Limitations in intense exercise performance of athletes – effect of speed endurance training on ion handling and fatigue development

Abstract

Mechanisms underlying fatigue development and limitations for performance during intense exercise have been intensively studied during the past couple of decades. Fatigue development may involve several interacting factors and depends on type of exercise undertaken and training level of the individual. Intense exercise (½–6 min) causes major ionic perturbations (Ca²⁺, Cl⁻, H⁺, K⁺, lactate⁻ and Na⁺) that may reduce sarcolemmal excitability, Ca²⁺ release and force production of skeletal muscle. Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise. Regular speed endurance training (SET), i.e. exercise performed at intensities above that corresponding to maximum oxygen consumption (${\dot{V}}_{{\mathrm{O}}_2,\max }$), enhances intense exercise performance. However, most of the studies that have provided mechanistic insight into the beneficial effects of SET have been conducted in untrained and recreationally active individuals, making extrapolation towards athletes’ performance difficult. Nevertheless, recent studies indicate that only a few weeks of SET enhances intense exercise performance in highly trained individuals. In these studies, the enhanced performance was not associated with changes in ${\dot{V}}_{{\mathrm{O}}_2,\max }$ and muscle oxidative capacity, but rather with adaptations in muscle ion handling, including lowered interstitial concentrations of K⁺ during and in recovery from intense exercise, improved lactate⁻–H⁺ transport and H⁺ regulation, and enhanced Ca²⁺ release function. The purpose of this Topical Review is to provide an overview of the effect of SET and to discuss potential mechanisms underlying enhancements in performance induced by SET in already well‐trained individuals with special emphasis on ion handling in skeletal muscle.

Abbreviations

ClC‐1

Cl⁻ channel isoform 1

FXYD1

phospholemman

K_IR2.1

strong inward rectifying K⁺ channel

K_ATP

ATP‐sensitive K⁺ channel

K_IR6.2

inward rectifying channel K⁺ subunit 6.2

MCT1

monocarboxylate cotransporter isoform 1

MCT4

monocarboxylate cotransporter isoform 4

NHE1

Na⁺–H⁺ exchanger isoform 1

NKCC1

Na⁺–K⁺–2Cl⁻ exchanger isoform 1

ROS

reactive oxygen species

RyR1

ryanodine receptor isoform 1

SERCAI

sarcoplasmic reticulum Ca²⁺ ATPase isoform 1

SERCAII

sarcoplasmic reticulum Ca²⁺ ATPase isoform 2

SET

speed endurance training

SR

sarcoplasmic reticulum

${\dot{V}}_{{\mathrm{O}}_2,\max }$

maximum oxygen consumption

Introduction

Training is fundamental to improve exercise performance in the athlete. Optimization of training strategies for performance‐enhancements is thus an important area of research within exercise physiology and sports medicine (Faria et al. 2005; Laursen, 2010; Bishop et al. 2011). In recent years, much research has focused on speed endurance training (SET), with brief intervals (≤ 60 s) performed at intensities above that corresponding to maximum oxygen consumption (${\dot{V}}_{{\mathrm{O}}_2,\max }$) (Ross & Leveritt, 2001; Iaia & Bangsbo, 2010; Gibala & Jones, 2013; Bangsbo, 2015). As presented in Table 1, SET is an efficient way to enhance intense exercise performance (½–6 min) in already well‐trained individuals and athletes, even with marked reductions in weekly training volume. It should be emphasised that in several of the studies presented in Table 1, training was also intensified by introducing or extending aerobic high‐intensity training, which is characterized by interval training with intensities close to those eliciting ${\dot{V}}_{{\mathrm{O}}_2,\max }$.

Table 1.

Adaptations to SET in performance during intense exercise (½–6 min) and skeletal muscle of trained individuals

