Type 2 diabetes is characterized by reduced aerobic exercise capacity (Wei et al., 1999) and poor glycemic control (Unwin et al., 2010). Both reduced aerobic exercise capacity and elevated glycosylated hemoglobin (Hb[A.sub.1c]) are associated with increased cardiovascular risk in type 2 diabetes (Emerging Risk Factors Collaboration et al., 2010; Wei et al., 2000; Zhang et al., 2012). Compared with healthy individuals, type 2 diabetics have a 2-4 times increased risk of developing cardiovascular disease (Emerging Risk Factors Collaboration et al., 2010), and more than 70 % of this patient population die of cardiovascular causes (Laakso, 2001).
Exercise training can improve aerobic exercise capacity ([VO.sub.2peak]) (Boule et al., 2003) and glycemic control (Boule et al. 2001) in type 2 diabetes. The benefits of exercise training on overall mortality and cardiovascular risk exceed those explained by glucose lowering alone (Wei et al., 2000, Church et al., 2005). However, two out of three individuals with type 2 diabetes do not exercise regularly (Thomas et al., 2004), and very few meet current exercise recommendations (Colberg et al., 2010). Lack of time is reported as one of the main reasons for the inactivity (Korkiakangas et al., 2009). This calls upon alternative exercise strategies that are less time consuming, yet effective to improve [VO.sub.2peak], glycemic control and other cardiometabolic risk factors.
In type 2 diabetes, high intensity interval exercise seems to be superior to continuous moderate exercise in improving aerobic exercise capacity and reducing several cardiovascular risk factors (Backx et al., 2011; Hollekim-Strand et al., 2014; Mitranun et al., 2014). Recent research indicates that low- and even extremely low volume intermittent high intensity exercise can be effective in improving glycemic control in type 2 diabetes (Little et al., 2011) as well as insulin sensitivity and [VO.sub.2peak] in sedentary, healthy men (Metcalfe et al., 2012).
Thus, the aim of this pilot study was to investigate the effects of low volume high intensity interval exercise (HIIE; 10x1-minute intervals at 90 % of maximum heart rate ([HR.sub.max])) and extremely low volume sprint interval exercise (SIE; 2x20-seconds intervals at maximum achievable intensity) on aerobic exercise capacity ([VO.sub.2peak]), glycemic control (Hb[A.sub.1c]), insulin resistance (HOMA-IR), blood pressure and body composition in individuals with type 2 diabetes. The main hypothesis was that in type 2 diabetes individuals, time effective exercise of HIIE and SIE is effective in improving cardiometabolic risk factors, but HIIE more than SIE.
In this 12 week randomized exercise trial, individuals with type 2 diabetes were recruited through a local newspaper and advertising at the St. Olavs Hospital, Trondheim University Hospital, Norway. The study was conducted between August 2013 and January 2014.
Inclusion criteria were age 20-65 years, diagnosed with type 2 diabetes within the past 10 years with no use of insulin. Exclusion criteria were known cardiovascular disease or lung disease, untreated hypertension of [greater than or equal to] 140/90 mmHg, orthopedic or neurological restrictions, severe obesity (BMI [greater than or equal to] 35), pregnancy, inability to exercise, drug or alcohol abuse, and reluctance to sign the consent form. Subjects were not eligible if they were more physically active than recommended in current exercise guidelines, e.g. more than 150 min/week of exercise at moderate intensity or greater (Colberg et al., 2010), assessed by a self-reported activity diary.
A total of 21 subjects were included. The unit of Applied Clinical Research at the Norwegian University of Science and Technology performed the randomization procedure. The protocol was approved by the Regional Committee for Medical and Health Research Ethics of Central Norway and was registered with the Clinical Trials Registry (ClinicalTrials.gov identifier: NCT02340260). Informed consent was obtained from all participants and all participants were insured.
The primary outcome measure was glycosylated hemoglobin (Hb[A.sub.1c]). Secondary outcome measures were aerobic exercise capacity, measured as peak oxygen uptake ([VO.sub.2peak]), blood glucose, insulin resistance (HOMA-IR), blood pressure and body composition.
Exercise training protocols
The HIIE protocol was described in detail previously (Little et al., 2011). Participants performed 3 minutes warm up at 70 % of [HR.sub.max] (determined from the exercise capacity test, methodology described later), 10x1-minute intervals of fast uphill walking or running at approximately 90 % of [HR.sub.max] with 75 seconds of active recovery between each interval, and 3 minutes cool down. Heart rate was continuously monitored by an experienced exercise physiologist throughout the intervention period, and treadmill speed and/or inclination was successively adjusted throughout the exercise intervention to make sure heart rate levels were met.
The SIE protocol was adapted to treadmill from a protocol previously described for stationary bicycle (Metcalfe et al., 2012). Participants performed 3 minutes warm up at 70 % of [HR.sub.max], 2x20-seconds sprint intervals at maximum achievable intensity, with approximately 3 minutes active recovery between intervals at an intensity of 70 % of [HR.sub.max], and subsequently 3 minutes cool down. The treadmill was set to 20 % incline and the first one or two sessions were used to find maximum achievable running speed. Speed was successively adjusted to ensure the intensity was according to the protocol throughout the intervention period.
Both groups exercised 3 times a week throughout the 12 week intervention. Exercise times per session in the HIIE and SIE group were 27 minutes and 10 minutes, respectively. It was not the aim of this trial to compare two isocaloric exercise protocols, but rather two protocols with different potential to reduce the time-burden associated with exercise in type 2 diabetes. Thus, the exercise protocols were not matched for energy expenditure.
Clinical and laboratory examinations
All measurements were performed at baseline and after completion of the 12 weeks of exercise. Post test measurements were made between 48 and 72 hours after the last exercise bout, in order to reduce the acute effects of the final exercise session.
Peak oxygen uptake
Exercise capacity ([VO.sub.2peak]) was measured during...