Molecular Mechanisms behind Muscles Growth in Young Subjects: Comparison
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Muscle growth, or muscle hypertrophy, is a complex process regulated by several molecular pathways. The IGF-1/PI3K/Akt/mTOR pathway is a vital signaling cascade in muscle growth that involves various interconnected mechanisms. Its activation increases protein synthesis, reduces protein degradation, and improves cell growth. Akt activation is crucial in promoting muscle protein synthesis in response to exercise and nutrient intake in young individuals. The timing of exercise and protein intake also affect Akt activation and subsequent muscle protein synthesis. While exercise alone did not increase Akt and mTOR phosphorylation, protein ingestion afterward did so in a dose-dependent manner. Growth hormone (GH) promotes the uptake of essential nutrients, such as glucose and amino acids, into muscle cells for energy production and protein synthesis. Testosterone is one of the most potent naturally secreted androgenic-anabolic hormones, and its biological effects include promoting muscle growth. 

  • exercise
  • nutrition
  • skeletal muscle

1. The Molecular Mechanisms behind Muscles Growth in Young Subjects: Exercise, Nutrients, and Hormones

Muscle growth, or muscle hypertrophy, is a complex process regulated by several molecular pathways. The primary pathways that promote muscle growth in response to exercise, nutrients, and growth signals include the IGF-1/PI3K (phosphoinositide 3-kinase)/Akt/mTOR pathway and hormones like testosterone and GH (Figure 1).
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Figure 1. Here is a diagram showing the typical triggers for muscle growth in young people, which include exercise and high-quality protein intake. These stimuli activate various molecular pathways that lead to muscle hypertrophy. However, on the right-hand side of the diagram, you can see that the same stimuli do not have the same effect on skeletal muscle in cases of anabolic resistance.
The IGF-1/PI3K/Akt/mTOR pathway is a vital signaling cascade in muscle growth that involves various interconnected mechanisms. Its activation increases protein synthesis, reduces protein degradation, and improves cell growth. The molecular mechanisms and interactions within this pathway are still being studied in human muscle physiology. When activated, mTOR promotes muscle hypertrophy by stimulating protein synthesis and cell growth [1]. The mTOR kinase is the main regulator of muscle protein synthesis and responds to the availability of nutrients, particularly amino acids and growth factors. Indeed, resistance exercise and nutrient intake, especially leucine-rich amino acids, activate the mTOR pathway essential for exercise-induced muscle protein synthesis and are, thus, necessary for increasing muscle protein synthesis following resistance exercise in young men [2]. The key importance of mTOR is exemplified by an experiment, where young subjects were treated with a potent mTORC1 inhibitor (rapamycin) before performing a series of high-intensity muscle contractions to demonstrate this activation. Rapamycin treatment blocked the early, acute (after 1–2 h) contraction-induced increase in human muscle protein synthesis. Other studies have also shown that a high-protein diet enhances mTOR-mediated protein synthesis in young men following endurance exercise [3]. These studies indicate that mTOR activation is crucial in mediating the anabolic response to exercise and diet in humans (Figure 1).
Akt activation is crucial in promoting muscle protein synthesis in response to exercise and nutrient intake in young individuals [4]. In response to resistance training, significant hypertrophy (+10%) in the human quadriceps was demonstrated, with a parallel increase in phospho-Akt, phospho-GSK-3beta, and phospho-mTOR protein, and a decrease in the nuclear protein content of Foxo1, which is the master regulator of the atrophy program [4]. Akt is also influenced by nutrition, as excess leucine intake has been shown to enhance Akt signaling in young individuals, suggesting that Akt is sensitive to dietary amino acids and can modulate protein synthesis accordingly [5].
The timing of exercise and protein intake also affect Akt activation and subsequent muscle protein synthesis. While exercise alone did not increase Akt and mTOR phosphorylation, protein ingestion afterward did so in a dose-dependent manner. As a matter of fact, Akt activation is a complex process influenced by exercise, nutrition, and specific amino acids, and further research is being conducted to fully understand its role in muscle growth and adaptation [6].
