Recent advances in molecular biology have elucidated some of the mechanisms that regulate muscle growth. This progress has opened many avenues in the study of resistance exercise (RE)-induced hypertrophy and enhanced understanding of this complex phenomenon. However, as this line of research progresses to in vivo RE models, investigators must not overlook the fundamental principles of RE. Without an appropriate RE stimulus, downstream signalling leading to muscle growth will be suboptimal. Therefore, the purpose of this article is to acquaint the reader with the fundamental principles of RE and place this information in context of cellular and molecular events. Our intent is that this conceptual paradigm (figure 1) will bridge the gap between molecular biology and RE theory to foster future research into the mechanisms of RE-induced muscle growth.

Fig. 1
figure 1

(a) Theoretical response of a physiological system to a given stimulus. In general, an ‘upstream’ stimulus activates a cascade of subsequent ‘downstream’ events for a desired response. (b) Paradigm of resistance exercise biology. The resistance exercise stimulus is an upstream signal that determines specific downstream events. Therefore, manipulation of the acute resistance exercise programme variables (i.e. exercise choice, load, volume, rest period lengths and exercise order) alters the unique ‘fingerprint’ of the resistance exercise stimulus, thus modifying downstream responses. If the resistance exercise stimulus is applied serially over several weeks (i.e. long-term training), then eventually muscle growth will occur. However, the degree of adaptation is highly dependent on other factors such as nutrition, age and training status.

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1. Cascade of Events Induced by Resistance Exercise

RE-induced muscle growth is a complex phenomenon that depends on numerous physiological systems and signalling pathways. Muscle growth occurs following a sequential cascade of: (i) muscle activation; (ii) signalling events arising from mechanical deformation of muscle fibres, hormones, and immune/inflammatory responses; (iii) protein synthesis due to increased transcription and translation; and (iv) muscle fibre hypertrophy.

1.1 Muscle Activation

During RE, α-motorneurons activate muscle fibres to produce force. The neuromuscular interaction determines which muscle fibres are activated and the amount of force exerted. Specifically, two variables regulate the force of muscle contraction: neural firing frequency and number of motor units recruited. Henneman’s size principle describes the latter method, dictating that neural recruitment of muscle tissue begins with the smallest motor units (primarily type I) and progresses to larger motor units (primarily type II) until force production matches force requirements.[1] The size principle is a critical and often under-appreciated feature of motor unit recruitment. Practically, the size principle ensures that low-force activities recruit primarily type I, fatigue-resistant motor units. As the force requirements of the activity increase (e.g. by increasing the load for a given RE), additional, progressively higher-threshold motor units are recruited. RE that utilize very heavy, near-maximal loads activate the entire spectrum of motor units.

Although the size principle appropriately describes the relationship between exercise load and muscle activation, two important caveats exist: the influence of performing exercises ‘explosively’ (e.g. Olympic weight-lifting), and the influence of muscle fatigue/failure. Rapidly accelerating the load increases muscle force production (as force = mass × acceleration) and therefore, as predicted by the size principle, recruits high-threshold motor units. Performing explosive concentric muscle actions using a light load (∼40% of maximal isometric force) increases electromyographic (EMG) activity when compared with performing the same exercise with a heavier load (∼67% of maximal isometric force) at a slower velocity.[2] Additionally, as fatigue ensues during repetitive muscular activity, EMG activity increases.[3] Presumably this indicates increased contribution of high-threshold motor units to maintain force output as lower-threshold motor units begin to fatigue. Therefore, in addition to exercise load, the intent to perform the exercise explosively and muscle fatigue can modify muscle recruitment patterns during RE.

In summary, exercise load, rate of force development, and muscle fatigue affect motor unit recruitment during RE. Motor unit recruitment (and the phenotype of the recruited motor units) must be carefully considered for responses and adaptations to RE because: (i) only those motor units recruited will respond and adapt to RE; (ii) heavy loads, explosive exercises, and/or significant muscle fatigue is necessary to activate type II motor units; (iii) type I and type II muscle fibres display differential signalling responses to muscle contraction;[4] (iv) type II muscle fibres have a greater capacity for hypertrophy following RE training than type I fibres;[5] and (v) different muscle groups possess varied percentages of type I and type II muscle fibres (for example, in humans the gastrocnemius is ∼60% type I muscle fibres and the soleus is ∼85% type I muscle fibres).[6]

1.2 Signalling Events

Muscle fibre recruitment consequently activates several systems that signal muscle growth: (i) mechanical deformation of muscle fibres; (ii) hormonal responses; and (iii) immune/inflammatory responses.