Study	Subjects (no., M/F, exercise type)	${\dot{V}}_{{\mathrm{O}}_2,\max }$	Intervention (duration, sets/reps, work/rest, frequency, Δ% volume)	Performance measure	Effect (Δ%)	Muscle adaptation	Effect (Δ%)
Houston & Thomson, 1977	5 M runners	59	6 wks of 3×60 s/120 s + 5×6 s/24 s + 2×90 s/180 s (4/wk) + leg press (15 RM)	Intense exercise (∼1 min) 60 s running distance 90 s running distance	+17 +14 +13	ATP content	+15
Nevill et al. 1989	4 M/4F runners	55	8 wks of 2×30 s/10 min + 6–10×6 s/54 s + 2–5×2 min/5 min (3–4/wk)	30 s all out	+6	Rate of anaerobic glycolytic ATP production In vivo buffering capacity	+14 +44
Shepley et al. 1992	9 M runners	67	1 wk of 1–5×∼70 s/6–12 min (5/wk) −91% training volume	Intense exercise (∼5 min)	+22	CS	+18
Dawson et al. 1998	9 M trained	57	6 wks of 3–5×4‐8×10 s/40–60 s (3/wk)	Repeated sprint Intense exercise (∼1 min)	+2 +11	Phosphorylase activity CS activity	+40 −36
Ørtenblad et al. 2000	9 M trained	61	5 wks of 20×10 s/50 s (3/wk)	Repeated sprint	+12	Ca2+ release SERCAI SERCAII RyR1	+9 +41 +55 +48
Bickham et al. 2006	7 M runners	58	6 wks of 4 × 14–30 × 40–100 m (3/wk)	Intense exercise (∼2½ min)	+11	Type II fibres MCT1	+6 +50
Bangsbo et al. 2009	12 M runners	63	6‐9 wks.of 8–12×30 s/180 s (2–3/wk) + 4×4 min (1–2/wk) −30%, training volume	Intense exercise (∼2 min) 30 s all‐out	+36 +5–7	Na+–K+ subunit α2 Na+–K+ subunit β1	+68 (+10)
Iaia et al. 2008, 2009	8/9 M runners	56	4 wks of 8–12×30 s/180 s (3–4/wk) −64% training volume	Intermittent running Intense exercise (∼1½ min) 30 s all‐out	+19 +19–27 +7	Na+–K+ subunit α1 Na+–K+ subunit α2 NKCC1 NHE1	+29 (+16) (+14) +30
Thomassen et al. 2010; Christensen et al. 2011	7 M football	55	2 wks of 10–12×30 s/180 s (2/wk) −30% training volume	Repeated sprint Intermittent running	+2 (+6)	Na+–K+ subunit α2 FXYD1‐phos MCT1 PDH	+15 +27 (+13) +17
Gunnarsson et al. 2012	7 M football	60	5 wks of 5–9×30 s/180 s (1/wk) +11% training volume	Intermittent running	+11	MCT1 Na+–K+ subunit β1	+9 −13
Gunnarsson et al. 2013; Thomassen et al. 2016	8 M cyclists	59	7 wks of 10–12×30 s/270 s (2–3/wk)+ 4–5×3‐4 min (1–2/wk) −70% training volume	Repeated sprint Intense exercise (∼4 min)	+4 +18	In vivo buffering capacity CS COX4 KIR2.1 KIR6.2 NHE1 FXYD1 FXYD1‐phos PLN PLN‐phos CaMKII CaMKII‐phos	+18 −16 −11 (+18) −14 (+30) +30 +40 +16 +42 +25 +300
Puype et al. 2013	9 M trained	53	6 wks of 4–9×30 s/270 s (3/wk)	–		PFK MCT1	+17 +70
Skovgaard et al. 2014	12 M runners	59	8 wks of 4–12×30 s/180 s + resistance training (4/wk) −42% training volume	1500 m running (∼5 min) Intermittent running	+5 +44	NHE1 SERCAI	+35 −15
Nyberg et al. 2016	16 M football	–	9 wks of 2–3×8–10×5 s/10 s (1½/wk)	Intermittent running	+12	HAD COX4 OXPHOS Capillarization	−8 −9 −16 −15
Vorup et al. 2016	9 M runners	60	8 wks of 4–8×30 s/180 s (2/wk) + resistance training/aerobic moderate or high intensity training (2/wk) −58% training volume	400 m running (∼1 min) Intermittent running	+5 +19	Na+–K+ subunit β1 LDH	+15 +17

Relative effect size for performance and muscle changes from each study are only presented for statistical significant measures (P ≤ 0.05) or when trending towards significance (P = 0.05–0.10) in parentheses. Abbreviations: CaMKII, Ca²⁺–calmodulin‐dependent protein kinase II; COX4, cytochrome c oxidase subunit 4; CS, citrate synthase, HAD, hydroxyacyl‐CoA dehydrogenase, K_IR2.1, K⁺ inward rectifier channel 2.1; K_IR6.2, K⁺ inward rectifier channel 6.2; LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; NHE1, Na⁺–H⁺ exchanger isoform 1; NKCC1, Na⁺–K⁺–2Cl⁻ channel isoform 1; OXPHOS, oxidative phosphorylation complex I–V enzymes; PFK, phosphofructokinase; PLN, phospholamban; RyR1, ryanodine receptor 1, SERCA, sarcoplasmic reticulum Ca²⁺ ATPase; ${\dot{V}}_{{\mathrm{O}}_2,\max }$, maximum oxygen consumption (ml kg⁻¹ min⁻¹).

The mechanisms underlying performance enhancements induced by a period of SET are not completely clear. Although several advanced human physiological experiments have been undertaken to investigate the effects of SET, a limitation of the research within this area is that most studies included untrained or recreationally active individuals (Iaia & Bangsbo, 2010; Gibala & Jones, 2013; Weston et al. 2014; Bangsbo, 2015), which makes translation towards athletes’ performance difficult. In untrained and recreationally active individuals, SET augments ${\dot{V}}_{{\mathrm{O}}_2,\max }$ by 7–15% after only few weeks of training despite a low volume of training (Burgomaster et al. 2008; Jacobs et al. 2013). This observation is not surprising, since the heart rate of the individuals during a SET session with 6–10 × 30 s sprints, separated by 90 s of recovery, may reach mean and peaks of around 85 and 95% of maximum heart rate, respectively (Mohr et al. 2007). Furthermore, faster ${\dot{V}}_{{\mathrm{O}}_2}$ kinetics, increased capillarization, and augmented mitochondrial function of skeletal muscle have been observed in untrained and recreationally active individuals following a period of SET (Jensen et al. 2004; Gibala et al. 2006; Jacobs et al. 2013; Christensen et al. 2016). In contrast, SET does not increase ${\dot{V}}_{{\mathrm{O}}_2,\max }$, capillarization or oxidative capacity of skeletal muscle in already trained individuals (Bangsbo et al. 2009; Iaia et al. 2009; Christensen et al. 2015). Actually, SET has been shown to enhance exercise performance in spite of a concurrent reduction in expression of oxidative enzymes and capillarization of skeletal muscle in well‐trained individuals (Dawson et al. 1998; Nyberg et al. 2016). On the other hand, performance enhancements in trained individuals elicited by a period of SET are associated with several adaptations related to ion handling in skeletal muscle. SET may augment K⁺ handling (Bangsbo et al. 2009; Thomassen et al. 2010, 2016), lactate⁻–H⁺ transport capacity (Bickham et al. 2006; Gunnarsson et al. 2012, 2013; Puype et al. 2013), H⁺ regulation (Nevill et al. 1989; Gunnarsson et al. 2013; Skovgaard et al. 2014), and Ca²⁺ handling function (Ørtenblad et al. 2000) of trained individuals. The importance of these adaptations for enhancements in performance during intense exercise (½–6 min) following a period of SET in already trained individuals will be discussed in the present Topical Review.