The upstream controller in this axis is the insulin-like growth factor-1 (IGF-1), which is crucial in promoting growth and anabolic processes in skeletal muscles [1]. IGF-1 is a key growth factor that regulates both anabolic and catabolic pathways in skeletal muscle.
Studies indicate that IGF-1 induces hypertrophy of human myotubes in vitro, characterized by an increase in the mean number of nuclei per myotube, an increase in the fusion index, and an increase in myosin heavy chain (MyHC) content [7]. IGF-1 contributes to muscle protein turnover, protein synthesis, and the adaptation of skeletal muscles to resistance training [8]. Hormonal factors, such as ethinyl estradiol administration, can affect IGF-1 synthesis and degradation in skeletal muscles, potentially modulating muscle protein turnover and influencing responses to exercise and nutrient intake [9]. IGF1 is closely connected with growth hormone (GH), which stimulates muscle growth through several mechanisms and interactions with other signaling pathways. It stimulates protein synthesis by enhancing the uptake of amino acids into muscle tissue, providing the building blocks for muscle protein synthesis. This process is mediated, at least in part, by the GH/IGF-1 axis. GH stimulates the liver and other tissues to produce the insulin-like growth factor 1 (IGF-1), which promotes cell growth, protein synthesis, and the proliferation of satellite cells involved in muscle repair and growth [10].
GH promotes the uptake of essential nutrients, such as glucose and amino acids, into muscle cells for energy production and protein synthesis. It also affects metabolism by enhancing the breakdown of fats, providing additional energy sources for muscle growth [11]. However, pharmacological GH supplementation only increases muscle strength or size in individuals with clinical GH deficiency, and there is no evidence that exercise-induced changes in GH have the same effects in individuals with normal GH levels [12].
Testosterone is one of the most potent naturally secreted androgenic-anabolic hormones, and its biological effects include promoting muscle growth. In muscle, testosterone stimulates protein synthesis (anabolic effect) and inhibits protein degradation (anti-catabolic effect) [13][14][15][16][17]. Testosterone plays a crucial role in muscle growth in response to exercise and nutrition. Various studies have shed light on this topic. Vingren et al. (2010) explored the physiological aspects of testosterone in resistance exercise and training, highlighting its upstream regulatory elements [16]. They found that testosterone enhances muscle protein synthesis, stimulates satellite cell activation and proliferation, and modulates anabolic signaling pathways such as mTOR and IGF-1. These findings suggest that testosterone plays a key role in mediating the anabolic response to resistance exercise [16]. West and Phillips (2010) discussed the anabolic processes in human skeletal muscle, emphasizing the roles of growth hormone and testosterone (24). They concluded that testosterone acts directly on muscle tissue to promote muscle protein synthesis and hypertrophy.
The interactions between testosterone, growth hormone, and anabolic signaling pathways, such as mTOR and IGF-1, contribute to the overall anabolic response in muscle. West and Philips highlighted the importance of optimizing testosterone levels for maximizing muscle growth in response to exercise and nutrition [12]. In their work, the exercise paradigms are designed based on the assumption (not necessarily evidenced-based mechanisms) that GH and testosterone facilitate anabolic processes that lead to skeletal muscle protein accretion and hypertrophy. Furthermore, they pointed out that exercise-induced hormonal stimulation does not enhance intracellular markers of anabolic signaling or the acute post-exercise elevation of myofibrillar protein synthesis. Furthermore, they demonstrated that exercise-induced increases in GH and testosterone availability are unnecessary and do not enhance strength and hypertrophy adaptations. So, they concluded that local mechanisms, intrinsic to the skeletal muscle tissue performing the resistive contractions (i.e., weight lifting) are predominant in stimulating anabolism [12]. The regulation of satellite cells following myotrauma caused by resistance exercise is also related to testosterone, which plays a critical role in activating satellite cells, which are crucial for muscle repair and growth [17]. Finally, Bhasin et al. (2001) examined the dose–response relationships of testosterone in healthy young men, and they found that testosterone administration at varying doses increased dose-dependently muscle protein synthesis rates, resulting in greater muscle mass and strength gains [18]. These findings suggest that higher testosterone levels promote anabolic processes and contribute to muscle growth in response to exercise and nutrition.

