The Effects of Low-Volume High Intensity Interval Training and Circuit Training on Maximal Oxygen Uptake
Phase: NA
Conditions: High Intensity Interval Training
Sponsor: University of Central Lancashire

Cover Sheet 
 
 
Study Protocol 11/05/2017 
 
Study title 
 
The Effects of Low-Volume High Intensity Interval Training and Circuit Training 
on Maximal Oxygen Uptake 
 
Trial Number: NCT03700671 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Design  
Regular exercise is well known to have a positive effect on health [1,2]. Many of these benefits are 
associated with improving maximal oxygen uptake (V̇O2max) [3,4]. Increasing V̇O2max through exercise 
training may improve cardiometabolic health, quality of life and increase life-expectancy [5,4]. 
Popular training methods aimed at improving V̇O2max are circuit and high intensity interval training 
(HIIT) and are routinely adopted by the general population, health and fitness professionals and 
researchers [6]. Circuit training (CT) is typically performed at a moderate or high intensity, over a 
period of 30-50 minutes and involves a range of aerobic, body weight and resistance exercises with 
minimal rest [7,8]. Low-volume HIIT is defined as ‘brief, intermittent bursts of vigorous activity, 
interspersed by periods of rest or low intensity exercise’ [9], typically prescribed at a training 
intensity between 80 and 100% of heart rate maximum (HRmax) [10].  
 
Whole-body CT and low volume HIIT consisting of eight to twelve one-minute interval bouts, 
interspersed with a similar recovery time have been shown to improve cardiometabolic health and 
cardiorespiratory fitness (CRF) [8,11-16]. Increases in V̇O2max and the anaerobic threshold (AT) have 
been observed following CT, however published studies are limited to sedentary and older 
populations, with further markers of CRF not fully explored [8,17]. Comparable improvements in 
V̇O2max and the AT have been shown following low volume HIIT, with increases in maximum oxygen 
pulse (V̇O2 /HR) also observed [16,18,19].  However, there is limited evidence directly comparing 
both training methods across a range of CRF markers, as such it is unknown which approach is most 
beneficial [20,21].   
 
Direct comparisons have been made investigating low volume HIIT and moderate intensity 
continuous training (MICT), which is typically 30-60 minutes in duration adopting the same modality 

of exercise [15,16,14]. The results demonstrate low volume HIIT to be a time efficient method, 
eliciting greater improvements in V̇O2max, although these findings are not consistently shown [22].  
While HIIT and CT are feasible and effective at improving CRF, no study has directly investigated the 
two. Therefore, the aim of this study was to compare the effect of two weekly sessions of low 
volume HIIT and CT over an eight-week period on V̇O2max in apparently healthy middle-aged adults. 
We also investigated changes in other markers of CRF such as the first ventilatory anaerobic 
threshold (V̇O2 at VAT) and maximum V̇O2 /HR. 
 
Study Design 
Participants were enrolled in a randomised control trial at the University of Hull to either eight 
weeks of HIIT or CT (two supervised sessions per week, accompanied by an exercise physiologist). A 
sample size of 38 using G*Power 3.1 software was calculated based on previously published data in 
which the mean difference between HIIT and MICT was 3.2 ml.kg-1.min-1 with a pooled standard 
deviation of 3 ml.kg-1.min-1 [19]. Statistical significance was set at  = 0.05 and power set to 0.95. To 
allow for 10% attrition 42 individuals were recruited to the study. To assess the effectiveness of the 
interventions as determined by V̇O2max, a maximal cardiopulmonary exercise test (CPET) to volitional 
exhaustion on an electronically braked cycle ergometer at baseline (visit one), and following an 
eight-week exercise intervention of HIIT or CT (visit two) was conducted. When attending the 
assessments participants were asked not to take part in any strenuous exercise 24 hours prior to the 
appointment, to wear suitable comfortable clothing and avoid a large meal. Visit two CPET was 
performed within six days of completing the exercise interventions. A thorough warm-up and cool 
down before and after each exercise session. All were asked to maintain their habitual physical 
activity patterns during the intervention. Body mass index (BMI) was calculated by dividing body 
weight [23] by height in meters squared and was presented as kg.m-2. Resting blood pressure was 
measured after 15 minutes of rest using a sphygmomanometer (A.C. Cossor & Son Ltd, London UK) 

and stethoscope (3M Healthcare, St Paul, MN). To provide a comprehensive account of the study the 
Consensus on Exercise Reporting Template (CERT) was consulted [24].  
 
Participants  
Ethical approval was provided by the School of Life Sciences ethics committee at the University of 
Hull which was in accordance with the 1964 Helsinki declaration and its later amendments or 
comparable ethical standards. A total of 42 apparently healthy men and women between the age of 
18-65 years were recruited to the study. Participant characteristics are given in Table 1. Enrolled 
individuals reported no medical history of cardiometabolic or limiting respiratory disease, were non-
smokers, had a body mass index <30 kg.m-2, classified as recreationally active [25] and none were 
taking any medication that would affect heart rate. As a condition of enrolment, those over 45 years 
obtained written medical clearance from a general practitioner and underwent resting and exercise 
12-lead electrocardiogram (ECG) (GE Healthcare, Chalfont St Giles, United Kingdom). Written 
informed consent and a pre-exercise medical questionnaire was completed by all. 
 
