There is currently insufficient evidence of molecular interference occurring in human skeletal muscle following a single bout of concurrent exercise to explain the attenuated hypertrophy and strength response following concurrent training. However, it is unclear whether this is a consequence of a lack of the proposed mechanisms operating in human skeletal muscle, and/or the limitations of existing evidence, which are briefly discussed below.
2.8.2.1 Relationship between molecular responses to single bouts of exercise and chronic training adaptations
Most molecular concurrent training studies provide a brief ‘snapshot’ of the adaptive events occurring in close proximity to single bouts of concurrent exercise (Apro et al., 2015; Apro et al., 2013; Carrithers et al., 2007; Coffey et al., 2009a; Coffey et al., 2009b; Donges et al., 2012; Fernandez-Gonzalo et al., 2013; Lundberg et al., 2012; Lundberg et al., 2014b; Pugh et al., 2015; Wang et al., 2011). Many questions remain, however, with regards to the long-term molecular regulation of skeletal muscle adaptations and interference with concurrent training. Firstly, there is limited data indicating a direct coupling between molecular responses to single bouts of exercise and the long-term phenotypic adaptations associated with chronic exercise training (Hawley, 2009; Mitchell et al., 2014). It is therefore unclear whether skeletal muscle molecular responses in the hours following single bouts of concurrent exercise provide a valid indication of the adaptive phenotype, and potential interference, which might be induced if training was repeated long-term. Indeed, the efficacy of the molecular markers commonly used to gauge the anabolic response to exercise and nutritional stimuli has been questioned (Atherton et al., 2010; Phillips et al., 2013), while even rates of MPS following a single bout of RE do not correlate with muscle hypertrophy after 16 weeks of RT (Mitchell et al., 2014). Only four human studies have directly measured protein synthesis rates following single bouts of concurrent exercise (Apro et al., 2015; Camera et al., 2015; Carrithers et al., 2007; Donges et al., 2012), while others have instead utilised proxy markers of MPS and/or MPB (Apro et al., 2013; Coffey et al., 2009a; Coffey et al., 2009b; Fernandez-Gonzalo et al., 2013; Lundberg et al., 2012; Pugh et al., 2015; Wang et al., 2011). Importantly however, a direct coupling between mTORC1 signalling and protein synthesis rates does not always exist in humans
(Atherton et al., 2010), and rates of protein synthesis can be saturated at approximately 30% of the maximal phosphorylation of p70S6K1 in rodents (Crozier et al., 2005). Given these apparent discordances, any minor interference to anabolic signalling responses following single bouts of concurrent exercise may not reflect any potential interference to protein synthesis, and subsequently chronic muscle hypertrophy. Studies extrapolating the anabolic response and potential interference effect from early signalling responses alone must therefore be interpreted with caution. Nevertheless, p70S6K1 phosphorylation following a single bout of RE correlates well (r = 0.82 to 0.99) with chronic hypertrophy in both rodents (Baar & Esser, 1999) and humans (Mayhew et al., 2011; Terzis et al., 2008), supporting this as a proxy marker for chronic hypertrophy and potentially interference with concurrent training. Regardless of the methods used to gauge the post-exercise anabolic response, most concurrent exercise studies have been characterised by limited post-exercise time courses. For example, most existing studies have examined early molecular responses up to 6 h post-exercise, whereas mTORC1 signalling can be sustained for up to 24 h post-exercise (Deldicque et al., 2008; Drummond et al., 2011). These studies may therefore have overlooked any potential effects of concurrent exercise on mTORC1 signalling occurring later than 6 h post-exercise. Further work employing extended post-exercise time-courses is required to determine whether mTORC1 signalling is altered by concurrent exercise during the later recovery period.