2. Effects of Substrates and Exercise on Skeletal Muscle Protein Synthesis in Young, Middle-Aged Subjects

Accretion of muscle mass depends on physiological, metabolic, and hormonal factors, as well as on physical activity. Conversely, it is hindered by inactivity, malnutrition, overt diseases, and/or subtle, chronic pathological conditions [19][20][21][22][23][24]. The main protein-anabolic factors, at both the whole-body and skeletal muscle level, are the proteins and/or the amino acids themselves (i.e., the protein building blocks), as well as physical activity/exercise. In addition, energy availability, anabolic hormones (insulin, human Growth Hormone (hGH), IGF-1, β-agonists, anabolic steroids, and see also above), adequate tissue perfusion, and, in general, a “healthy status” (i.e., the absence of both overt diseases and subtle pathological conditions, such as a chronic sub-inflammatory status) also condition skeletal muscle accretion. Furthermore, the effects of any of these factors could be divided into either “acute” (i.e., detectable under acute experimental conditions) or “chronic” (i.e., following repeated stimuli, with end-point results tested sometime after). Protein anabolism is thus achieved through the stimulation of protein synthesis and/or the inhibition of protein degradation. MPS can be determined in vivo either by measuring the incorporation of infused amino acid stable isotopes into muscle by biopsy (Figure 2) and/or through measurements of the A-V difference in labeled as well as unlabeled amino acid(s) across a sampled district predominantly constituted by skeletal muscle, typically a limb (either the leg or the forearm) [25] (Figure 3).
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Figure 2. The figure schematically illustrates the methodology commonly employed to determine skeletal muscle fractional protein synthesis rate (FSR) with the combined infusion of an amino acid tracer (in this example, phenylalanine, (phe)) and its timed incorporation into skeletal muscle (measured by biopsy). Absolute Muscle Protein Synthesis (MPS) rate is calculated as the product of FSR times muscle protein mass. The asterisks (*) indicate the labeled amino acid. Abbreviations used in the Figure: art: artery; m: muscle; prec.: the enrichment (or the specific activity) of the “precursor” (i.e., the labeled amino acid), used in the calculations.
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Figure 3. Measurement of skeletal muscle protein synthesis and degradation with arterial/venous combined with isotope infusion. The figure schematically depicts the measurements performed in the perfusing artery and in the deep vein draining blood from the sampled muscular-rich district (i.e., the leg or the forearm). In this example, the indicator essential amino acid is phenylalanine (Phe), which is utilized by muscle only for protein synthesis and is released from muscle only from protein degradation. Blood flow across the muscle district has to be measured too. Thus, from the measured phenylalanine utilization and release, it is possible to extrapolate protein degradation and synthesis. The asterisk (*) indicates the labeled amino acid. Abbreviations used in the Figure: art: artery; ven: vein; ic: intracellular. The equations used in the calculations are not reported here.