1.2.1 Mechanical Deformation of Muscle Fibres

Mechanical deformation of muscle fibres (i.e. contraction and/or stretching) stimulates various muscle signalling pathways independently of changes in hormones and growth factors.[7,8] In particular, mechanical deformation activates the protein kinase B (Akt; also known as PKB)-mammalian target of rapamycin (mTOR) pathway, the adenosine monophosphate-activated protein kinase (AMPK) pathway, and the mitogen-activated protein kinase (MAPK) pathways. The Akt-mTOR pathway is crucial for adaptations to RE;[9] however, the importance of AMPK and MAPK signalling for adaptations to RE remains to be elucidated.

Protein Kinase B-Mammalian Target of Rapamycin Signalling

When muscle fibres contract, Akt-mTOR signalling increases dramatically; this response is critical for increasing muscle protein synthesis[7] and for subsequent growth.[9] Akt phosphorylates and activates mTOR during muscle overload.[9] However, the increase in mTOR signalling following mechanical deformation can also occur independently of Akt.[7] Akt-independent, mechanically induced mTOR signalling occurs via phospholipase D-generated phosphatidic acid production.[10] At rest, α-actinin in the z-band of the sarcomere associates with and inhibits phospholipase D.[11] However, as proposed by Hornberger et al.,[10] phospholipase D dissociates from α-actinin during repetitive mechanical deformation; this relieves the inhibition of phospholipase D by α-actinin and subsequently promotes phosphatidic acid production and mTOR activation.

mTOR signalling increases protein synthesis by enhancing translational efficiency (i.e. messenger RNA [mRNA] translated per ribosome). When activated, mTOR phosphorylates two primary targets: the 70 kDa ribosomal protein S6 kinase (p70 S6K) and eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1). Phosphorylation stimulates p70 S6K activity, causing it to subsequently phosphorylate the S6 subunit of the 40S ribosomal protein. Phosphorylation of S6 increases the translation of mRNAs encoding ribosomal proteins and translational factors.[12] 4E-BP1 is normally bound to eIF4E; however, hyperphosphorylation of 4E-BP1 by mTOR causes it to release from eIF4E. Subsequently, eIF4E binds mRNA and congregates with eIF4A (an RNA helicase) and eIF4G (a scaffolding protein) to form the heterotrimeric translation initiation complex eIF4F.[13] In summary, muscle contraction activates mTOR signalling (via Akt and/or phosphatidic acid). mTOR subsequently phosphorylates downstream targets (i.e. p70 S6K and 4E-BP1) that, in turn, activate ribosomal proteins (i.e. S6) and translation initiation factors (i.e. eIF4E). Ultimately, mTOR signalling enhances translational efficiency, increases protein synthesis, and promotes muscle growth.

Besides mTOR, Akt also phosphorylates glycogen synthase kinase-3β (GSK-3β) and the fork-head box O family of transcription factors (FOXO). Akt phosphorylates and inhibits GSK-3β, which relieves the inhibition of eIF2B by GSK-3β. Subsequently, eIF2B chaperones methionyl-mRNA to the 40S ribosomal subunit for translation initiation. Phosphorylation of FOXO by Akt prevents FOXO from stimulating transcription of proteolytic ubiquitin ligases.[14] Figure 2 summarizes the Akt-mTOR pathway. Hormone- and diet-induced activation of Akt-mTOR signalling will be discussed in the following sections.

Fig. 2
figure 2

Resistance exercise stimulates muscle growth and inhibits muscle atrophy via the protein kinase B-mammalian target of rapamycin (Akt-mTOR) pathway. Resistance exercise incites considerable hormonal responses, including growth hormone (GH) and insulin-like growth-factor-1 (IGF-1). Additionally, muscle contraction per se stimulates this pathway. Muscle contraction can paradoxically also inhibit Akt-mTOR signalling by stimulating adenosine monophosphate-activated protein kinase (AMPK); however, the importance of brief, transient increases in AMPK activity in mediating responses and adaptations to resistance exercise remains to be elucidated. Dietary intake potentiates the resistance exercise-induced increase in Akt-mTOR signalling and is necessary for a net increase in protein synthesis following resistance exercise. Arrows signify a stimulatory response; blocked lines indicate an inhibitory response. 4E-BP1 = eukaryotic initiation factor 4E binding protein-1; Akt = protein kinase B; CHO = carbohydrate; EAAs = essential amino acids; eIF2B = eukaryotic initiation factor 2B; FOXO = fork-head box O transcription factor; GSK-3B = glycogen synthase kinase-3B; mTOR = mammalian target of rapamycin; PA = phosphatidic acid; PI-3K = phosphatidylinositol-3 kinase; p70 S6K = the 70 kDa ribosomal protein S6 kinase.