Development of fatigue during intense exercise

Skeletal muscle fatigue can be defined as a disruption of the force production needed to meet the demand for a given exercise intensity (McKenna et al. 2008). The mechanisms underlying muscle fatigue development are highly debated and complex (Allen et al. 2008; Cairns & Lindinger, 2008; McKenna et al. 2008; MacIntosh & Shahi, 2011). Thus, fatigue development may involve several interacting factors and also depends on exercise modality and inter‐individual differences (Knicker et al. 2011). Intense exercise (½–6 min) causes major metabolic and ionic perturbations (Ca²⁺, Cl⁻, H⁺, K⁺, lactate⁻, Mg²⁺, Na⁺ and P_i) that impair excitation–contraction coupling of skeletal muscle, decrease mechanical efficiency and lead to force decline (i.e. skeletal muscle fatigue) (Cairns & Lindinger, 2008; Cairns et al. 2015; Grassi et al. 2015). Furthermore, accumulation of metabolites and ions in muscle interstitium stimulates sensory feedback from group III/IV muscle afferents to the central nervous system, thereby inducing the sensation of muscle ache associated with intense exercise (Pollak et al. 2014) and providing a regulatory input for motor command centres (Amann et al. 2011). Maintenance of ion homeostasis is thus essential to sustain force production and power output during intense exercise.

Skeletal muscle has several systems that regulate ion homeostasis of which some are presented in Fig. 1. The figure also provides information about the effect of a period of SET on adaptations in ion transporters, exchangers, and channels as well as in metabolic enzymes of trained individuals. The importance of these adaptations for performance during intense exercise will be discussed in the following sections.

Figure 1. Overview of metabolic and ion handling systems in skeletal muscle of potential importance for performance during intense exercise, and their adaptations to a period of speed endurance training (SET).

Blue (continuous) arrows, direction of flux. Green (dashed) arrows, potentiation of a given protein/enzyme. Red (dashed) arrows, inhibition of a given protein/enzyme. Green (continuous) arrows, augmented by a period with SET. Red (continuous) arrows, lowered by a period with SET. Question marks (?), the effect of a period with SET is unknown. ClC‐1, Cl⁻ channel isoform 1; DHPR, dihydropyridine receptor; K_IR2.1, K⁺ inward rectifier channel 2.1; K_IR6.2, K⁺ inward rectifier channel 6.2; LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; NHE1, Na⁺–H⁺ exchanger isoform 1; NKCC1, Na⁺–K⁺–2Cl⁻ exchanger isoform 1; PFK, phosphofructokinase; RyR1, ryanodine receptor 1, SERCA, sarcoplasmic reticulum Ca²⁺ ATPase; SR, sarcoplasmic reticulum; TCA, tricarboxylic acid cycle.

Effect of speed endurance training on muscle K⁺ homeostasis

Intense exercise causes major perturbations in the rundown of transmembrane ion gradients that may reduce sarcolemmal excitability and impair force production of skeletal muscle (Cairns & Lindinger, 2008; McKenna et al. 2008). Specifically, contraction‐induced accumulation of extracellular K⁺ depolarizes sarcolemma, which causes inexcitability and muscle force decline in isolated muscle preparations (Clausen, 2013, 2015; Cairns et al. 2015). During intense exercise in humans, the extracellular concentrations of K⁺ rise from 4 mm to critical levels (> 10 mm), which, based on studies in muscle preparations, contribute to development of muscle fatigue (Cairns et al. 1995, 1997; Cairns & Lindinger, 2008; McKenna et al. 2008). Accordingly, concentrations of K⁺ have been shown to exceed 12 mm in muscle interstitial fluid during intense one‐legged knee‐extensor exercise and cycling when measured with the microdialysis technique (Juel et al. 2000; Nielsen et al. 2003, 2004a; Nordsborg et al. 2003; Gunnarsson et al. 2013). Interstitial concentrations of K⁺ are likely to be much higher than that observed because of the low time resolution, geometric placement of probes (e.g. outside the transverse‐tubular system), and probe‐to‐probe variation associated with the microdialysis technique (Green et al. 2000; Juel et al. 2000; Nielsen et al. 2004a). In support of this hypothesis, extracellular concentrations of K⁺ as high as 50–80 mm have been reported in rat extensor digitorum longus muscle following repeated stimulations, which were associated with pronounced muscle force decline (Clausen, 2008, 2011, 2013). K⁺ handling is therefore of importance for maintenance of sarcolemmal excitability during muscle contractions and any system that counteracts accumulation of extracellular K⁺ could potentially delay fatigue development and enhance performance during intense exercise.

The Na⁺–K⁺ pump counteracts rundown of transmembrane gradients of Na⁺ and K⁺, and is essential for muscle functioning during sustained contractions (Clausen, 2015). Regulation of Na⁺–K⁺ pump activity is multifactorial, with activity being stimulated by muscle contractions, intracellular Na⁺ concentrations, and hormones, such as adrenaline and insulin (Clausen & Flatman, 1977; Clausen & Kohn, 1977; Flatman & Clausen, 1979; Clausen, 2003). Even a few days of training are capable of increasing the abundance of muscle Na⁺–K⁺ pumps by ∼15% in recreationally active individuals (Green et al. 1993), and trained aged individuals have been shown to have a higher Na⁺–K⁺ pump abundance than untrained elderly subjects (Klitgaard & Clausen, 1989). Intensified training has been shown to increase the abundance of muscle Na⁺–K⁺ pumps by ∼10–70% (McKenna et al. 1993; Nielsen et al. 2004a; Bangsbo et al. 2009) when determined by both the [³H]‐ouabain‐binding technique (Madsen et al. 1994; Evertsen et al. 1997) and Western blotting (Iaia et al. 2008; Bangsbo et al. 2009; Thomassen et al. 2010; Vorup et al. 2016). Even in already well‐trained individuals, a period of SET effectively increases expression of the catalytic α subunits and structural β subunits of the Na⁺–K⁺ pump. For instance, in trained runners, SET, with a concomitant reduction in training volume, has been shown to elevate expression of Na⁺–K⁺ pump subunits α₁ (29%), α₂ (15–68%) and β₁ (10–15%) (Iaia et al. 2008; Bangsbo et al. 2009). The increased abundance of muscle Na⁺–K⁺ pumps induced by SET is associated with a lower rate of K⁺ accumulation in arm venous blood during intense exercise and improved performance (Iaia et al. 2008; Bangsbo et al. 2009).