2.1. Effects of Proteins and Amino Acids

Definitely, protein ingestion stimulates skeletal muscle protein synthesis (MPS) [26]. The stimulation’s magnitude and duration depend on both the protein dose and its type/quality, which is closely associated with the concurrent post-ingestion rise in amino acid plasma concentrations [27][28]. Hyperaminoacidemia [29][30][31], specifically that of essential amino acids (EAAs) [32], among them, of the branched chain ones and of leucine in particular [19][33][34], largely condition tissue protein synthesis.
In non-exercising young subjects (i.e., “at rest”), the acute administration of either a protein-containing mixed meal or a pure protein load, stimulated skeletal muscle protein synthesis [35][36]. In dose–response studies, intake of 20 g of high-quality proteins, i.e., whey protein [37] or mixed egg protein [38], was sufficient to maximally stimulate postabsorptive rates of myofibrillar MPS in resting young men over 4 h [37]. In middle-aged men however, following the ingestion of graded amounts of beef (from 57 g, i.e., 12 g protein; 113 g, i.e., 24 g protein; or 170 g, i.e., 36 g protein), the stimulation of myofibrillar MPS was the greatest with 170 g of beef [39], thus apparently not achieving a plateau. Notably, in this study, exercise further and significantly enhanced the beef protein effect only at the highest administered dose (170 g beef). The administration of free amino acids, either as bolus ingestion of 15 g EAA [40], or of a leucine-rich EAA and carbohydrate mixture [41], increased human muscle protein synthesis too. When dose–response curves between crystalline EAA and MPS were constructed, the literature data somehow contrasted. It was initially reported that 2.5 g crystalline EAA was sufficient to elicit an increase above basal of MPS in young subjects [42]. However, in subsequent studies using intact whey protein, the lower dose capable of eliciting a response in MPS in young muscle was set at >10 g (=5 g EAA), reaching saturation at 20–40 g EAA. The type of the administered protein is important too. Since the amount of the EAAs, and/or their relative proportions, are greater and/or more balanced in high- than in low-quality proteins, the amount and quality of the protein are relevant in the stimulation of protein synthesis. This issue is important in ageing, because the estimated recommended daily protein intake in aged people is ≈50% greater (1.2 g/kg BW) than that of young-adult subjects (0.8 g/kg BW) [43], and it could be better achieved by the intake of high-quality protein, such as whey protein, albumin, egg, or, to a lesser extent, of mixed milk protein, thus helping to maintain muscle mass and prevent sarcopenia. Ingestion of a single dose of 38 g mixed milk protein (i.e., including both fast- and slow-absorbable proteins) in young men, resulted in a time-dependent increase in postprandial muscle protein synthesis, detectable as soon as 60′ after and lasting at least up to 5 h. [44]. Also, the ingestion of 30–40 g of a vegetal protein, mycoprotein, stimulated skeletal muscle protein synthesis to an extent comparable to that of an isonitrogenous omnivorous diet [45]. In young subjects, the total protein-anabolic effect of meal ingestion, measured using leucine tracers at the whole-body level, was more pronounced with “slow” (i.e., casein) than with “fast” (i.e., whey) protein administration [46][47]. The mechanisms leading to these effects were different, too: the “fast” protein markedly stimulated amino acid oxidation and protein synthesis but did not change proteolysis, whereas the “slow” protein increased amino acid oxidation and protein synthesis to a lesser extent but strongly inhibited proteolysis. Therefore, since the effects of whey proteins may be more rapidly vanishing, it would be useful to combine the effects of fast and slow proteins, irrespective of whether they are vegetal or animal ones. In young adults, a supplement containing a vegetal, antioxidant-rich soy protein mixed with whey protein, could prolong the improvement of the AA net balance across the leg up to ≈ 2 h post-ingestion, compared with the 20 min attained with whey alone [48].

2.2. Effects of Exercise and Nutrition on Muscle Protein Synthesis and Accretion

Exercise is a potent stimulator of muscle protein synthesis, particularly in the recovery phase [49], and it positively interacts with protein/amino acid ingestion in the stimulation of skeletal muscle anabolism. Muscle mass accretion following resistance exercise combined with food ingestion is observed even following an adequate habitual protein intake (≥0.8 g kg−1 day−1) [50]. A greater protein intake (1.8–3.0 g kg−1 day−1) further augments lean body mass (i.e., protein) accretion without increasing fat mass, when compared to an energy-rich, low-protein intake (≈5% of energy as protein) [50]. The high-quality whey protein, combined with resistance exercise in young adults, exerted a greater effect on MPS than equivalent doses of lower-quality proteins, such as soy protein or casein, an effect, however, still present up to 3–5 h post-exercise [38]. As reported above, a 20 g dose of whey protein was sufficient for the maximal stimulation of post-absorptive myofibrillar MPS in young men, whether or not exercising [37], and it was effective up to 3–5 h after exercise [28][51]. Conversely, others reported that the response of muscle protein synthesis following whole-body resistance exercise is greater following 40 g rather than 20 g of ingested whey protein [52]. After the ingestion of incremental doses of mixed milk protein (0, 15, 30, or 45 g) together with 45 g carbohydrate, the 30 g protein dose was sufficient to maximize the myosin synthesis rates during recovery from a single bout of endurance exercise in young men [53]. The intake of another high-quality protein, i.e., egg protein, as low as 5–10 g (approximating that contained in a single ≈ 60 g egg: ≈ 6.8 g total protein), increased MPS above basal following resistance exercise, reaching a maximum with a dose of 20 g egg protein [54].

