High-intensity interval training (HIT) describes exercise that is characterized by brief, intermittent bursts of vigorous activity, interspersed by periods of rest or low-intensity exercise. HIT is infinitely variable, with the specific physiological adaptations induced by this form of training determined by various factors including the precise nature of the exercise stimulus (i.e., the intensity, duration and number of intervals performed, as well as the duration and activity patterns during recovery). When compared on a matched-work basis, or when estimated energy expenditure is equivalent, HIT can serve as an effective alternative to traditional endurance training, inducing similar or even superior changes in a range of physiological, performance and health-related markers in both healthy individuals and diseased populations.
Less is known about the effects of low-volume HIT, but growing evidence suggests this type of training stimulates physiological remodeling comparable to moderate-intensity continuous training despite a substantially lower time commitment and reduced total exercise volume. The most common model employed in low-volume HIT studies has been the Wingate Test, which consists of a 30-second "all out" cycling effort against a supra-maximal workload. Subjects typically perform 4-6 work bouts, separated by approximately four minutes of recovery, for a total of 2-3 minutes of intense exercise during a training session that lasts approximately 20 minutes. As little as three sessions per week of this type of training induces skeletal muscle and cardiovascular adaptations comparable to several hours per week of continuous moderate training, as generally recommended by public health agencies.
Wingate-based HIT is, however, extremely demanding and may not be safe, tolerable or appealing for some individuals. We therefore sought to design a more practical model of low-volume HIT that is time-efficient while also having wider application to different populations. The new HIT model consists of 10 x 60 second work bouts at a constant-load intensity that elicits approximately 90 percent of maximal heart rate, interspersed with 60 seconds of recovery. The protocol is still time-efficient in that only 10 minutes of exercise is performed over a 20-minute training session. Importantly, this practical, time-efficient HIT model is still effective at inducing rapid skeletal muscle remodeling towards a more oxidative phenotype, similar to our previous Wingate-based HIT studies and high-volume endurance training. These findings are noteworthy from a public health perspective, given that "lack of time" remains one of the most commonly cited barriers to regular exercise participation.
Relatively high-volume HIT protocols that are comparable to traditional endurance training have been shown to improve cardiorespiratory fitness and other health status markers in a fairly large number of studies, including those conducted on people with coronary artery disease, congestive heart failure, middle age adults with metabolic syndrome and obese individuals. In contrast, low-volume HIT studies in persons who might be at risk for cardiometabolic disorders or patients with chronic disease are very limited. Our group's recent study (Hood et al. 2011), reported in the Oct. issue of MSSE, showed that six sessions of the practical HIT model over two weeks improved estimated insulin sensitivity in previously sedentary, overweight individuals. Insulin sensitivity was calculated based on single fasting glucose and insulin measurements and therefore primarily reflects hepatic as opposed to peripheral insulin sensitivity. We also recently showed that low-volume HIT was effective and well-tolerated in people with type 2 diabetes (Little et al. 2011). Two weeks of HIT reduced average 24-hour blood glucose concentration and the magnitude of glucose spikes after meals, measured via continuous glucose monitoring under standardized diet but otherwise free-living conditions. While the preliminary data from these small, proof-of-principle studies are intriguing, large-scale studies are clearly needed to resolve whether low-volume HIT is a safe, realistic and time-efficient exercise alternative to reduce the risk of cardiometabolic disease or improve health and wellbeing in patients with chronic disease.