The question is, however, whether adaptations in muscle Na⁺–K⁺ pumps are essential for the improved performance observed following SET in trained individuals, since other studies did not find any change in Na⁺–K⁺ pump subunit abundances despite improved K⁺ handling (Gunnarsson et al. 2013) and performance during intense exercise (Gunnarsson et al. 2012, 2013; Skovgaard et al. 2014). In trained cyclists, Gunnarsson et al. (2013) observed that 7 weeks of intensified training consisting of SET and aerobic high‐intensity training, with a 70% reduction in training volume, increased performance by 18% during intense exercise (∼4 min) and reduced femoral venous plasma K⁺ concentrations by 0.7–0.8 mm in recovery from exercise, without alterations in expression of muscle Na⁺–K⁺ pump subunits. Thus, the lower femoral venous plasma K⁺ concentration in recovery from intense exercise following the intervention period may be related to other factors than changes in muscle Na⁺–K⁺ pump content. In support of this proposition, maximal in vitro Na⁺–K⁺ pump activity of muscle homogenate has been shown to be augmented with no contaminant change in abundance of Na⁺–K⁺ pump subunits in endurance athletes after 3 weeks of aerobic high intensity training (Aughey et al. 2007).

One such factor that regulates Na⁺–K⁺ pump activity and is induced by SET is phospholemman (FXYD1), which interacts with the α and β subunits of the Na⁺–K⁺ pump (Crambert et al. 2002; Thomassen et al. 2010, 2016). Phosphorylation of FXYD1 increases affinity for Na⁺ (lowers K _m) of the Na⁺–K⁺ pump and augments maximal ATPase activity (${\dot{V}}_{\max }$) (Crambert et al. 2002; Bibert et al. 2008; Rasmussen et al. 2008; Walas & Juel, 2012). In the aforementioned study in trained cyclists (Gunnarsson et al. 2013), expression of FXYD1 increased by 30% with the training period (Thomassen et al. 2016). In addition, non‐specific FXYD1 phosphorylation was higher following the training at rest and during repeated intense exercise, mainly because of increased phosphorylation at serine68 (Thomassen et al. 2016), which regulates K _m of the Na⁺–K⁺ pump (Han et al. 2006; Juel et al. 2013). Similarly, 2 weeks of SET was shown to elevate phosphorylation of FXYD1 by 27% in elite football players, which correlated with enhancements in repeated sprint performance (10 × 20 m, 15 s active recovery) (Thomassen et al. 2010). Thus, SET‐induced increases in expression and phosphorylation of FXYD1 may accentuate Na⁺–K⁺ pump activity and explain the lower femoral venous concentrations of K⁺ in recovery from intense exercise observed by Gunnarsson et al. (2013).

Aside from Na⁺–K⁺ pumps, skeletal muscle expresses Na⁺–K⁺–2Cl⁻ exchangers (NKCC1), strong inward rectifying K⁺ channels (K_IR2.1), and ATP‐sensitive K⁺ (K_ATP) channels, all of which are involved in K⁺ homeostasis of skeletal muscle and may be affected by SET (Spruce et al. 1985; Barrett‐Jolley et al. 1999; Nielsen et al. 2003; Kristensen & Juel, 2010). However, only a few studies have investigated the effect of SET on muscle expression of NKCC1, K_IR2.1 and K_ATP channels in well‐trained individuals. In trained runners, 4 weeks of SET was shown to increase expression of NKCC1 by 14% (Iaia et al. 2008). A higher expression of NKCC1 induced by SET may benefit ion homeostasis during intense exercise in two ways. Firstly, NKCC1 regulates myocellular volume via influx of K⁺ and Na⁺ with Cl⁻ and water (Gosmanov et al. 2003, 2004; Cairns et al. 2015), thereby counteracting the osmotic changes associated with ionic perturbations during exercise. Secondly, NKCC1‐mediated influx of Na⁺ stimulates Na⁺–K⁺ pump activity and thereby further re‐uptake of K⁺ (Clausen & Nielsen, 2007). In addition, 7 weeks of SET have been shown to increase expression of K_IR2.1 by 18% (P = 0.06) and to reduce expression of the K_ATP channel subunit K_IR6.2 by 14% in trained cyclists (Gunnarsson et al. 2013). A higher expression of K_IR2.1 may augment K⁺ re‐uptake when the electrochemical gradient for K⁺ is reduced because of interstitial K⁺ accumulation (Wallinga et al. 1999; Kristensen et al. 2006; Kristensen & Juel, 2010), whereas a lower density of K_ATP channel K_IR6.2 potentially reduces K⁺ extrusion to the interstitium during intense exercise, since critically low myocellular ATP levels cause opening of K_IR6.2 (Davies, 1990; Davies et al. 1992, Xu et al. 2001). The concurrent increase in expression of K_IR2.1 and reduction in expression of K_IR6.2 may therefore also explain the lower femoral venous plasma K⁺ observed in recovery from intense exercise and the improved performance observed by Gunnarsson et al. (2013). Nevertheless, the role of NKCC1, K_IR2.1 and K_ATP channels in K⁺ handling and performance during intense exercise is not completely clear. Studies using muscle preparations indicate the importance of these channels, since pharmacological manipulation of NKCC1, K_IR2.1 and K_ATP channels affects fatigue development (Gong et al. 2003; Kristensen et al. 2006; Selvin & Renaud, 2015). And although muscle fatigue processes in non‐functioning K_IR6.2 mice models are uncertain (Boudreault et al. 2010; Bruton, 2010), K_ATP channels are highly expressed in the transverse‐tubular system (Nielsen et al. 2003) and have been shown to serve a myoprotective role against high cytosolic Ca²⁺ fluctuations and fibre damage during intense muscle activity (Selvin & Renaud, 2015).

