Jonathan P. Little

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

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 ∼250% with 4 min recovery (SIT) or 90–120 min continuous cycling at ∼65% (ET). Training time commitment over 2 weeks was ∼2.5 h for SIT and ∼10.5 h for ET, and total training volume was ∼90% lower for SIT versus ET (∼630 versus∼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.

A practical model of low-volume high-intensity interval training induces mitochondrial biogenesis in human skeletal muscle: potential mechanisms

High‐intensity interval training (HIT) induces skeletal muscle metabolic and performance adaptations that resemble traditional endurance training despite a low total exercise volume. Most HIT studies have employed ‘all out’, variable‐load exercise interventions (e.g. repeated Wingate tests) that may not be safe, practical and/or well tolerated by certain individuals. Our purpose was to determine the performance, metabolic and molecular adaptations to a more practical model of low‐volume HIT. Seven men (21 ± 0.4 years, ml kg−1 min−1) performed six training sessions over 2 weeks. Each session consisted of 8–12 × 60 s intervals at ∼100% of peak power output elicited during a ramp peak test (355 ± 10 W) separated by 75 s of recovery. Training increased exercise capacity, as assessed by significant improvements on both 50 kJ and 750 kJ cycling time trials (P < 0.05 for both). Skeletal muscle (vastus lateralis) biopsy samples obtained before and after training revealed increased maximal activity of citrate synthase (CS) and cytochrome c oxidase (COX) as well as total protein content of CS, COX subunits II and IV, and the mitochondrial transcription factor A (Tfam) (P < 0.05 for all). Nuclear abundance of peroxisome proliferator‐activated receptor γ co‐activator 1α (PGC‐1α) was ∼25% higher after training (P < 0.05), but total PGC‐1α protein content remained unchanged. Total SIRT1 content, a proposed activator of PGC‐1α and mitochondrial biogenesis, was increased by ∼56% following training (P < 0.05). Training also increased resting muscle glycogen and total GLUT4 protein content (both P < 0.05). This study demonstrates that a practical model of low volume HIT is a potent stimulus for increasing skeletal muscle mitochondrial capacity and improving exercise performance. The results also suggest that increases in SIRT1, nuclear PGC‐1α, and Tfam may be involved in coordinating mitochondrial adaptations in response to HIT in human skeletal muscle.

Acute high‐intensity interval exercise reduces the postprandial glucose response and prevalence of hyperglycaemia in patients with type 2 diabetes

High‐volume endurance exercise (END) improves glycaemic control in type 2 diabetes (T2D) but many individuals cite ‘lack of time’ as a barrier to regular participation. High‐intensity interval training (HIT) is a time‐efficient method to induce physiological adaptations similar to END, but little is known regarding the effect of HIT in T2D. Using continuous glucose monitoring (CGM), we examined the 24‐h blood glucose response to one session of HIT consisting of 10 × 60 s cycling efforts at ∼90% maximal heart rate, interspersed with 60 s rest. Seven adults with T2D underwent CGM for 24‐h on two occasions under standard dietary conditions: following acute HIT and on a non‐exercise control day (CTL). HIT reduced hyperglycaemia measured as proportion of time spent above 10 mmol/l (HIT: 4.5 ± 4.4 vs. CTL: 15.2 ± 12.3%, p = 0.04). Postprandial hyperglycaemia, measured as the sum of post‐meal areas under the glucose curve, was also lower after HIT vs. CTL (728 ± 331 vs. 1142 ± 556 mmol/l·9 h, p = 0.01). These findings highlight the potential for HIT to improve glycaemic control in T2D.

Intermittent and continuous high‐intensity exercise training induce similar acute but different chronic muscle adaptations

What is the central question of this study? How important is the interval in high‐intensity interval training (HIIT)? What is the main finding and its importance? The intermittent nature of HIIT is important for maximizing skeletal muscle adaptations to this type of exercise, at least when a relatively small total volume of work is performed in an ‘all‐out’ manner. The protein signalling responses to an acute bout of HIIT were generally not predictive of training‐induced outcomes. Nonetheless, a single session of exercise lasting <10 min including warm‐up, performed three times per week for 6 weeks, was sufficient to improve maximal aerobic capacity.