2.8.2.2 Effect of training status on early molecular responses to exercise
In addition to the exercise modality, the adaptive state of skeletal muscle appears to impact upon molecular responses to single bouts of exercise (Benziane et al., 2008; Coffey et al., 2006a; Coffey et al., 2006b; McConell et al., 2005; Yu et al., 2003). Both short periods of exercise training (Benziane et al., 2008; McConell et al., 2005; Wilkinson et al., 2008), and years of training in a single exercise modality (Coffey et al., 2006a; Coffey et al., 2006b; Yu et al., 2003), modulate early molecular responses to single bouts of exercise. Merely ten days of endurance training can attenuate AMPK responses to prolonged and sub-maximal intermittent endurance exercise completed at the same absolute pre-training workload (Benziane et al., 2008; McConell et al., 2005). Moreover, while there are little divergences in early mTORC1 signalling responses to single bouts of resistance and endurance exercise in relatively untrained subjects
(Camera et al., 2010; Vissing et al., 2011; Wilkinson et al., 2008), the mTORC1 pathway can be preferentially induced by resistance, but not endurance exercise, in training-accustomed individuals (Vissing et al., 2011). Additionally, while RE performed in the untrained state elicits comparable increases in rates of both myofibrillar and mitochondrial protein synthesis, these responses become more refined following training, whereby only myofibrillar protein synthesis rates are increased in response to RE (Wilkinson et al., 2008). Data from highly strength- or endurance- trained athletes (Coffey et al., 2006a; Coffey et al., 2006b) also supports the notion the skeletal muscle phenotype, rather than the mode of exercise per se, can influence the molecular responses to divergent exercise modes. For example, highly-trained endurance cyclists lacked the ability to induce AMPK activation in skeletal muscle following continuous sub-maximal exercise in their habitual discipline (i.e., endurance exercise), while AMPK was activated after exercise in their non-habitual discipline (i.e., RE) (Coffey et al., 2006b). Similarly, an identical endurance training bout performed at the same relative workload was sufficient to activate AMPK in highly-trained powerlifters exhibiting extreme resistance-trained phenotypes (Coffey et al., 2006b). These observations suggest the novelty of the exercise stimulus and associated stressors, not merely the exercise stimulus per se, influences early molecular responses to exercise. It therefore appears likely long-term concurrent training would modulate early post-exercise exercise responses (and potentially any interference effect) in skeletal muscle over time, whereby the early molecular profile to unaccustomed exercise bouts may represent a generalised, unrefined adaptive response (Mahoney & Tarnopolsky, 2005; Wilkinson et al., 2008). In this case, evidence regarding molecular responses to single bouts of concurrent exercise in relatively untrained subjects should be interpreted with caution. While most molecular concurrent training studies have utilised participants who were recreationally undertaking both resistance and endurance training (Apro et al., 2015; Apro et al., 2013; Carrithers et al., 2007; Coffey et al., 2009a; Coffey et al., 2009b; Lundberg et al., 2012), some have used sedentary participants (Donges et al., 2012; Pugh et al., 2015; Wang et al., 2011) or participants not accustomed to both exercise modalities (Lundberg et al., 2012). Observations that early concurrent exercise bouts promote cumulative effects on protein synthesis and/or mitochondrial biogenesis signalling (Donges et al., 2012; Lundberg et al., 2012; Wang et al., 2011) may therefore be more reflective of the unfamiliarity to the exercise bout (Atherton & Smith, 2012) rather than suggesting enhanced potential for chronic adaptation. Moreover, often
overlooked is that the original concurrent training study by Hickson (1980) showed no detectable interference effect until the eighth week of training, suggesting any interference effect may not manifest until a certain training status is attained. Taken together, these results suggest participant training status is an independent influence on exercise-induced molecular responses in skeletal muscle, and potentially the interference effect, and must be taken into consideration when interpreting existing concurrent training evidence. Future work examining the existence of molecular interference should employ participants who are accustomed to both exercise modes to account for the potentially confounding effects of training status on early post-exercise molecular responses to exercise (Coffey et al., 2006a; Coffey et al., 2006b). Moreover, this further exemplifies the need for longer-term (>8 weeks) training studies examining the potential modulation of interference following periods of concurrent training.