As outlined above, SET elicits several adaptations related to K⁺ handling of skeletal muscle in already trained individuals. SET may augment abundance and regulation of Na⁺–K⁺ pumps, increase expression of NKCC1 and K_IR2.1, and reduce expression of K_IR6.2. These adaptations are associated with lower concentrations of interstitial and femoral venous K⁺ during exercise and a more rapid decline in recovery from exercise (Nielsen et al. 2004a; Gunnarson et al. 2013). Given that the concentrations of interstitial and femoral venous K⁺ reach same levels at exhaustion after compared to before a period of SET, the adaptations related to K⁺ handling induced by SET may, at least in part, explain the improved performance during intense exercise in already trained individuals.

Effect of speed endurance training on accumulation of lactate⁻ and H⁺

A major contributor to muscular energy production during intense exercise (½–6 min) is glycolysis, leading to production of lactate⁻ and accumulation of H⁺ (Hultman & Sahlin, 1980; Cheetham et al. 1986; Nevill et al. 1989; Gaitanos et al. 1993; Juel et al. 2004). Estimates of energy production derived from glycolysis range from 60–70% during 30 s of maximal cycling (Bogdanis et al. 1995; Kalsen et al. 2016) to 30–40% during 2–5 min of intense knee extensor exercise (Bangsbo et al. 1990, 1992). Thus, muscle lactate⁻ may accumulate from ∼1 to > 25 mmol (kg ww)⁻¹ and pH may decline from ∼7.2 to < 6.6 during intense exhaustive exercise lasting a few minutes (Juel et al. 1990; Bangsbo et al. 1993; Hostrup et al. 2014). Skeletal muscle has several transport systems that facilitate efflux of lactate⁻ and H⁺ from myocellular compartments to the interstitium (Juel, 1998a; Juel & Halestrap, 1999; Pilegaard et al. 1999b). The main muscle transporters of lactate⁻ and H⁺ during exercise are the lactate⁻–H⁺ monocarboxylate cotransporters (MCT) of which the predominant isoforms 1 (MCT1) and 4 (MCT4) account for 70–80% of the lactate⁻ and H⁺ efflux (Mason & Thomas, 1988; Juel, 1997; Pilegaard et al. 1999b; Thomas et al. 2012).

While several studies have investigated the effect of SET on muscle lactate⁻ and H⁺ transport capacity and regulation in humans (Thomas et al. 2012), only few have been conducted in trained individuals. Unlike the marked improvements SET exerts in lactate⁻–H⁺ transport capacity of skeletal muscle in untrained and recreationally active individuals (Pilegaard et al. 1999a; Juel et al. 2004; Burgomaster et al. 2007), the adaptive response to SET in trained individuals is moderate and sometimes not evident. A few weeks of SET with reduced training volume have been shown to increase expression of MCT1 by ∼10% in elite football players and by up to ∼70% in trained men (Bickham et al. 2006; Thomassen et al. 2010; Gunnarsson et al. 2012; Puype et al. 2013), whereas no changes were observed in trained runners and cyclists (Bangsbo et al. 2009; Gunnarsson et al. 2013; Vorup et al. 2016). Unlike MCT1, expression of MCT4 does not seem to change with a period of SET in trained individuals (Bickham et al. 2006; Thomassen et al. 2010; Gunnarsson et al. 2013; Puybe et al. 2013; Skovgaard et al. 2014). However, it may be that alterations in MCT4 only occur in sarcolemma, since Dubouchaud et al. (2000) observed that endurance training increased expression of MCT4 in sarcolemmal fractions, but not in whole‐muscle homogenate.

Apart from MCT, skeletal muscle has several systems that regulate myocellular H⁺, including Na⁺–H⁺ exchangers (NHE1), carbonic anhydrase enzymes, HCO₃ ⁻ transporters, and H⁺ buffers (Sahlin & Henriksson, 1984; Geers & Gros, 2000; Messonnier et al. 2007). While data on the effect of SET on carbonic anhydrase enzymes and HCO₃ ⁻ transporters are scant, studies have shown that SET increases content of NHE1 (Iaia et al. 2008; Skovgaard et al. 2014) and H⁺ buffering capacity of skeletal muscle (Parkhouse & McKenzie, 1984; Bell & Wenger, 1988; Gibala et al. 2006; Baguet et al. 2011). NHE1 was shown to increase by 30–35% during a period of intensified training with reduced volume when SET was performed alone or in combination with resistance training in trained runners (Iaia et al. 2008; Skovgaard et al. 2014). Given that a higher expression of NHE1 may increase Na⁺ uptake and H⁺ efflux, it could be speculated that such adaptation may counteract contraction‐induced accumulation of interstitial K⁺ and sarcolemmal depolarization during intense exercise, since intracellular Na⁺ stimulates Na⁺–K⁺ pump activity (Nielsen et al. 2004b) and H⁺ accumulation causes opening of K_ATP channels and hence extrusion of K⁺ (Davies, 1990; Davies et al. 1992, Xu et al. 2001; Street et al. 2005). While most studies found no change in in vitro muscle H⁺ buffering capacity of muscle homogenate in trained individuals during a period of SET (Nevill et al. 1989; Iaia et al. 2008), in vivo muscle buffering capacity has been shown to be increased in some studies (Nevill et al. 1989; Gunnarsson et al. 2013). Thus, Nevill et al. (1989) observed no change in buffering capacity of muscle homogenate in vitro, whereas the estimated in vivo muscle buffering capacity from H⁺ and lactate⁻ during 30 s of maximal cycling was 44% higher after 8 weeks of SET in male and female runners. In line with this, Gunnarsson et al. (2013) observed that in vivo muscle buffering capacity estimated during intense exercise (∼4 min) increased by ∼18% with 8 weeks of combined SET and aerobic high‐intensity training in trained cyclists. The difference between the in vitro and in vivo measurements observed may be due to a relative greater increase in the capacity of the trained muscle to release H⁺ than lactate (Juel et al. 1998a,b), e.g. due to an increased expression of NHE1 (Gunnarsson et al. 2013), which per se will increase the estimated in vivo buffer capacity but not the in vitro equivalent.