Just HIT it! A time‐efficient exercise strategy to improve muscle insulin sensitivity

Muscle insulin resistance plays a major pathophysiological role in type 2 diabetes and is associated with major public health problems, including obesity and coronary artery disease. Given the dire consequences associated with sedentary living (in terms of individual disease risk and the economic burden on health care systems), the promotion of an active lifestyle is an international priority. Public health guidelines generally recommend that adults perform at least 150 min week−1 of ‘moderate-intensity’ aerobic physical activity (typically defined as 40–60% of maximal aerobic power ()) or a minimum of 60 min week−1 of ‘vigorous-intensity’ exercise (>60%) to promote health. These recommendations are based on robust evidence that suggests endurance training reduces the risk for chronic disease through the same general mechanisms that lead to improved athletic performance; for example, exercise-induced increases in muscle oxidative and glucose transport capacities have been linked to improved insulin sensitivity and glycaemic control. Unfortunately, most people fail to meet even the minimum physical activity guidelines, citing ‘lack of time’ as the major barrier to regular exercise participation. Innovations in exercise prescription that show benefits despite a minimal time commitment therefore represent a valuable strategy to encourage physical activity participation and reduce the risk of chronic diseases. n nA growing body of evidence suggests that high-intensity interval training (HIT) induces numerous physiological adaptations that are similar to traditional endurance training despite a lower total exercise volume and training time commitment (Gibala & McGee, 2008). Low-volume HIT is characterized by brief repeated ‘bursts’ of vigorous exercise interspersed with periods of rest or low-intensity exercise for recovery. A common model employed in many HIT studies is the Wingate test, which consists of a 30 s ‘all-out’ cycling effort against a standardized resistance. In a typical training session, subjects complete four to six Wingate tests interspersed with 4 min of rest, for a total of only 2 to 3 min of maximal exercise spread over a ∼15–30 min period. As little as six sessions of this low-volume HIT protocol over 2 weeks is a potent stimulus to increase muscle oxidative and glucose transport capacities (Gibala & McGee, 2008), but little is known about the effect of this type of training on common health status markers linked to disease risk. n nIn a recent issue of The Journal of Physiology, Richards et al. (2010) report that a Wingate-based HIT protocol consisting of only 16 min of all-out cycling over 14 days improved insulin sensitivity in previously sedentary or recreationally active young adults. Babraj and colleagues (2009) previously provided indirect evidence of improved insulin sensitivity based on oral glucose tolerance tests (OGTT) performed before and several days after an identical HIT protocol. However, the data from Richards et al. (2010) are more compelling, since insulin sensitivity was determined using the hyperinsulinaemic euglycaemic clamp technique, which is widely accepted as the reference standard for direct measurement in humans. Short-term HIT would not be expected to influence body composition and Richards et al. (2010) reported no change in several circulatory markers linked to insulin action, providing support for their conclusion that skeletal muscle adaptations probably contributed to the improved insulin sensitivity. As recognized by the authors, an important question pertinent to studies of this sort is whether the change in insulin sensitivity is due to training per se, or to the preceding exercise bout. While acute exercise effects are detectable for up to 48 h, the rigorous study design by Richards et al. (2010) suggests that the improved insulin sensitivity after HIT was a training-induced effect. Post-training measurements were made 72 h after the final training session, and no change in insulin sensitivity was observed in a control group that performed a single bout of Wingate-based exercise. However, the results are in contrast to a recent study (Whyte et al. 2010) that reported improved insulin sensitivity based on OGTT results 24 but not 72 h following a similar HIT training protocol in overweight and obese men. It has been suggested that the insulin-sensitizing effects of this type of exercise may be attenuated in obese and/or insulin-resistant adults and additional work is needed to clarify the effectiveness of HIT in different populations. n nWingate tests require a specialized cycle ergometer and the ‘all-out’ maximal effort necessitates an extremely high level of subject motivation. Therefore, it may not be safe or practical to implement this form of training in the general population. A recent study (Little et al. 2010) evaluated whether a more practical model of low-volume HIT could elicit metabolic and performance adaptations similar to Wingate-based HIT studies. The modified protocol involved eight to twelve 1 min intervals at an intensity that corresponded to ∼100% with 75 s of rest in between. While still a demanding form of exercise, the absolute work intensity corresponded to less than half of that achieved during an all-out Wingate test. The protocol was also time efficient in that only ∼10 min of exercise was performed over a 15–25 min period during each training session. Similar to Wingate-based HIT, six sessions of this modified HIT protocol over 2 weeks was a sufficient stimulus to increase skeletal muscle oxidative capacity and GLUT4 protein content. Unpublished work from our laboratory shows this HIT model is well tolerated and reduces hyperglycaemia in people with type 2 diabetes. Additional studies are needed to resolve whether low-volume HIT is a realistic, time-efficient exercise alternative to reduce the risk of metabolic disease in various populations.