Low-volume ‘sprint’ interval training (SIT) stimulates rapid improvements in muscle oxidative capacity that are comparable to levels reached following traditional endurance training (ET) but no study has examined metabolic adaptations during exercise after these different training strategies. We hypothesized that SIT and ET would induce similar adaptations in markers of skeletal muscle carbohydrate (CHO) and lipid metabolism and metabolic control during exercise despite large differences in training volume and time commitment. Active but untrained subjects (23 ± 1 years) performed a constant-load cycling challenge (1 h at 65% of peak oxygen uptake ( ˙ VO2peak) before and after 6 weeks of either SIT or ET (n = 5 men and 5 women per group). SIT consisted of four to six repeats of a 30 s ‘all out’ Wingate Test (mean power output ∼500 W) with 4.5 min recovery between repeats, 3 days per week. ET consisted of 40‐60 min of continuous cycling at a workload that elicited ∼65% ˙ VO2peak (mean power output ∼150 W) per day, 5 days per week. Weekly time commitment (∼1.5 versus ∼4.5 h) and total training volume (∼225 versus ∼2250 kJ week −1 ) were substantially lower in SIT versus ET. Despite these differences, both protocols induced similar increases (P < 0.05) in mitochondrial markers for skeletal muscle CHO (pyruvate dehydrogenase E1α protein content) and lipid oxidation (3-hydroxyacyl CoA dehydrogenase maximal activity) and protein content of peroxisome proliferator-activated receptor-γ coactivator-1α. Glycogen and phosphocreatine utilization during exercise were reduced after training, and calculated rates of whole-body CHO and lipid oxidation were decreased and increased, respectively, with no differences between groups (all main effects, P < 0.05). Given the markedly lower training volume in the SIT group, these data suggest that high-intensity interval training is a time-efficient strategy to increase skeletal muscle oxidative capacity and induce specific metabolic adaptations during exercise that are comparable to traditional ET.

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Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans

Brief, intense exercise training may induce metabolic and performance adaptations comparable to traditional endurance training. However, no study has directly compared these diverse training strategies in a standardized manner. We therefore examined changes in exercise capacity and molecular and cellular adaptations in skeletal muscle after low volume sprint-interval training (SIT) and high volume endurance training (ET). Sixteen active men (21 +/- 1 years, ) were assigned to a SIT or ET group (n = 8 each) and performed six training sessions over 14 days. Each session consisted of either four to six repeats of 30 s 'all out' cycling at approximately 250% with 4 min recovery (SIT) or 90-120 min continuous cycling at approximately 65% (ET). Training time commitment over 2 weeks was approximately 2.5 h for SIT and approximately 10.5 h for ET, and total training volume was approximately 90% lower for SIT versus ET ( approximately 630 versus approximately 6500 kJ). Training decreased the time required to complete 50 and 750 kJ cycling time trials, with no difference between groups (main effects, P </= 0.05). Biopsy samples obtained before and after training revealed similar increases in muscle oxidative capacity, as reflected by the maximal activity of cytochrome c oxidase (COX) and COX subunits II and IV protein content (main effects, P </= 0.05), but COX II and IV mRNAs were unchanged. Training-induced increases in muscle buffering capacity and glycogen content were also similar between groups (main effects, P </= 0.05). Given the large difference in training volume, these data demonstrate that SIT is a time-efficient strategy to induce rapid adaptations in skeletal muscle and exercise performance that are comparable to ET in young active men.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.2006.112094

Short‐term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance

Short-duration sprints (<10 seconds), interspersed with brief recoveries (<60 seconds), are common during most team and racket sports. Therefore, the ability to recover and to reproduce performance in subsequent sprints is probably an important fitness requirement of athletes engaged in these disciplines, and has been termed repeated-sprint ability (RSA). This review (Part I) examines how fatigue manifests during repeated-sprint exercise (RSE), and discusses the potential underpinning muscular and neural mechanisms. A subsequent companion review to this article will explain a better understanding of the training interventions that could eventually improve RSA. Using laboratory and field-based protocols, performance analyses have consistently shown that fatigue during RSE typically manifests as a decline in maximal/mean sprint speed (i.e. running) or a decrease in peak power or total work (i.e. cycling) over sprint repetitions. A consistent result among these studies is that performance decrements (i.e. fatigue) during successive bouts are inversely correlated to initial sprint performance. To date, there is no doubt that the details of the task (e.g. changes in the nature of the work/recovery bouts) alter the time course/magnitude of fatigue development during RSE (i.e. task dependency) and potentially the contribution of the underlying mechanisms. At the muscle level, limitations in energy supply, which include energy available from phosphocreatine hydrolysis, anaerobic glycolysis and oxidative metabolism, and the intramuscular accumulation of metabolic by-products, such as hydrogen ions, emerge as key factors responsible for fatigue. Although not as extensively studied, the use of surface electromyography techniques has revealed that failure to fully activate the contracting musculature and/or changes in inter-muscle recruitment strategies (i.e. neural factors) are also associated with fatigue outcomes. Pending confirmatory research, other factors such as stiffness regulation, hypoglycaemia, muscle damage and hostile environments (e.g. heat, hypoxia) are also likely to compromise fatigue resistance during repeated-sprint protocols.