2.8.2.3 Effect of nutrient availability on early molecular responses to exercise
It has become increasingly clear nutrient availability exerts a profound effect on the adaptive responses to exercise training in human skeletal muscle (Beelen et al., 2010; Hawley et al., 2011). For example, the availability of muscle glycogen has been reported to modulate early molecular responses to both endurance and RE in a divergent manner (Cochran et al., 2010; Creer et al., 2005; Yeo et al., 2010). Low carbohydrate availability in close proximity to endurance exercise appears to augment early signalling responses governing skeletal muscle mitochondrial biogenesis and metabolic adaptation (Cochran et al., 2010; Psilander et al., 2012; Yeo et al., 2010), while commencing RE with low muscle glycogen may compromise post-exercise Akt signalling (Creer et al., 2005). Any potential negative effect of low muscle glycogen on anabolic responses in skeletal muscle was, however, recently questioned by a study showing no effect of muscle glycogen depletion on anabolic responses to RE (Camera et al., 2012). Low muscle glycogen is associated with fatigue development (Hulston et al., 2010; Ortenblad et al., 2013) and increased AMPK activity (Derave et al., 2000), which might inhibit anabolic responses induced by RE (Creer et al., 2005; Thomson et al., 2008). As aforementioned (see section 2.5.2.3), amino acids can independently stimulate mTORC1 activation and subsequently increase protein synthesis rates (Blomstrand et al., 2006; Deldicque et al., 2005; Rennie et al., 2006), while branched-chain amino acid provision reduces post-RE increases in Atrogin-1 mRNA and MuRF-1 protein
(Borgenvik et al., 2012). Nutrient availability is therefore a potent modulator of molecular responses to exercise and skeletal muscle adaptations following chronic exercise training (Beelen et al., 2010; Hawley et al., 2011) and must be considered when interpreting the concurrent training literature.
Most existing molecular concurrent training studies have employed designs whereby participants performed exercise in the fasted state (Apro et al., 2013; Carrithers et al., 2007; Coffey et al., 2009a; Coffey et al., 2009b; Wang et al., 2011), or were not provided with nutrients upon cessation of exercise (Lundberg et al., 2012), presumably to control for the independent effects of nutrient availability on molecular responses within skeletal muscle (Hawley et al., 2011). Performing exercise in the fasted state undoubtedly presents a heightened metabolic challenge within the muscle milieu, presumably increasing energy-sensing kinase activity (e.g., AMPK and eEF2k) with the capacity to suppress protein synthesis (Atherton et al., 2005; Thomson et al., 2008), and promote autophagy (Jamart et al., 2013). The ability of amino acid ingestion to independently stimulate activation of anabolic signalling responses (Blomstrand et al., 2006; Deldicque et al., 2005; Rennie et al., 2006) suggests adequate nutrient availability may be essential for attenuating any potential negative impact of endurance exercise and the associated molecular responses on protein synthesis (Coffey et al., 2011). It is well established that ingestion of sufficient protein in the early recovery period following RE is required to maximise MPS and subsequently muscle hypertrophy (Areta et al., 2013; Hawley et al., 2011). Recent evidence also suggests protein ingestion following concurrent exercise is sufficient to maximise rates of MPS compared with placebo (Camera et al., 2015). Further, as muscle hypertrophy is an energetically-demanding process, a positive energy balance may also be required to support increases in muscle mass (Lambert et al., 2004). Although empirical evidence for this premise is lacking, endurance exercise nevertheless likely disrupts this balance via continual substrate depletion and/or amino acid oxidation (Blomstrand & Saltin, 1999). The potentially confounding effects of altering nutrient availability on molecular responses to exercise should therefore also be considered when interpreting the concurrent training literature. Additionally, further work is required to fully elucidate the importance of nutrient availability on modulating interference during concurrent training (Perez-Schindler et al., 2015).