The implications of adaptations in muscle lactate⁻–H⁺ transport and H⁺ regulation for performance during intense exercise following a period of SET in trained individuals are not completely clear. While SET increases rate of H⁺ release during intense exercise and lactate⁻–H⁺ clearance in the first few minutes of recovery from exercise, lactate⁻ and H⁺ formation greatly exceed muscle transport capacity during intense exercise (Bangsbo et al. 1990, 1993; Juel et al. 1990, 2004). To what extent accumulation of lactate⁻ and H⁺ contribute to fatigue development is also controversial (Korzeniewski & Zoladz, 2002; Pedersen et al. 2004; Bangsbo & Juel, 2006; Fitts, 2006; Lamb & Stephenson, 2006; Lindinger, 2007). Studies in muscle preparations have challenged the importance of lactate⁻ and H⁺ as muscle fatigue inducers (de Paoli et al. 2007), since low pH has little detrimental effect on fatigue development at physiological relevant temperatures, and lactate⁻ has been shown to restore the depressing effect of raised extracellular K⁺ on sarcolemmal excitability because of its inhibitory effect on Cl⁻ channels (ClC‐1) (Nielsen et al. 2001; Pedersen et al. 2004; Hansen et al. 2005, 2005; de Paoli et al. 2010). In simulation models, Korzeniewski & Zoladz (2002) also demonstrated that H⁺ accumulation may counteract disturbances in energy system integrity, especially oxidative phosphorylation in working muscles, in that glycolytic production of H⁺ compensates the cytosolic alkalization caused by H⁺ consumption of creatine kinase at the onset of exercise. Nevertheless, although these studies in muscle preparations in vitro and in simulation models imply that lactate⁻ and H⁺ may even act protectively against fatigue (Allen & Westerblad, 2004), studies in humans indicate that accumulation of lactate⁻ and H⁺ may be detrimental for performance during intense exercise. For instance, accumulation of lactate⁻ induced by intense arm ergometer exercise was associated with greater accumulation of myocellular H⁺ (pH: 6.65 vs. 6.82), reduced lactate⁻ release (19 vs. 50 mmol), and a ∼30% reduction in performance of the quadriceps during subsequent intense knee‐extensor exercise (Bangsbo et al. 1996; Nordsborg et al. 2003). Notably, preceding arm ergometer exercise also caused a higher rate of extracellular K⁺ accumulation during the subsequent knee‐extensor exercise (Bangsbo et al. 1996; Nordsborg et al. 2003). The latter observation may be related to the greater myocellular accumulation H⁺, thus causing opening of K_ATP channels (Davies, 1990; Davies et al. 1992, Xu et al. 2001; Street et al. 2005). In addition, lactate⁻–H⁺ transport and buffering capacity of skeletal muscle are good predictors for intense exercise performance (Nevill et al. 1989; Pilegaard et al. 1994; Bishop et al. 2003, 2004; Thomas et al. 2005; Messonnier et al. 2007), and athletes have higher lactate⁻–H⁺ transport and buffering capacity of skeletal muscle than trained and untrained individuals (McKenzie et al. 1982; Sahlin & Henriksson, 1984; Pilegaard et al. 1994; Edge et al. 2006). Lastly, β‐alanine and sodium bicarbonate supplementation enhance intense exercise performance in trained individuals (Carr et al. 2011; Hobson et al. 2012). These effects are respectively related to increased muscle content of the H⁺ buffer carnosine (Hill et al. 2007) and systemic HCO³⁻ concentrations (Sostaric et al. 2006). Augmented muscle carnosine content may also increase Ca²⁺ sensitivity and affect cross‐bridge formation because of carnosine's potential role as a diffusible Ca²⁺–H⁺ exchanger (Dutka et al. 2012; Swietach et al. 2013), whereas increased systemic HCO³⁻ concentrations have been shown to reduce the rate of accumulation of extracellular H⁺ and K⁺ (Street et al. 2005; Sostaric et al. 2006). Collectively, these observations underpin the importance of improved muscle lactate⁻–H⁺ transport and H⁺ regulation induced by a period of SET for performance during intense exercise in already trained individuals.

Effect of speed endurance training on sarcoplasmic reticulum Ca²⁺ handling and myofibrillar Ca²⁺ sensitivity

Skeletal muscle force production and relaxation are regulated by myoplasmic concentrations of Ca²⁺ and myofibrillar Ca²⁺ sensitivity. Force production is controlled by Ca²⁺ release via opening of sarcoplasmic reticulum (SR) ryanodine receptors (RyR1) and relaxation by subsequent re‐sequestration of Ca²⁺ from the myoplasm into the SR, which predominantly is controlled by the Ca²⁺ uptake rate of SR Ca²⁺‐ATPase isoforms SERCAI and SERCAII, but also by the dissociation off‐kinetics of troponin C, and the Ca²⁺ loading rate of Ca²⁺‐binding proteins (Westerblad & Allen, 1996a,b; Nogueira et al. 2013). Based on studies in muscle preparations, impairment of Ca²⁺ release in particular appears to contribute to muscle fatigue development (Allen et al. 2008, 2011; Nogueira et al. 2013). However, while the importance of Ca²⁺ handling for fatigue resistance is well described in muscle preparations, surprisingly few studies have investigated the effect of training on SR Ca²⁺ handling function and myofibrillar Ca²⁺ sensitivity in humans, and only two studies have, to our knowledge, been undertaken with SET in trained individuals (Ørtenblad et al. 2000; Thomassen et al. 2016).