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Adenosine Monophosphate-Activated Protein Kinase Signalling

AMPK is postulated to be the ‘energy sensor’ of the cell, as high AMP and low glycogen concentrations (i.e. markers of low cellular energy) activate AMPK.[15] In response to this decrease in energy, AMPK promotes energy-releasing pathways (e.g. glucose and fatty acid oxidation) and inhibits energy-consuming pathways (e.g. protein synthesis) during times of energy shortage (e.g. exercise).[15] Much research focuses on the role of AMPK in promoting adaptations to endurance exercise.[15] However, AMPK might also play an important role in affecting RE adaptations. For example, AMPK reduces protein synthesis via inhibition of the mTOR pathway,[16] potentially via tuberin phosphorylation and/or mTOR Thr 2446 phosphorylation (for details see Kimball[17]). AMPK activity transiently increases following an acute bout of RE and returns to (or is at least statistically similar to) resting values within the initial 2 hours post-RE.[18] The relative importance of AMPK activity for mediating responses and adaptations to RE remains to be elucidated.

Mitogen-Activated Protein Kinase Signalling

MAPK signalling pathways are a network of several parallel phosphorylation cascades. These pathways are generally divided into four major subfamilies: extracellular signal-regulated kinases 1 and 2 (ERK1/2), p38 MAPK, c-Jun NH2-terminal kinase, and ERK5. Muscle contraction and growth factors stimulate these pathways. MAPKs exert cellular effects by phosphorylating and activating various transcription factors and co-activators.[19] Additionally, MAPKs also phosphorylate histones. Phosphorylation of histones alters the chromatin environment of specific genes and enhances transcription.[20] Although MAPK signalling increases following acute RE in humans,[21] the importance of MAPK signalling for RE-induced adaptations remains unclear.[19]

1.2.2 Hormonal Responses

Depending on the exact RE protocol, RE incites considerable anabolic hormonal responses, including growth hormone (GH), insulin-like growth factor-1 (IGF-1), and testosterone.[22] The acute RE programme variables (discussed in detail in section 2) dictate the magnitude of this hormonal response. RE that utilizes large muscle masses, moderate loads (10-repetition maximum [RM]), short rest periods (1 minute) and high total work, maximize the hormonal response to exercise.[22,23] Hansen et al.[23] cleverly demonstrated the importance of exercise-induced hormonal responses by examining two groups of subjects who performed identical upper-body RE programmes for 9 weeks. One group performed additional lower-body RE to stimulate large increases in circulating hormones in conjunction with the standard upper-body RE protocol. Subjects training only the upper body increased arm strength by ∼9%, while subjects training the upper and lower body (and thus experiencing greater circulating hormonal concentrations) increased arm strength by ∼37%. These results indicate that RE-induced hormonal responses potentiate strength gains following long-term training. Importantly, though, gains in strength[23] and hypertrophy[24] can occur with little to no change in circulating hormones; indicating that hormonal responses potentiate, but are not solely responsible for, adaptations to RE.

Growth Hormone

GH is actually a ‘family of hormones’ as >100 different variant forms exist in circulation.[25] These variants include 22 kDa GH monomers (the most frequently studied form), 20 kDa mRNA splice variants, disulfide-linked homodimers and heterodimers, glycosylated GH, high molecular weight oligomers, GH bound to GH-binding protein, and hormone fragments (e.g. 5 kDa and 17 kDa fragments) resulting from proteolysis. The specific biological activity of each GH variant awaits complete elucidation; however, acute RE dramatically affects concentrations of these GH variants.[26–28] Additionally, we recently demonstrated that long-term RE training increases the biological activity of circulating GH molecular weight variants.[29]