Repeated-Sprint Ability – Part I: Factors Contributing to Fatigue

This review has grouped many studies on different populations with different protocols to show the interactive effects of intensity, frequency and duration of training as well as the effects of initial fitness levels and programme length on cardiorespiratory fitness as reflected by aerobic power (VO2max). Within each level of exercise duration, frequency, programme length or initial fitness level, the greatest improvements in aerobic power occur when the greatest challenge to aerobic power occurs i.e., when intensity is from 90 to 100% of VO2max. The pattern of improvement where different intensities are compared with different durations suggests that when exercise exceeds 35 minutes, a lower intensity of training results in the same effect as those achieved at higher intensities for shorter durations. Frequencies of as low as 2 per week can result in improvements in less fit subjects but when aerobic power exceeds 50 ml/kg/min, exercise frequency of at least 3 times per week is required. As the levels of initial fitness improve, the changes in aerobic power decreases regardless of the intensity, frequency or duration of exercise. Although these pooled data suggest that maximal gains in aerobic power are elicited with intensities between 90 to 100% VO2max, 4 times per week with exercise durations of 35 to 45 minutes, it is important to note that lower intensities still produce effective changes and reduce the risks of injury in non-athletic groups.

The interactions of intensity, frequency and duration of exercise training in altering cardiorespiratory fitness

Short-duration sprints, interspersed with brief recoveries, are common during most team sports. The ability to produce the best possible average sprint performance over a series of sprints (≤10 seconds), separated by short (≤60 seconds) recovery periods has been termed repeated-sprint ability (RSA). RSA is therefore an important fitness requirement of team-sport athletes, and it is important to better understand training strategies that can improve this fitness component. Surprisingly, however, there has been little research about the best training methods to improve RSA. In the absence of strong scientific evidence, two principal training theories have emerged. One is based on the concept of training specificity and maintains that the best way to train RSA is to perform repeated sprints. The second proposes that training interventions that target the main factors limiting RSA may be a more effective approach. The aim of this review (Part II) is to critically analyse training strategies to improve both RSA and the underlying factors responsible for fatigue during repeated sprints (see Part I of the preceding companion article). This review has highlighted that there is not one type of training that can be recommended to best improve RSA and all of the factors believed to be responsible for performance decrements during repeated-sprint tasks. This is not surprising, as RSA is a complex fitness component that depends on both metabolic (e.g. oxidative capacity, phosphocreatine recovery and H+ buffering) and neural factors (e.g. muscle activation and recruitment strategies) among others. While different training strategies can be used in order to improve each of these potential limiting factors, and in turn RSA, two key recommendations emerge from this review; it is important to include (i) some training to improve single-sprint performance (e.g. 'traditional' sprint training and strength/power training); and (ii) some high-intensity (80-90% maximal oxygen consumption) interval training to best improve the ability to recover between sprints. Further research is required to establish whether it is best to develop these qualities separately, or whether they can be developed concurrently (without interference effects). While research has identified a correlation between RSA and total sprint distance during soccer, future studies need to address whether training-induced changes in RSA also produce changes in match physical performance.

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Repeated-Sprint Ability – Part II: Recommendations for Training

Fourteen Subjects (6 male, 8 female) participated in a training program upon a bicycle ergometer for 7 weeks. Group CT followed a continuous training regimen 4 days per week at 70% 
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 O2 max. Group IT trained by an interval method at 100% 
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 O2 max. The duration of each training session was assigned so that each subject would complete 10,000 kpm of work per session during the first week. Each subsequent week, the work load was increased 3000 kpm. Pretraining tests included 
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 O2 max, standard 7 min tests at 80% 
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 O2 and 90% 
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 O2, an endurance test at 90%, and an intense anaerobic work bout at 2400 kpm. Variables assessed were 
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 O2, HR, and blood lactic acid concentrations. The mean increase in 
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 O2 max was 5.1 ml/kg min (15%) for both groups with a corresponding increase in maximal lactate of 20 mg-%. The response to the post-training tests was nearly identical for both groups: submaximal heart rate at the same absolute work load declined 17 beats/min (CT) and 15 beats/min (IT), submaximal lactate levels declined significantly, endurance ride duration increased 26 min. Continuous and interval training at 70% and 100% 
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 O2 max respectively produce identical changes in heart rate response, blood lactic acid concentration and 
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 O2 max when the total work load is equated per training session.

Kenneth L. Sparks

Papers

The effects of continuous and interval training in women and men