2.3. Effect of Other Substrates

Protein synthesis is an energy-requiring process [55]; therefore, energy-providing substrates such as glucose and fat may affect protein turnover too. The activities of the cellular pathways controlling protein turnover are bio-energetically expensive and therefore depend on intracellular energy availability (i.e., macronutrient intake) [56].

Glucose

Glucose increased muscle protein synthesis in vitro [57]. Testing glucose-induced or derived substrates separately, tissue ATP decreased during incubation with lactate, and lactate + pyruvate supported protein synthesis better than pyruvate or glucose. The data on the effects in humans of either glucose or fat on protein metabolism, specifically on skeletal muscle, are scarce, complex, and not univocal [30]. Enteral glucose administration did not affect either duodenal mucosal protein FSR or the activities of mucosal proteases [58]. An oral glucose load, and the simultaneous glucose-induced stimulation of insulin secretion, did not alter the rate of whole-body protein synthesis or breakdown [59].

Lipids and Ketones

The effects of either lipid or ketone infusion/administration in humans are complex. Lipid infusion in humans did not affect proteolysis [60]. In contrast, medium-chain fatty acid infusion apparently increased leucine oxidation and, therefore, net protein catabolism [61]. Thus, the effects on whole-body protein degradation may depend on the fatty acid length [62]. The increase in FFA decreased basal muscle protein synthesis, but not the anabolic effect of leucine [63]. When associated with dietary protein ingestion, neither acute nor short-term dietary fat overload impaired skeletal MPS in overweight/obese men in the post-prandial phase, thus excluding a role by dietary accumulation of intramuscular lipids on the anabolic response to meal ingestion [64]. The infusion of 3OHButyrate decreased both whole body and forearm protein turnover (measured by phenylalanine/tyrosine tracers), as well as phenylalanine catabolism, in post-absorptive conditions, whereas it did not modify the insulin-induced effects following an euglycemic clamp [60].

Other Nutritional Interventions

β-alanine supplementation may increase physical performance in middle-aged individuals [65] and improve physical performance during exercise [66]. Creatine may increase muscle mass in combination with resistance exercise, although the mechanism(s) of action remain elusive. Short-term creatine monohydrate (CrM) supplementation may exert anti-catabolic actions on selected proteins in men, but it did not enhance either whole-body or mixed-muscle protein synthesis [67]. Acute metabolic studies testing various substances may provide useful information for estimating the efficacy of potentially anabolic agents [68].

3. Hormones and Related Drug Interventions

3.1. Insulin

Although insulin has an undisputed anabolic effect on tissue protein, its experimental demonstration in vivo has challenged the investigators over time, mainly because its administration induces a decrease in amino acid plasma concentration, thus possibly obscuring its direct effect in muscle. A major advancement was the maintenance of the “amino acid “clamp” at baseline during insulin infusion or injection, also achieved when insulin was directly infused into the artery perfusing a muscle-rich organ (such as the leg or the forearm), thus avoiding a system perturbation of the aminoacidemia and allowing, at the same time, the insulin effect in the perfused limb to be studied selectively, using any of these techniques. Muscle protein synthesis and degradation were determined by combining amino acid isotope infusion with arterial–venous limb catheterization, often complemented by muscle biopsy in some studies.