Binding of GH to its membrane-bound receptor initiates janus kinase 2 (JAK2) signalling. JAK2 has several downstream substrates that signal a variety of cellular functions (for a review see Piwien-Pilipuk et al.[30]). Of particular importance, GH-induced JAK2 signalling activates phosphatidylinositol-3 kinase (PI-3K). Because PI-3K is proximal to Akt-mTOR signalling, it is likely that the GH response to RE promotes translational efficiency and muscle anabolism (see figure 2). In vivo research supports this hypothesis, as GH injection (150 μg/kg/day injected into pigs) increases skeletal muscle eIF2B activity and 4E-BP1 phosphorylation, resulting in enhanced translational efficiency and protein synthesis.[31]

Insulin-Like Growth Factor-1

RE increases concentrations of circulating[22] and muscle[32] IGF-1. Moreover, RE alters concentrations of IGF binding proteins that influence the biological activity of IGF-1.[33] IGF-1 stimulates muscle hypertrophy via PI-3K-Akt-mTOR signalling (figure 2).[34,35] Additionally, IGF-1 stimulates the proliferation and differentiation of specialized stem cells located at the periphery of muscle fibres called satellite cells. Satellite cell activation, proliferation and differentiation contribute significantly to muscle growth following long-term training[36] and will be discussed in detail in ’Satellite Cell Activity’, section 1.2.3.

RE-induced GH responses would theoretically increase liver and/or muscle IGF-1 production. The somatomedin hypothesis states that GH stimulates the production and secretion of IGF-1; increased circulating IGF-1 subsequently inhibits further release of GH and IGF-1 in a negative feedback loop (for review see Le Roith et al.[37]). However, the in vivo relationship between GH and IGF-1 is incompletely delineated (especially with regards to exercise). For example, RE-induced increases in endogenous GH do not significantly affect circulating IGF-1 concentrations during the ensuing 24-hour recovery period.[38] Pharmacological GH injection increases skeletal muscle IGF-1 mRNA expression.[39] However, the effect of endogenous increases of GH on skeletal muscle IGF-1 protein content remains to be studied.

Testosterone

Numerous studies have demonstrated that RE increases circulating concentrations of testosterone.[22,40–43] Testosterone exerts its influence on skeletal muscle protein synthesis via androgen receptors (AR). Testosterone binds to and converts AR to a transcription factor capable of translocating to the nucleus and associating with DNA to regulate androgen-specific gene expression. AR blockade attenuates muscle protein accretion, indicating the physiological importance of testosterone-AR interactions for muscle hypertrophy.[44] Additionally, like IGF-1, testosterone exerts some of its influence on muscle growth via satellite cells. Supra-physiological doses of testosterone increase satellite cell number in a dose-dependent manner[45] and stimulate satellite cell proliferation and differentiation.[46]

1.2.3 Immune/Inflammatory Responses

Traditional RE involves concentric and eccentric muscle actions. During eccentric actions, the myofibrils of the muscle fibre stretch while producing force. Repetitive overstretching leads to sarcomere disruption and eventually membrane damage.[47] Muscle damage contributes to muscle soreness, but also provides an important stimulus for muscle growth. As reviewed by Peake et al.,[48] RE-induced muscle damage promotes neutrophil mobilization and invasion into the muscle tissue. Neutrophils and macrophages have the dual function of degrading damaged muscle tissue and producing pro-inflammatory cytokines (e.g. interleukin-6 [IL-6], transforming growth factor-β [TGF-β], and tumour necrosis factor-α [TNFα]). IL-6 and TGFβ are expressed in muscle in response to damage and stimulate satellite cell proliferation and differentiation, respectively.[36] Although muscle damage-induced increases in TNFα production can inhibit Akt signalling,[49] long-term exercise training suppresses TNFα expression in muscle,[50] which would promote an anabolic environment.

Satellite Cell Activity

The myonuclear domain theory proposes that each nucleus of a multi-nucleated muscle cell can transcribe mRNA for a finite volume of cytoplasm.[36] In other words, a nucleus can ‘manage’ only a certain amount of muscle tissue. Therefore, for a muscle fibre to hypertrophy, a corresponding increase in the number of nuclei must occur to regulate the increase in cytoplasm. This is achieved by incorporating local satellite cells (and their associated nuclei) into the existing muscle fibre.

