Adaptations to concurrent training in healthy active men: the role of exercise session order

Concurrently performing endurance and resistance exercise within the same training program presents a theoretically optimal training method for improving athletic performance, as well as attaining the multiple health benefits from both modes of training. However, many studies provide evidence demonstrating that concurrent training can attenuate the development of hallmark resistance training adaptations such as strength, muscle hypertrophy, and power, compared to performing resistance training alone. This phenomenon has been termed the “interference effect” or the “concurrent training effect”. Whilst much research has been dedicated to understanding this effect, the precise causes are not well known, and are further confounded by a growing body of conflicting literature. Given that endurance and resistance exercise transiently induce distinct molecular responses that govern their respective mode-specific phenotypic adaptations, it has been proposed that some degree of molecular incompatibility between the different exercise modes may contribute to the interference effect; however, supportive evidence in human studies is lacking. Furthermore, the nature of the interference effect may largely be dictated by the manipulation of training variables (e.g., exercise order, intensity, frequency, volume, mode, recovery duration) and non-training variables (e.g., training status, nutrient availability). The overarching aim of this thesis was to investigate the effects of concurrent endurance and resistance training on the development of hallmark resistance and endurance training adaptations, and the molecular responses that regulate them. A secondary aim was to investigate the effect of manipulating the order in which concurrent exercise sessions are performed. An in-depth review of the existing concurrent training literature was conducted (Chapter 2), followed by an original body of research designed to investigate how exercise-induced molecular responses to resistance-only and concurrent exercise differ before and after a period of a structured training (Chapter 4), whilst simultaneously assessing the effects of concurrent training (in both orders) on the development of whole-body training adaptations compared to resistance-only training (Chapter 5). As such, data in each chapter were derived from one major training study involving the same cohort of participants who performed acute, experimental exercise trials both before and after a 9-week period of training. Following familiarisation and baseline fitness testing, twenty-nine healthy, active men were ranked according to their baseline levels of maximal strength, aerobic fitness, and lean body mass, and allocated to one of three training groups in a counterbalanced order: 1) RO, resistance-only exercise; 2) ER, endurance prior to resistance exercise; or 3) RE, resistance followed by endurance exercise. On the first training day in both Weeks 1 and 10, twenty-five of these participants completed an “experimental” training day, during which muscle biopsies were obtained immediately before, after, and 3 hours after each exercise session, to characterise temporal changes in gene expression and protein phosphorylation across a full day in response to resistance-only and concurrent exercise, before and after a period of training. Between Weeks 1 and 10, all twenty-nine participants completed 8 weeks of structured training in their respective groups (Weeks 2 to 9). The training program was of a moderate frequency (3 days a week), and the same-day concurrent sessions were separated by 3 hours of recovery. The battery of anthropometric, physiological, and performance tests were repeated during, and after the training program to assess changes in whole-body adaptations in response to the different training programs. Chapter 4 represents the first study of its kind that attempts to elucidate the extended time-course of molecular responses to both concurrent and resistance-only exercise when performed in the fed-state, in both the untrained (Week 1) and training-accustomed states (Week 10). Following training, all groups demonstrated comparable increases in resting muscle glycogen concentration. Despite concurrent exercise (regardless of the order) inducing greater muscle glycogen depletion than resistance-only exercise by the end of each day, as well as transiently upregulating purported inhibitors of anabolic signalling pathways, the findings in this study do not clearly support the premise that concurrent exercise induces a molecular interference effect. Novel findings include the similar, rather than divergent, patterns of expression between AMPK and Akt, as well as the characterisation of Mighty mRNA expression, which has not been previously reported in resistance and concurrent exercise models in human skeletal muscle. This study also provides supportive evidence for resistance exercise-induced increases in PGC-1α mRNA, contraction-induced reductions in myostatin mRNA, and the differential regulation of ‘atrogenes’ (MuRF1 and MAFbx) in response to endurance and resistance exercise. Finally, this study also provides support for training-induced changes in molecular responses to exercise, whereby several genes and proteins (related to mitochondrial biogenesis, protein degradation and translation) elicited more transient, and smaller perturbations in the training-accustomed, compared to untrained state. Whilst the data gleaned from Chapter 4 did not clearly indicate that performing concurrent exercise would ‘acutely’ interfere with the molecular responses governing resistance training adaptations, the relationship between exercise-induced molecular responses and training-induced adaptations is not always clear. Therefore, the aim of Chapter 5 was to assess changes in hallmark endurance and resistance adaptations, following 9 weeks of resistance-only and concurrent training in both exercise orders. The main findings demonstrate that concurrent training, irrespective of the session order, led to comparable improvements in maximal strength and lean body mass to that of resistance-only training. Furthermore, independent of the session order, both concurrent groups similarly improved all markers of aerobic fitness more than resistance-only training. However, performing endurance training after resistance training (i.e., RE) attenuated the development of countermovement jump displacement, force, and power compared to resistance-only training; the reverse exercise order (i.e., ER) possibly had a negative effect on these parameters. In addition, only the RE group displayed a meaningful reduction of total fat mass following training. This chapter also provides novel data regarding the participants subjective wellbeing and “readiness-to-train” prior to all exercise sessions, as well as their training load (both internal and external). In combination with the performance and physiological data, this study indicates that whilst all three groups completed similar volumes of resistance training, performing endurance training before resistance training may lead to greater perceptions of internal training load, and more negative perceptions of total wellbeing, muscle soreness, stress and mood. Collectively, the results from this body of work do not support the premise of compromised molecular responses, or subsequent strength and lean mass gains, following concurrent training, compared to only performing resistance training. In healthy, active men, a short-term concurrent training program, regardless of exercise order, presents a viable strategy to improve lower-body maximal strength and total lean body mass comparably to resistance-only training, whilst also improving aerobic fitness. However, improvements in some measures of countermovement jump performance were attenuated with concurrent training, particularly when resistance exercise was performed first. There were also possible effects of exercise order on changes in countermovement jump performance (favouring ER) and reductions in fat mass (favouring RE); however, more data are required to determine the importance of these effects. For healthy, active individuals engaging in same-day concurrent training, with short recovery durations, the choice of exercise order could be dictated by personal preference, given that the exercise order may affect perceptions of “readiness-to-train” prior to, and perceptions of effort after, resistance exercise. Perhaps more importantly, the exercise order should be periodised according to the specific goals of an individual training cycle. However, in environments where the exercise order may be dictated by external factors (e.g., congested competition schedules, restricted availability of training facilities), careful consideration should also be given to the effects of other training and non-training variables, to minimise potential interference effects and maximise concurrent training adaptations.