3.2. Glucocorticoids

Cortisol, often referred to as a stress hormone, has both catabolic and anabolic effects, depending on the context. Cortisol-induced secondary sarcopenia (i.e., specifically induced either by either an exogenous administration or an endocrine disease) is a frequent finding in the clinical setting. Chronic glucocorticoid exposure induces loss of lean body mass by decreasing protein synthesis and increasing degradation [69][70][71]. The protein-catabolic effect of prednisone is antagonized by growth hormone [70]. In humans, the administration of 8 mg dexamethasone daily for 4 days antagonized the anti-proteolytic effect of insulin in the forearm [72].

3.3. Human Growth Hormone (hGH) and IGF-1

In humans, hGH administration increased whole-body [70] and muscle protein synthesis [73]. The insulin-like growth factor-binding proteins (IGFBPs) bind to IGFs, modulating their activity and availability. They help in regulating IGFs’ actions in various tissues, including muscle. When rhIGF-I was infused at a rate achieving plasma IGF-I concentrations close to those observed following rhGH treatment, and yet avoiding the IGF-1-induced hypoglycemia, proteolysis and protein synthesis were not affected, even in the presence of prednisone treatment [74]. However, when rhGH and rhIGF-I were administered simultaneously, nitrogen balance was remarkably improved [74]. IGF-1 exhibits several splicing variants, IGF-1Ea, which is a circulating factor synthesized in the liver, and IGF-1Eb and IGF-1Ec, recognized as Mechano-Growth Factors (MGFs), which manifest in skeletal muscles of rodents and humans, respectively [75]. The act of stretching or overloading skeletal muscles triggers a rise in IGF-1 mRNA, particularly emphasizing the specific IGF-1Ec variant (MGF) [75]. The extent to which IGF-1 alternative splicing variants might induce greater hypertrophy compared to IGF-1 per se remains partially unresolved [76]. Notably, the effects of the synthetic E-domain peptide mimetic have been elucidated: the MGF-24aa-E peptide (YQPPSTNKNTKSQRRKGSTFEEHK) activates satellite cells, prompting their replication [75]. 

3.4. Catecholamines

Epinephrine has both an α- and β-receptor affinity. Epinephrine infusion in humans depressed plasma amino acid concentrations, particularly the essential ones, however without changing leucine or phenylalanine flux [77], nor did it impair the disposal of exogenous amino acids in humans [78]. However, in another study, the increase in plasma epinephrine concentrations inhibited proteolysis and leucine oxidation in humans via beta-adrenergic mechanism, compatible with an anabolic effect [79].

3.5. Estrogens

Although estrogens are female sex hormones, they are also present in smaller amounts in males. Estrogen plays a role in maintaining bone health, promoting protein synthesis and muscle growth, and influencing body composition, in both males and females. While the exact mechanisms by which estrogens affect muscle growth are still being studied, research suggests that they can, directly and indirectly, affect muscle tissue. One way estrogens may influence muscle growth is by promoting protein synthesis, interacting with receptors in muscle cells, and activating signaling pathways involved in protein synthesis [80]. Decreased estrogen-associated signaling impairs mitochondrial function leading to muscle atrophy [81]. Estrogens can also indirectly affect muscle growth by modulating the production and activity of other hormones, such as growth hormone and insulin-like growth factor 1 (IGF-1), which are important for muscle development. Estrogens may regulate the release of these hormones from the pituitary gland and the liver, thereby influencing muscle growth [82].

3.6. Androgens

Testosterone is a potent anabolic stimulus primarily through improvement in the re-utilization of amino acids released from protein degradation [83] (see also the above paragraph), and this will be further discussed below. Testosterone and progesterone, but not estradiol, stimulated muscle protein synthesis in postmenopausal women [84].