As previously mentioned in sections 1.2.2 and 1.2.3, RE-induced hormonal and immune responses increase satellite cell activity (i.e. activation, proliferation and differentiation). Hormones and cytokines activate these normally quiescent satellite cells and subsequently cause them to proliferate, differentiate and fuse to existing muscle fibres.[36] Satellite cells thus contribute new nuclei to the existing pool within the muscle fibre. This is critical, because increasing the number of myonuclei enhances the fibre’s capacity for transcription, protein synthesis and growth. The importance of satellite cells for muscle hypertrophy has been clearly demonstrated, as irradiation (a potent inhibitor of satellite cell activity) negates the increase in myonuclei, DNA content and muscle size that normally occurs following muscular overload.[51]

1.3 Protein Synthesis

RE-induced stimulation of cellular signalling pathways and satellite cells subsequently increases muscle protein synthesis. Although not completely defined, it appears that a temporal relationship exists among the mechanisms of this response. Adams et al.[51] subjected rat plantaris to 90 days of muscle overload and detailed the mechanisms of protein accretion. Immediately (within 1 day) after ablation-induced overload, phosphorylation of p70 S6K and eIF4E increased ∼8-fold. These changes in translational efficiency eventually returned to baseline 15–90 days later. Alternatively, muscle DNA content continued to rise throughout the 90-day protocol; and total muscle RNA (an index of translational capacity) increased after 3 days, continued to rise until 7 days, and remained elevated at 90 days. Together, these results indicate that translational efficiency (i.e. mRNA translated per ribosome) is important for acute increases in protein synthesis during the initial hours/days of overload; however, increased transcriptional capacity (i.e. quantity of nuclei) via satellite cell fusion, and perhaps increased translational capacity (i.e. quantity of ribosomes), critically regulate long-term gains in muscle size.

1.4 Muscle Growth

Significant muscle protein accretion can occur if bouts of RE are performed serially for an extended period of time (i.e. long-term training). Upstream signalling events arising from mechanical deformation of muscle fibres, hormonal responses, immune responses, cellular signalling pathways, and satellite cell activity support this adaptation. Figure 3 provides a summary of the RE-induced signalling responses that lead to muscle growth.

Fig. 3
figure 3

Summary of the signalling responses to resistance exercise. Resistance exercise stimulates muscle fibre contraction and evokes endocrine and immune responses. These various signals stimulate transcription and translation and, over time, muscle hypertrophy. Corresponding increases in satellite cell-derived myonuclei accompany muscle fibre hypertrophy. 4E-BP1 = eukaryotic initiation factor 4E binding protein-1; Akt = protein kinase B; AMPK = adenosine monophosphate-activated protein kinase; AR = androgen receptor; GH = growth hormone; IGF-1 = insulin-like growth factor-1; JAK = janus kinase; MAPKs = mitogen-activated protein kinases; mTOR = mammalian target of rapamycin; PI-3K = phosphatidylinositol-3 kinase; p70 S6K = 70 kDa ribosomal protein S6 kinase.

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Muscle growth depends on the relationship between protein synthesis and protein breakdown: only when synthesis exceeds breakdown can muscle growth occur. Although an acute bout of RE stimulates both synthesis and breakdown, the disproportionate increase in protein synthesis (if provided adequate dietary intake) favours muscle growth. Following long-term RE, muscle growth results primarily, if not entirely, from hypertrophy of existing muscle fibres. Hyperplasia does occur in avian models of chronic stretch,[52] but no evidence conclusively documents hyperplasia in human muscle. Following long-term RE training in humans there is: (i) hypertrophy of type I and type II fibres (although type II fibres demonstrate greater hypertrophy); (ii) increased cross-sectional area of the whole muscle group (as measured by magnetic resonance imaging); and (iii) no evidence of muscle fibre hyperplasia.[5]

2. Principles of Resistance Exercise and their Relationship to Muscle Signalling

Adequate study of the cellular and molecular mechanisms of RE-induced muscle growth requires an appropriate RE stimulus. The acute RE programme variables, originally defined by Kraemer in 1983,[53] characterize the type and magnitude of the RE stimulus. These variables are: (i) choice of exercises; (ii) load (i.e. the amount of resistance); (iii) volume (i.e. number of repetitions × number of sets × load); (iv) rest period lengths between sets and exercises; and (v) order of exercises. Although other exercise variables can be modified (e.g. time under tension, the occurrence of volitional muscle failure[54]), the implementation of the acute programme variables primarily determines the responses and adaptations to RE. This section introduces the principles of RE, discussing (and sometimes speculating on) the implications for muscle signalling events.

