and can promote more cell swelling than strength RT.

Finally, another aspect that we should consider, especially among well-trained subjects, is RT with moderate/high repetitions until failure. Recent studies show that, when RT is executed with low load (30%-50% 1RM and 25-35 repetitions) until failure, hypertrophy is similar when compared to high load (70%-90% 1RM and 8-12 repetitions)[59-61]. Although no studies have confirmed this hypothesis, we believe that muscular failure can exert additional metabolic stress and then induce anabolic signaling. These findings suggest that the greater time under tension with moderate/high repetitions without relaxation combined with short rest interval and muscular failure can generate a strong hypertrophic response similar to RT with high loads. However, caution should be taken, because restricting rest periods would cause a reduction in the volume performed during a RT session, thus affecting hypertrophy process negatively[62].

This effect can be caused by high metabolic stress, leading to anabolic signaling through hypoxia, hormonal release, ROS production and cell swelling (Figure 1).

Open in New Tab   Full Size Figure   Download Figure

Figure 1 Role of metabolic stress induced by different kinds of training (resistance, blood flow restriction and high intensity interval intraining) for enhancing muscle adaptations. ROS: Reactive oxygen species.

During the last decade, blood flow restriction training (BFRT), also known as KAATSU or occlusion[63], combined with low-intensity strength training (20%-30% 1RM), has been shown to increase strength and muscle size among trained/untrained athletes[64-66] injured[67] and the elderly[68]. This training model requires the use of cuffs that are placed at the proximal ends of the upper arms or thighs reducing blood flow from the muscle (approximately 100-200 mmHg). Thus, the external pressure applied maintains arterial inflow while blocking venous outflow of blood[69], resulting in an ischemic/hypoxic environment that enhances the training effect[70].

Several studies have compared low-intensity strength training with BFRT and high-intensity without BFRT and demonstrated a significant increase in muscle cross-section area in both exercise protocols[64,69,71,72]. However, RT performed with moderate/high intensities seems to lead to similar degrees of muscle hypertrophy when combined with BFRT. It is not clear if the maximal degree of muscle hypertrophy can be optimized by increasing external loads or if the ceiling for maximal hypertrophy is achieved with low-moderate loads[73].

Cumming et al[74] performed a study with nine healthy volunteers performing five sets of unilateral knee extension at 30% of 1RM until failure combined with BFRT and the same workout without BFRT. Analysis of muscle biopsies revealed a rapid translocation of heat-shock proteins (HSP27 and aB-crystallin) from cytosol to cytoskeletal structures, both of which have been identified as important HSPs for repair and stabilization of stressed and damaged proteins[75]. This indicates that cytoskeletal proteins are stressed during BFRT even without myofibrillar disruptions. Thus, muscle hypertrophy induced by BFRT seems to be mediated by metabolic stress and mechanical tension, and sarcolemmal-bound mechanosensors (i.e., integrins) stimulate intracellular anabolic and catabolic pathways, which convert mechanical energy into chemical signals, promoting protein synthesis instead of degradation[76].

Suga et al[77] investigated metabolic stress (intramuscular phosphocreatine (PCr), Pi, Diprotonated phosphate-H₂PO₄ and Intramuscular pH) in subjects that performed four unilateral plantar flexion (two min of 30 repetitions/min) using three different intensities (20%, 30% and 40% 1RM) with two resistance exercises (20% 1 RM and 65% 1RM) without BFRT. They concluded that 30% of 1RM induced a similar intramuscular metabolites and pH response than high-intensity RT without BFRT. In addition, Suga et al[31] also showed that multiple low-intensity BFRT sets increase fast-twitch fiber recruitment that could assist the slow twitch fiber to keep the strength during training, however, the authors did not observe statistical significance between multiple sets of high intensity exercise without BFRT. Therefore, these results suggest that multiple-set exercise are more effective than single-set RT.

Previous studies have shown that metabolic stress induced by low-intensity plus BFRT increases GH secretion and muscle hypertrophy[64,65,78], furthermore, this could stimulate metabolic stress markers, such as IL-6[79,80]. The recovery process is initiated by IL-6 by modulating muscle regulatory genes (i.e., MyoD)[81-83] and activating muscle satellite cells[80], and therefore may play a role in regulating muscle growth/hypertrophy[80].

An acute increase in anabolic hormones (e.g., testosterone, GH) has been found during short rest periods (30 to 60 s)[84], however, regarding cytokine production, a recent study compared 30 s vs 90 s of rest after four sets of squat and four sets of bench press with 70% of 1RM until failure without BFRT in healthy adults and observed higher IL-6 levels when 90 s rest was used[85]. In addition, Phillips et al[86] reported greater post-exercise IL-6 concentrations with 65% of 1RM compared to 85% of 1RM with two minutes of recovery. Thus, short rest period induce an acute increase in anabolic hormones, however, it seems that longer recovery intervals combined with higher loads contribute to an increase in IL-6 concentration during RT.