4. Exercise

Exercise is a powerful stimulus to promote skeletal muscle protein synthesis and net protein anabolism, involving specific metabolic and morphological adaptations in muscle [49], also interacting with substrates [85][86]. Exercise produces diverse changes in amino acid metabolism and protein turnover in muscle, according to the exercise phase. Acute changes in amino acid metabolism during exercise are primarily catabolic (i.e., increased amino acid oxidation), yet exercise does not cause muscle wasting. This is because both immediate and later post-exercise phases are anabolic. Thus, regular exercise is essential for optimizing muscle growth and hypertrophy. The type of exercise also determines the magnitudes of these processes, and exercise requires a sequence of metabolic adjustments from the catabolic period of the ongoing exercise to the anabolic period of recovery. Two primary exercise types commonly associated with muscle growth are resistance and endurance. Resistance exercise, such as weightlifting, involves using external resistance to challenge the muscles. This type of exercise is a primary intervention used to develop strength and stimulate muscle hypertrophy. Muscle mass increases constitute key components of conditioning in the outcome of various sports due to the correlation between cross-sectional muscle area and muscle strength [87][88].

4.1. Resistance Exercise Can Be Further Classified into Two Main Categories

High-Intensity Resistance Exercise: This exercise type typically involves lifting heavy weights for a relatively low number of repetitions. It primarily targets fast-twitch muscle fibers, which have a higher potential for muscle growth. High-intensity resistance exercise promotes muscle hypertrophy by acutely causing mechanical tension and muscle damage, subsequently triggering muscle protein synthesis and adaptation [89]. High-intensity resistance exercise is a potent stimulus for skeletal MPS and hypertrophy in young adults during high-intensity exercise and post-exercise recovery [90][91][92][93][94][95]. Moderate-Intensity Resistance Exercise: This exercise type involves using moderate weights for more repetitions. While the hypertrophic response may be less pronounced than high-intensity resistance exercise, moderate-intensity resistance exercise still contributes to muscle growth and can benefit endurance and functional capacity [89].

4.2. Effects of Exercise in Conjunction with Nutrient Intake

The specific adaptation to each type of exercise can vary depending on factors such as exercise intensity, volume, frequency, and individual characteristics. Combining resistance and endurance exercises in a well-designed training program can provide comprehensive benefits for muscle growth, strength, endurance, and overall fitness. It has been shown that combined strength and endurance training in the evening may lead to larger gains in muscle mass [96]. Additionally, other factors such as nutrition, rest, and recovery play crucial roles in maximizing the benefits of exercise and promoting muscle growth. Adequate protein intake, overall caloric balance, and appropriate recovery periods are important factors to be considered when optimizing muscle growth in response to exercise. During the acute phase, the energy needs to stimulate amino acid oxidation/catabolism (together with that of glucose and fat), thus depleting intracellular amino acid pools. In contrast, in the recovery phase, the depleted amino acid pools and energy need to be reconstituted. Since the response of muscle protein metabolism to a resistance exercise bout lasts up to 5 h [38], such an ample post-exercise recovery phase allows a comfortable, positive anabolic interaction with food (protein) intake. Immediately following exercise, muscle protein turnover, i.e., protein synthesis, breakdown, and amino acid transport, are accelerated. However, the net protein balance remains negative (i.e., catabolic or not above zero) unless food is ingested [85]. Therefore, food intake associated with exercise is necessary to bring muscle protein balance to be positive.

4.3. Exercise–Insulin Interaction

Insulin and exercise can positively interact in the stimulation of protein anabolism, in a complex fashion. Both protein synthesis and degradation rates are ≥1-fold greater following the post-exercise recovery than at rest [97]. The effects of insulin on muscle protein synthesis and degradation, either without or with exercise, are complex. Insulin-stimulated protein synthesis at rest, but not in the recovery post-exercise phase. In contrast, insulin decreased protein degradation following exercise as opposed to no effect at rest. Thus, the post-exercise phase can be viewed as a (transient) insulin-resistant condition as regards protein synthesis that, in terms of the effects on net protein balance, is however overwhelmed by a greater insulin-mediated suppression of proteolysis.

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