2.1 Exercise Choice

Exercise choice determines the muscle groups activated and the type(s) of muscle actions used (i.e. concentric, eccentric and/or isometric). Generally, exercises can be classified as either large or small muscle group exercises. Large muscle group exercises (e.g. back squat, dead lifts and Olympic lifts) invoke significantly greater hormonal and metabolic response than small muscle group exercises (e.g. arm curl, bench press and shoulder press).[27,55,56] These findings suggest the amount of muscle mass recruited directly affects the metabolic and hormonal responses to RE. Therefore, increasing the hormonal responses to RE by using large muscle group exercises should provide favourable responses and adaptations to RE (because, for example, hormones increase translational efficiency via mTOR signalling,[31,34] signal satellite cell activity,[36] and promote gains in strength following long-term training[23]).

The type of muscle action also impacts signalling responses and adaptations to RE. Maximal eccentric actions noticeably increase p70 S6K phosphorylation, while maximal concentric and submaximal eccentric actions produce much smaller effects.[57] Additionally, eccentric actions potently stimulate MAPKs.[19] Evidence from animal studies demonstrate that concentric, eccentric and isometric actions induce similar signalling responses when force is equated.[58] However, skeletal muscle can generate ∼30% more tension during eccentric than concentric actions,[57] which explains why maximal eccentric actions evoke a greater signalling response in humans[57] and in animals.[59,60] Studies investigating the role of muscle action for RE adaptations show that RE composed only of isometric contractions fails to counteract muscle atrophy during unloading[61] and omission of eccentric muscle actions impairs strength increases during RE training.[62] Altogether, these studies indicate that, when muscle tension is equated, different muscle actions produce similar responses and adaptations. However, eccentric actions seem to be a more potent stimulus for muscle signalling because greater muscle tension can be developed. Although these findings indicate that eccentric muscle actions must be included to optimize adaptations to RE, disproportionate volumes of supplementary eccentric muscle actions might be counter-productive, as excessive muscle damage might ensue.

2.2 Load

Load represents the amount of weight lifted or the resistance used during exercise. The maximal load that can be used highly depends on other acute programme variables such as exercise volume, exercise order,[63] muscle action[64] and rest interval length.[65] Load is typically prescribed as a percentage of 1RM [e.g. 85% 1RM] or as a weight that allows a specific number of repetitions (e.g. 6RM). In traditional RE there is an inextricable relationship between load and volume (i.e. as the load increases the number of repetitions that can be performed decreases). Therefore, the independent effects of load and volume are rarely controlled for in RE research.

Certain muscular performance characteristics are best trained using specific RM loads. Traditionally, 1–6RM, 8–12RM and 15–25RM loads are recommended to maximize strength, hypertrophy and muscular endurance, respectively. Campos et al.[66] confirmed these suppositions by evaluating adaptations following 8 weeks of progressive resistance training in groups using 3–5RM loads (i.e. low repetitions), 9–11RM loads (i.e. intermediate repetitions) and 20–28RM loads (i.e. high repetitions). The investigation revealed: (i) step-wise increases in strength (low > intermediate > high repetitions); (ii) step-wise increases in local muscular endurance (high > intermediate > low repetitions); (iii) increased muscle fibre cross-sectional area only in the low and intermediate repetition groups; and (iv) increased maximal aerobic power only in the high-repetition group. These results support the notion that different RM load ranges maximize specific muscular adaptations. It is worth noting, however, that exercise volume was not controlled in the study of Campos et al.[66] Therefore, the prescription of RM loads to promote specific muscular adaptations reflects the combined influence of exercise load and exercise volume.

No studies have isolated the independent influence of exercise load on muscle signalling responses and subsequent adaptations to RE. However, use of high-frequency electrical stimulation (HFES, which stimulates high-force contractions) and low-frequency electrical stimulation (LFES, which stimulates low-force contractions)[8,67] provide an appropriate surrogate to study the influence of exercise load on muscle signalling responses. HFES potently stimulates Akt-mTOR signalling, while LFES increases AMPK activity. These results indicate that, as expected, heavy loads promote muscle growth and light loads promote muscle endurance (as AMPK activation promotes endurance adaptations[15]). Importantly, though, exercise volume (number of repetitions) and rest periods between sets and repetitions differed dramatically between the HFES and LFES protocols used in these investigations.[8,67] Therefore, to date, the independent influence of exercise load on muscle signalling is inconclusive.