Therefore, changes in variables, such as recovery intervals, volume, intensity, and repetition speed, could be used to optimize the specific adaptation during low-intensity RT plus BFRT.

Studies have investigated the benefits of metabolic stress on skeletal muscle remodeling, angiogenesis, mitochondrial biogenesis, performance, and high-intensity interval training (HIIT) has shown to be a promising training routine. This exercise/training routine is based on high-intensity exercise sets with passive or low-intensity intervals between them. Endurance training adaptations have been found with HIIT[87,88].

The HIIT configuration allows intervals of effort and pause, and the various forms of stimuli can cause adaptations, such as: (1) mechanical stretching and muscle tension; (2) increase of ROS; (3) increase of intramuscular calcium concentrations and (4) changes of energy “status” in the cell.

Two HIIT routines that are commonly used are: four sets of 30 s at 100%[88] and four sets of four minutes[89] at 90%-95% of the maximum power (Pmax), velocity (Vmax) or maximum heart rate (HR max). Wahl et al[90] compared the acute responses of these two routine with another routine done continuously (two hours at 55% Pmax) in triathlon athletes and found that the most intense stimulus (four sets of 30 s at 100%) generated higher metabolic acidosis (pH) and higher concentrations of anabolic hormones (testosterone and GH) after the session. Supporting these results, Wahl et al[91] compared the use of buffer solution (sodium bicarbonate) and placebo with HIIT (four sets of 30 s at 100%), and showed a significant decrease in pH in the placebo group with increases in GH compared to the buffer group. The elevation of these hormones mean hypertrophic adaptations and also important stimuli expression of oxidative enzymes and erythropoiesis, promoting improvements in aerobic performance. This can be explained by the direct stimulation of bone marrow by testosterone, supporting the synthesis of erythropoietin in kidney cells[92].

Mitochondrial biogenesis is another adaptation of great importance in this process and one of the most studied. A key molecule for this adaptation is PGC-1α, a coactivator of several transcription factors related to metabolic and mitochondrial adaptations[93]. Burgomaster et al[87] found that six weeks of HIIT (three times per week, four to six sets of 30 at 100%) and continuous training (five times per week, 40 to 60 min at 55% VO_2max) showed significant improvements in mitochondrial functions with optimization lipid oxidation, increased activity of oxidative enzymes (citrate synthase and 3-hydroxyacyl CoA dehydrogenase) and contents of PGC-1α. The important finding of this study was the difference in the duration of training sessions, ranging from approximately 1.5 h to 4.5 h per week for HIIT and continuous training, respectively.

Due to the importance of PGC-1α, the expression and activation of proteins that stimulate it has great relevance. Two proteins, which are unquestionably stimulated by metabolic stress, are p38MAPK and AMPK[94-96]. Gibala et al[97] showed a significant increase in phosphorylation of AMPK and p38MAPK after acute sessions of HIIT (four sets of 30 s at 100%), and despite a great increase in mRNA of PGC-1α, its protein content did not change. Additionally, Little et al[98], using the same protocol of exercises, showed significantly higher values of p38MAPK after exercise, as well as an increase of 750% of mRNA PGC-1α and 66% of protein already in the nucleus of muscle cells, confirming the potential of these training routine.

Mitochondrial biogenesis and angiogenesis are essential for aerobic adaptations and improvement of performance. Considering the efficiency of HIIT (short training repetitions and metabolic stress), with BFRT seems to be beneficial to increase vascular adaptations. Consequently, Taylor et al[32] compared acute HIIT (four sets of 30 s at 100%), with HIIT + BFR (cuff in the thigh, two minutes, 130 mmHg). The results of these biopsies (vastus lateralis) showed a significant increase in p38MAPK after HIIT and HIIT+BFRT, with no differences between them. After three hours of exercise, a significant increase in mRNA PGC-1α was observed, vascular endothelial growth factor (VEGF) and its receptor (VEGFR-2), however mRNA of HIF-1α only increased in HIIT + BFRT. These results indicate that HIIT by itself is capable of stimulating angiogenesis, but the fact that only HIIT + BFRT increased HIF-1α cannot be overlooked, because it is a key factor for hypoxia and metabolic stress. Low PO₂ increases concentrations, favoring translocation to the nucleus and subsequent activation of VEGF in the human skeletal muscle[99].

Changes in acute exercise routine variables, such as intensity, volume, recovery interval and type of training are determinants that influence the magnitude of metabolic stress. Despite, traditional training protocol, such as RT, increase metabolite accumulation and influence hormonal release, hypoxia, ROS production and cell swelling. In this review, we discussed that low-intensity RT plus BFRT and HIIT are alternative exercise routines that increase metabolic stress and muscle adaptation among different populations. However, the difference between exercise protocols used in literature and different levels of physical fitness should be considered when interpreting the results.