Ørtenblad et al. (2000) observed that 5 weeks of SET augmented maximal AgNO₃ stimulated Ca²⁺ release in SR vesicles of muscle homogenate by 9%, which was associated with a 12% better mean power output during a repeated sprint test (10 × 8 s, 32 s recovery) in highly trained men. The adaptations in Ca²⁺ release function were presumably related to a training‐induced increase in total SR‐volume, since expression of RyR1, SERCAI and SERCAII was increased by SET, whereas [H³]‐ryanodine‐specific binding remained unchanged (Ørtenblad et al. 2000). In spite of the increased expression of SERCAI and II, however, the authors observed no changes in SR vesicle Ca²⁺ uptake kinetics or Ca²⁺‐ATPase activity of muscle homogenate with the intervention. Because these measures were performed in in vitro preparations, extrapolation to in vivo Ca²⁺ release and uptake function during exercise is speculative. However, reduction in the in vitro SR Ca²⁺ release function of muscle homogenate has been shown to predict a decline in muscle torque after intense knee‐extensor exercise of untrained individuals (Hill et al. 2001). In addition, 7 weeks of SET, combined with aerobic high‐intensity training, was shown to increase total expression and level of phosphorylation of the SERCAII‐regulatory protein phospholamban and Ca²⁺–calmodulin‐dependent protein kinase II (CaMKII) in cyclists (Thomassen et al. 2016). A higher expression and phosphorylation of phospholamban and CaMKII may increase the rate of SR Ca²⁺ release and re‐uptake because of the reduced affinity for Ca²⁺ of SERCAII and increased opening probability of RyR1 (Suko et al. 1993; Rose et al. 2006; Kemi et al. 2007). Such effects might potentially improve SR Ca²⁺ handling during intense exercise. For myofibrillar Ca²⁺ sensitivity, currently no data are available regarding the effect of SET in trained individuals. In untrained individuals, Lynch et al. (1994) observed that sprint training lowered sensitivity for Sr²⁺, while sensitivity for Ca²⁺ remained unaltered at normal pH but was lower at pH 6.6, which reflects the pH during intense exercise.

The limited number of studies that have investigated the effects of SET on Ca²⁺ handling makes interpretation of its implications difficult. Cross‐sectional data have shown that endurance‐trained individuals have lower in vitro Ca²⁺ handling function than resistance‐trained and untrained individuals (Li et al. 2002). In line with this, aerobic training has been shown to reduce expression of SERCA and to lower in vitro Ca²⁺ release and uptake function of skeletal muscle (Majerczak et al. 2008; Green et al. 2011; Zoladz et al. 2013). Although these observations may imply that Ca²⁺ handling function is not the main determinant factor for endurance performance, studies have shown that Ca²⁺ uptake function may predict some of the variance during intense exercise (Harmer et al. 2014). Furthermore, 6 weeks of knee‐extensor training was shown to increase rate of Ca²⁺ release and reduce Ca²⁺ leak from the SR, which were associated with enhanced peak power in untrained men (Munkvik et al. 2010). Nonetheless, while potential enhancing effects of SET in SR Ca²⁺ release function (Ørtenblad et al. 2000) as well as in expression and phosphorylation of phospholamban (Thomassen et al. 2016) may augment Ca²⁺ handling during intense exercise in trained individuals, more studies are needed on this issue.

Influence of speed endurance training on energy metabolism and reactive oxygen species: interaction with ion handling

Myocellular energy metabolism is important for excitation–contraction coupling during intense exercise. Na⁺–K⁺ pumps and SERCA account for ∼40% of energy expenditure of skeletal muscle during repeated contractions (Clausen, 2003; Walsh et al. 2006; Barclay et al. 2007) and any local limitation in energy availability and/or accumulation of metabolites impairs ion handling and accelerates fatigue development (Matar et al. 2000; Gong et al. 2003; Nogueira et al. 2013; Ørtenblad et al. 2013; Place et al. 2015; Selvin & Renaud, 2015). Low levels of ATP with a concomitant rise in free Mg²⁺, H⁺, ADP, IMP and P_i activates K_ATP channels (Allard et al. 1995; Barrett‐Jolley & Davies, 1997; Pedersen et al. 2009), causes Ca²⁺–P_i precipitation inside the SR (Allen et al. 2011), impairs Ca²⁺ handling (Li et al. 2002; Allen et al. 2008; Nogueira et al. 2013), and reduces Na⁺–K⁺ pump activity (Juel et al. 2014). Furthermore, anaerobic energy production through glycolysis leads to lactate⁻ production and the accompanying H⁺ accumulation changes SERCA activity and K_ATP channel sensitivity for ATP (Davies, 1990, 1992, Westerblad & Allen, 1993, Xu et al. 2001; Flagg et al. 2010; Nogueira et al. 2013). Anaerobic enzymes lactate dehydrogenase and creatine kinase may also interact with K_ATP channels (Crawford et al. 2002a,b), and Na⁺–K⁺ pump activity and SR Ca²⁺ handling are regulated by glycogenolysis and glycolysis (James et al. 1999; Okamoto et al. 2001; Dutka & Lamb, 2007; Nogueira et al. 2013; Ørtenblad et al. 2013). As such, a higher capacity for glycolytic ATP provision following a period of SET may potentially improve regulation of ion homeostasis during intense exercise.