Exercise load might impact the detection of signalling responses following RE. Humans possess muscle groups of mixed fibre type, yet analysis of muscle signalling proteins typically requires homogenization of the muscle sample. Human muscle homogenate contains an array of type I, type IIa, and type IIx muscle fibres, eliminating the ability to analyse fibre type-specific results. This is relevant because the load employed during RE strongly affects motor unit (and hence, muscle fibre) recruitment. For example, low-load RE protocols might not recruit type II motor units (as suggested by the size principle), unless the exercise is performed explosively and/or there is significant muscle fatigue. Therefore, low-load RE protocols might result in significantly different signalling responses compared with heavy-load RE protocols. This is an important consideration, given that muscle contraction-induced increases in p70 S6K phosphorylation occur mainly in type II muscle fibres.[4]

2.3 Volume

Exercise volume is typically expressed as: volume = sets (number) × repetitions (number) × resistance (weight).[68] Thus, manipulation of training volume can be achieved by altering the number of exercises performed per session, the number of sets performed per exercise, the number of repetitions performed per set, or the resistance used. Changes in training volume influence hormonal,[22] neural[69] and metabolic[70] responses to RE. For example, multiple-set (i.e. high volume) bouts of RE invoke significantly greater GH and testosterone responses than single-set (i.e. low volume) programmes.[65] Moreover, long-term training that employs multiple sets, compared with single sets, is superior for RE adaptations.[71–74]

As with load, it is difficult to describe the independent effects of RE volume because most experiments manipulate load and volume simultaneously. Logically, however, altering RE volume would considerably impact muscle signalling: high-volume RE would deplete muscle glycogen stores, thus stimulating AMPK activity. Subsequently, increased AMPK activity would promote muscle endurance adaptations and inhibit protein synthesis (via inhibition of Akt-mTOR signalling). This theoretical relationship between RE volume, muscle glycogen, and AMPK signalling is supported by research demonstrating that low muscle glycogen potentiates exercise-induced AMPK activity[75,76] and attenuates RE-induced Akt signalling.[77] Paradoxically, though, adequate exercise volume (i.e. number of sets) is necessary for optimal gains in muscle strength and size.[71–74] Therefore, it is difficult to make firm conclusions about the exercise volume-muscle signalling relationship and its ultimate influence on muscle adaptations.

A series of studies by Wong and Booth[78–80] investigated the influences of exercise volume, exercise load, and muscle action on acute protein synthetic responses and chronic gains in muscle size. Rats performed one of four exercise regimes that differed by load and volume twice per week for 16 weeks. During the exercise protocol, the gastrocnemius activated concentrically, while the tibialis anterior activated eccentrically. To examine acute changes in protein synthesis, some animals were sacrificed after the first bout of exercise. This research demonstrated that changes in exercise volume, exercise load, and muscle action differentially affected protein synthesis responses and hypertrophic adaptations (table I). However, differences in rest period lengths and the relative intensity of the exercise load complicate the interpretation of the independent influences of each of these variables. Animals rested significantly longer during the 24-repetition protocol than the 192-repetition protocol, likely altering the cellular signalling response. Additionally, the gastrocnemius and tibialis anterior differ in maximal force-generating capacity (estimated as 1100 g and 300 g, respectively); suggesting that the relative load for the tibialis anterior greatly exceeded that of the gastrocnemius. Explaining and extrapolating these data is difficult because performing 192 near-maximal repetitions of a given muscle group per session, twice per week for 16 weeks is not a commonly used RE protocol, especially in humans. Therefore, direct application of these data is complicated.

Table I
figure Tab1

Summary of findings from research performed by Wong and Booth.[78–80] These studies investigated the effects of muscle action, exercise volume and exercise load on protein synthesis and hypertrophic responses to electrically induced resistance exercise in rats

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2.4 Rest Periods

Rest periods between sets and exercises significantly influence the responses and adaptations to RE. Short rest periods are typically recommended for RE programmes designed to maximize muscle hypertrophy because short rest periods augment the GH response when compared with long rest intervals.[22,43,81] However, short rest periods impair physical performance during subsequent sets[65] and, over several weeks, attenuate strength increases when compared with long rest periods.[3,82] Therefore, short rest periods are not recommended for optimizing gains in muscle strength.[83]