***Table 1 here*** 
  
Cardiopulmonary Exercise Testing 
Maximal CPETs were conducted in accordance with the American Thoracic Society (ATS) and the 
American College of Chest Physicians (ACCP) guidelines [26]. An Oxycon pro (Jaeger, Hoechburg, 
Germany) breath by breath metabolic cart was used to collect respiratory gas exchange data. 
Automatic and manual calibration evaluated ambient temperature, humidity, barometric pressure 
and altitude. Calibration of the air flow volume was conducted using a 3L syringe and by automatic 
calibration. Two-point gas calibration was also conducted to ensure accurate measures of inspired 

O2 and expired CO2 [27]. Tests were performed on a GE e-bike ergometer (GE Healthcare, 
Buckinghamshire, UK) using a ramp protocol [26]. The protocol consisted of a three-minute rest 
phase, three minutes of unloaded cycling, followed by a personalised ramp test [28] (ramp rate 
ranged between 15 and 30 watts) with work rate continually increased every one to three seconds. 
Participants performed the same ramp rate pre and post testing. Participants were asked to pedal at 
a cadence of 70 rpm until they reached volitional exhaustion at a protocol duration between eight to 
twelve minutes. Self-reported rating of perceived exertion (RPE) scores using the 6-20 scale [29] and 
heart rate (HR) (FT1 heart rate monitor, Polar Electro, OY, Finland) was recorded during the last five 
seconds of each minute of the test, at maximum exercise and during the recovery period. Together 
with verbal encouragement to volitional exhaustion, V̇O2max was attained by participants achieving at 
least two of the following criteria: V̇O2 plateau as determined by a failure of V̇O2  increase by 150 
ml/min with further increases in workload analysed by breath by breath gas exchange data averaged 
over 15 seconds [30], respiratory exchange ratio (RER) > 1.10, achieve > 85% age predicated HRmax 
and a RPE > 17 on the 6-20 Borg scale [27]. V̇O2 at VAT was defined using the ‘V slope’ method [31] 
and verified using ventilatory equivalents. Peak power output (PPO) (watts) and HRmax were defined 
as the highest value achieved during the CPET with maximum V̇O2 /HR determined by the ratio of 
V̇O2max and HRmax. 
 
Training Interventions 
The HIIT group were asked to perform ten one-minute HIIT intervals, each followed by one minute of 
active recovery (AR) (total exercise time 20 minutes). Resulting from the CPET, HIIT was set at > 85% 
HRmax with a specific HR designated for this criterion. Active recovery was set at a load corresponding 
to 25-50 watts. Sessions were performed on a Wattbike trainer (Wattbike Ltd, Nottingham, UK). The 
CT group completed a practical seven-station mixed modality exercise circuit (cycle ergometer, 
rower, treadmill, sit to stand/squats, knee to elbow and leg kickback with bicep curl) at an intensity 

of 60-80% HRmax (calculated from CPET). No resistance equipment was involved, only body weight. 
Participants initially performed 20 minutes of CT with duration increased by five minutes per week 
until the desired 40 minutes. Each station was occupied for three to six minutes depending on 
session duration, moving from one station to the next with minimal rest. During both interventions, 
HR was measured in last 5 seconds of each station/interval using a FT1 polar heart rate monitor 
(Polar Electro, OY, Finland) with each CT session timed using a stop watch (Axprod S.L, Guipuzcoa, 
Spain). Intensity for both interventions was adjusted throughout by the investigator to ensure an 
appropriate HR range and successful completion of the protocol. Participants were made aware of 
their HR ranges and verbal encouragement was given by the physiologist to help achieve and 
maintain these thresholds. Energy expenditure between HIIT and CT was not matched.  
 
To assess the validity of the exercise interventions, participant fidelity to the desired exercise 
intensity was determined using cut points of >85% HRmax and 60-80% HRmax for HIIT and CT 
respectively and reported using previous examples [32,33]. These values were calculated using the 
participants mean heart rate for each individual interval or station over the 16 sessions and was 
expressed as a percentage of HRmax as determined by CPET at visit 1. Specific fidelity thresholds were 
consulted to determine low (<50%), moderate (50-70%) and high (>70%) compliance [34]. 
Adherence was determined as a percentage of completed sessions, with 14 (> 85%) being the 
threshold for completion.  
 
Statistical Analysis 
Statistical analysis was conducted using SPSS version 24 (IBM, New York, USA). An independent t-
test was used to identify group differences at baseline. Assumptions of normality were verified using 
the Shapiro-Wilk test. Skewness and kurtosis of distribution was visually examined. Non-normally 

distributed data was presented as median and interquartile range (IQR). A two-way (condition x 
time) repeated measures analysis of variance (ANOVA) was used to compare CRF pre-and post-
training. Post-hoc analysis for the main effects and interactions was assessed using a Bonferroni 
adjustment. Group differences were compared using independent t tests. Variables were displayed 
as mean with 95% confidence intervals (95% CI) or standard deviation (±) where specified. Partial eta 
squared (p2) effect sizes were also calculated with 0.01, 0.06 and 0.14 representing small, medium 
and large effect sizes, respectively [35].