SET has been shown to accentuate glycolytic enzymes (Dawson et al. 1998; Iaia et al. 2008; Puype et al. 2013: Vorup et al. 2016), increase glycogen content, and augment capacity for glycolytic energy production of skeletal muscle in trained individuals (Nevill et al. 1989; Shepley et al. 1992). Furthermore, given that CaMKII regulates the rate of glycogenolysis and glycolysis (Rose et al. 2006), a higher expression and phosphorylation of CaMKII induced by SET may accelerate the rate of glycolytic ATP provision during intense exercise (Gunnarsson et al. 2013; Thomassen et al. 2016). Notably, Iaia et al. (2008) observed that glycogen and phosphocreatine utilization during a second bout of intense exercise to exhaustion (∼1½ min) were lower after a period of SET, despite of higher accumulation of blood lactate⁻ and longer time to exhaustion. This observation suggests that muscle oxidation and/or mechanical efficiency was improved by SET. Such adaptations may enhance metabolic stability and delay the impairments in contraction–excitation contraction coupling of skeletal muscle during intense exercise (MacIntosh & Shahi, 2011; Grassi et al. 2015). Furthermore, any potential glycogen‐sparing effect of a period of SET might potentially counteract exercise‐induced reductions in SR Ca²⁺ handling, since critically low glycogen levels in proximity to the SR have been shown to be associated with impaired Ca²⁺ handling function in highly trained individuals (Ørtenblad et al. 2011; Hostrup et al. 2014; Nielsen et al. 2014).

Intense exercise leads to formation of reactive oxygen species (ROS) and glutathione in skeletal muscle that reacts with thiol groups (glutathionylation) of various myocellular proteins of importance for ion handling (Juel et al. 2015; Cheng et al. 2016). Among these proteins, the Na⁺–K⁺ pump is subjected to glutathionylation in human skeletal muscle during intense exercise, which has been shown to attenuate maximal in vitro Na⁺–K⁺ pump activity by ∼25% in highly trained individuals (Hostrup et al. 2014; Juel et al. 2015); a phenomenon known as Na⁺–K⁺ pump inactivation (McKenna et al. 2008). Furthermore, ROS‐production reduces SR Ca²⁺ release function and myofibrillar Ca²⁺ sensitivity (Place et al. 2015; Cheng et al. 2016). Interestingly, in recreationally active individuals, muscle Ca²⁺ release channel RyR1 is subjected to fragmentation following one session of SET, whereas endurance athletes have no fragmentation (Place et al. 2015). This observation indicates that training augments muscle oxidant defence and protects the integrity of proteins related to ion handling during intense exercise. In support of these observations, a few weeks of SET has been shown to increase activity of antioxidant enzymes glutathione peroxidase and glutathione reductase in recreationally active individuals (Hellsten et al. 1996; Bogdanis et al. 2013). In addition, Green et al. (1998) observed that 7 and 11 weeks of high‐intensity resistance training attenuated reductions in Ca²⁺‐ATPase activity induced by intense cycling. While it could be speculated that SET would have similar adaptive effects in already trained individuals, future studies are warranted to elucidate whether SET augments muscle oxidant defence and counteracts glutathionylation of the Na⁺–K⁺ pump and fragmentation of RyR1 in the trained population.

Conclusion and perspectives

As presented in this review, only a few weeks of SET leads to significant performance enhancements during intense exercise in already trained individuals. Given that a period of SET has no apparent effects on ${\dot{V}}_{{\mathrm{O}}_2,\max }$, haemoglobin mass, plasma volume, muscle mass, capillarization or oxidative capacity in trained individuals, the enhancements in performance during intense exercise induced by SET may be related to muscle adaptations in ion handling, including improved regulation of K⁺ homeostasis, enhanced lactate⁻–H⁺ transport, and in some instances augmented H⁺ buffering capacity and SR Ca²⁺ handling. The implications of these adaptations with relevance for performance during intense exercise may involve:

• Lower rate of accumulation of interstitial K⁺ of exercising muscles due to increased Na⁺–K⁺ pump activity and repression of K_ATP channels.

• Reduced extrusion of K⁺ from K_ATP channels because of augmented lactate⁻–H⁺ transport and H⁺ regulation.

• Augmented Ca²⁺ handling.

• Higher capacity for glycolytic ATP provision.

Because of the marked perturbations of multiple ions (Cairns & Lindinger, 2008; McKenna et al. 2008) and their interactions with energy production and ROS formation during muscle activity (MacIntosh & Shahi, 2011; Place et al. 2015; Cheng et al. 2016), fatigue processes during intense exercise in trained individuals are complex, and further studies are needed to elucidate the exact mechanisms underlying the enhancing effects of SET on performance. For instance, recent observations in muscle preparations indicate that extracellular Ca²⁺ interacts with lowered transsarcolemmal K⁺ gradients during muscle contractions, where a reduction in extracellular Ca²⁺ lowers the intracellular activity of K⁺ and contributes to fatigue development (Cairns et al. 2015). In this perspective, studies are warranted that investigate the importance of muscle Ca²⁺–H⁺ and Na⁺–Ca²⁺ exchangers, as well as ClC‐1 channels for exercise performance in humans and their potential adaptability to training. For example, ClC‐1 channels are of interest because of their role in regulating Cl⁻ conductance and sarcolemmal excitability during muscle activity (Dutka et al. 2008; de Paoli et al. 2010, 2013). Furthermore, improvements related to ion handling, lactate⁻ transport, and H⁺ regulation after a period of SET could possibly affect regulation of myocellular and plasma osmolarity during exercise, and the implication of such changes for performance should be studied (Lindinger et al. 1995, 2013). Despite the difficulty in recruiting athletes for advanced physiological training interventions, more studies are needed in the athlete population. Lastly, it is relevant to probe the long‐term effects of SET, since the intervention period of most studies that have provided an emphasis for this training modality only lasted a few weeks.

Additional information

Competing interests

None declared.

Biographies

Morten Hostrup, Ph.D., is Assistant Professor in the Department of Nutrition, Exercise and Sports (NEXS), University of Copenhagen and the Department of Respiratory Research at Bispebjerg Hospital in Copenhagen, Denmark. He studies fatigue processes and limitations in exercise performance of humans using various interventions, including training, nutrition and pharmacology.

Jens Bangsbo, Ph.D., is Professor and Deputy Head at NEXS, University of Copenhagen, Denmark, and leader of the Integrative Physiology Group at NEXS. His research focuses on optimizing exercise performance, including how development of muscle fatigue is affected by high‐intensity training.