With regards to muscle signalling, rest period length might dramatically affect AMPK activity. In theory, because short rest periods increase the metabolic stress associated with RE,[84] shortened rest periods would enhance AMPK activity and inhibit protein synthesis following acute RE. This would explain why short rest periods attenuate strength gains with long-term training.[3,82] However, this theoretical relationship between short rest periods, AMPK activity and impaired protein synthesis is debatable because: (i) the relative importance of brief, transient increases (∼1–2 hours post-exercise) in AMPK activity for mediating muscle growth following long-term RE remains unclear; and (ii) short rest periods increase GH responses to RE, which would promote muscle hypertrophy. In summary, the direct influence of rest periods on mediating RE-induced muscle signalling responses is largely unexplored. Therefore, it is recommended[83] that short rest periods can be used to stimulate hypertrophy and that long rest periods are used to maximize strength gains.

2.5 Exercise Order

Exercise order (the sequencing of specific exercises within a session) significantly affects exercise performance, force production, and rate of muscle fatigue during a RE session.[85,86] For example, performing the squat exercise last, as compared to first, during a RE session dramatically reduces the number of repetitions that can be performed.[85] Attenuated muscle activation (as measured via EMG[87]) and metabolic fatigue (reduced muscle glycogen and/or phosphocreatine concentrations) likely explain the reduction in performance. The degree to which exercise order alters signalling responses to RE likely depends on the magnitude of change required of the other exercise variables (e.g. reduced load and/or volume, increased rest) to compensate for altered neuromuscular performance.

It is beyond the scope of this article to discuss important details regarding the implementation and progression of the acute RE programme variables for long-term gains in muscle size and strength. For this, we refer readers to the American College of Sports Medicine Position Stand.[83] In general, though, we recommend employing a progressive RE programme emphasizing large muscle group exercises, moderate-to-heavy loads (12RM to 3–12RM), and multiple sets to maximize gains in muscle size. A RE routine such as this would recruit large amounts of muscle tissue, including high-threshold motor units, and stimulate considerable hormonal responses.

Finally, one cannot ignore the importance of dietary intake for modulating RE-induced responses and adaptations. Although RE in a fasted state increases protein synthesis, net protein balance remains negative unless nutrients are provided.[88] In particular, it seems that provision of essential amino acids drives the net increase in muscle protein synthesis following RE.[89] Essential amino acids serve two critical functions for protein synthesis:

  1. 1.

    Essential amino acids are substrates for protein synthesis. mRNA translation cannot progress unless the entire array of amino acids are immediately available, which highlights the importance of ingesting essential amino acids (i.e. those that cannot be synthesised de novo).

  1. 2

    Essential amino acids are signals for protein synthesis. Several recent studies have highlighted the role of essential amino acids as independent signals for translation initiation,[90] and ingesting essential amino acids before and/or after RE potentiates the RE-induced Akt-mTOR response.[91] Dietary carbohydrate ingestion might also be important for promoting responses and adaptations to RE, as low muscle glycogen prevents RE-induced Akt activation.[77] Therefore, although RE is a potent stimulus for muscle protein synthesis and muscle growth, dietary intake critically mediates these responses (see figure 2).

3. Conclusion

RE-induced muscle growth is an intricate, multi-faceted process. Recruitment of motor units to produce force causes mechanical deformation of muscle fibres and stimulates hormonal and immune/inflammatory responses. These ‘upstream’ factors independently (and in some cases, inter-dependently) influence various muscle cell signalling pathways. In particular, the immediate response of the mTOR pathway and the prolonged influence of satellite cell activity are critical for mediating muscle growth. Manipulation of the acute RE programme variables (i.e. exercise choice, load, volume, rest periods and exercise order) dramatically impacts the signalling responses and subsequent adaptations to RE. Of particular importance to the RE practitioner is that specific combinations of RE variables must be utilized to optimize the desired functional outcome (muscle strength, muscle power, muscle size or muscle endurance).

Applying the RE paradigm presented herein will provide a basic forum from which to begin systematic investigations into the mechanisms of RE-induced muscle growth. We hope that future research will isolate the independent influences of each of these exercise variables to determine their specific role in mediating responses and adaptations to RE. Future work should also consider biological markers (e.g. cell signalling proteins, gene expression) as well as functional changes (e.g. strength, size). Such a systematic and holistic approach would greatly expand our understanding of the mechanisms behind RE-induced muscle growth and, more importantly, would improve the implementation of RE interventions to enhance physical health and to offset muscle wasting that occurs with aging and various disease states.