Insufficient recruitment

Many people do not manage to do the recommended amount of physical activity for improving general health and wellbeing, and a common reason for this is lack of time. Sprint interval training (SIT) has been suggested to be a time-efficient alternative to current exercise recommendations, but most SIT protocols are not actually as time-efficient as claimed. However, it has previously been shown that the training time commitment of common SIT protocols can be substantially reduced while remaining effective at improving key health markers such as aerobic fitness, insulin function and blood pressure. For example, the reduced-exertion high-intensity interval training (REHIT) protocol consists of two 20-second 'all-out' cycle sprints within a 10-minute low-intensity exercise session. There is some evidence that REHIT is just as effective at improving aerobic fitness with 2 exercise sessions per week compared to 3 or 4 sessions. However, it remains unknown if improvements in aerobic fitness are reduced if just a single REHIT session is performed each week. Therefore, the aim of the present study is to compare improvements in aerobic fitness levels between a control group (no training intervention), a group performing a single REHIT session per week, and a group performing 2 REHIT sessions per week.

Up to 45 apparently healthy participants will be recruited at multiple sites. Eligible participants will complete a fitness test to measure pre-training maximal aerobic capacity (V̇O2max). Participants will start cycling on a stationary bike at a low intensity (30 W). The intensity will increase by 1 W every 3 seconds until volitional exhaustion or an inability to maintain a pedalling frequency of >60 rpm. Expired O2 and CO2 will be continuously measured breath-by-breath using an online gas analyser. V̇O2max will be determined as the highest value for a 15-breath rolling average of V̇O2. V̇O2max will be accepted if at least 2 of the following criteria are met: volitional exhaustion / inability to maintain a pedal frequency of 60 rpm, RER>1.10, a plateau in V̇O2, and/or heart rate within 10 bpm of the age predicted maximum (220-age).

Participants will then be randomised into one of 3 groups, performing either 1 or 2 REHIT training sessions per week, or no training intervention (control group). Randomisation will be performed using the sealed envelope method. Participants in the control group will be asked to maintain their regular lifestyle for 6 weeks. Participants in the training groups will perform 6 weeks of REHIT. Each REHIT session involves 10 min of unloaded pedalling interspersed with 2 all-out cycle sprints against a resistance equivalent to 7.5% of the participant's body weight. Sprint duration will be 10 s in week 1, 15 s in week 2, and 20 s in the remaining 4 weeks. The first sprint will finish at 2 min and the second sprint will finish at 6 min. Participants will be instructed to start pedalling as fast as they can ~2-3 s before applying the sprint resistance, and to keep pedalling as fast as they can during the sprint. Verbal encouragement will be provided. Heart rate and power output will be measured throughout each 10-minute exercise session. The post-training V̇O2max test will be scheduled 3 days following the last training session.

Inclusion Criteria:

• Apparently healthy
• Sedentary or recreationally active

Exclusion Criteria:

• Classified as highly physically active on the International Physical Activity Questionnaire (IPAQ)
• Answering 'Yes' to any of the questions of a standard Physical Activity Readiness Questionnaire (PAR-Q)
• Resting heart rate >100 bpm
• Resting blood pressure > 140/90 mm Hg
• BMI > 35 kg/m2
• Testing positive for Covid-19
• Pregnant women

Anonymised IPD can be provided to other researchers on request, if the data are of use to answer a valid research question.

After completion of data collection.

The nature of the intervention does not allow for blinding.