High Intensity Interval Training Versus Circuit Training: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for High Intensity Interval Training Versus Circuit Training (NCT03700671), a randomized na study. The primary outcome is Maximal Oxygen Consumption (ml.Kg-1.Min-1).

2. Clinical Question

3. Trial Design

The registry describes a randomized, parallel-group trial comparing high intensity interval training with circuit training. Participants were assigned to one of the two training groups, but the method used to generate the random sequence and the process used to conceal allocation are not reported in the registry record. The planned allocation ratio for the trial itself is not reported; equal allocation is stated only as an assumption for planning illustrations.

The recorded enrollment is 42. That figure is not a substitute for the number of participants analyzed in each group. Group-specific counts for the primary outcome, withdrawals, and analysis populations are not reported in the registry record. The adverse-event risk denominators are reported separately and should not be taken as the primary outcome analysis sizes.

Masking is recorded as none. The registry record does not explain which people, if any, knew group assignments during assessment or analysis. The model is parallel, meaning the comparison is between groups assigned to different training approaches rather than a crossover design.

The study is marked completed. Its recorded start date is 2016-05-11, and the primary completion date is 2017-05-11. The phase is listed as NA. The sponsor is the University of Central Lancashire.

The HIIT description specifies cycle ergometry, an intensity above 85% HRmax, and active recovery at 25-50 watts. CT is described as a mixed-modality circuit with intensity of 60-80%, initially performed for 20 minutes, with duration gradually increased. Session frequency, attendance, adherence, and the full progression schedule are not reported in the registry record. The details therefore describe the planned approaches but do not establish how much training participants completed.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

Reasons for not completing this period:

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: no race or ethnicity exclusion

6. Primary Endpoint

The registry lists the primary outcome as Maximal Oxygen Consumption (ml.Kg-1.Min-1), measured over Baseline and 8 weeks.

The primary endpoint is maximal oxygen consumption, also called VO2max in the registry description. It is a continuous measure reported in ml/kg/min, with the endpoint specified as a mean and standard deviation. It was assessed at baseline and 8 weeks. The description says it is determined during a cardiopulmonary exercise test (CPET) and represents the upper limit of aerobic fitness.

The registry’s rationale describes VO2max as relevant to cardiorespiratory fitness. It states that low VO2max is associated with greater risk of premature all-cause and cardiovascular mortality, independent of traditional risk factors and physical activity status. It also describes the possibility that increasing VO2max through exercise training may improve cardiometabolic health, quality of life, and life-expectancy. These statements explain why the measure may be of interest; they are not results from this comparison.

A comparison of group means could describe the average outcome in each group, while a between-group estimate could quantify the difference associated with assignment to the training approaches. Precision would depend on the observed variability and the number of participants contributing outcome data. The registry record does not report the means, standard deviations, changes from baseline, or comparative estimate, so the size and precision of any observed difference cannot be assessed.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes a randomized parallel-group design and a continuous primary endpoint. General statistical explanation: random allocation aims to make the groups comparable on average at the point of assignment, including for characteristics that are not measured. It does not guarantee that every measured or unmeasured characteristic will be balanced in a particular trial. The registry record does not report the sequence-generation method, allocation concealment, or implementation details, so these aspects cannot be evaluated from the record.

How to interpret a continuous outcome

For a continuous measure such as maximal oxygen consumption, a mean describes the center of the observed values, while a standard deviation describes their spread around that mean. The endpoint is specified as a mean with standard deviation, but the registry record does not report the group values. Without those summaries, the degree of variability and the difference between group averages are unknown.

A trial can compare the groups at the 8-week assessment, compare changes from baseline, or use another model that accounts for repeated measurements. These approaches answer related but not identical questions and may rely on different assumptions. The registry record names baseline and 8 weeks as time points, but does not report the analysis model or the exact contrast. It is therefore not possible to determine which method was used or whether the reported analysis, if any, followed a prespecified plan.

What uncertainty measures add

General explanation: a confidence interval describes uncertainty around an estimated effect under the assumptions of the analysis. It is useful to consider both the values covered by the interval and the estimate’s size, rather than treating statistical significance as the only criterion. A p-value describes how compatible the data are with a specified null model; it is not the probability that the null hypothesis is true and does not measure the size of an effect. The registry record does not report a confidence interval or p-value for the primary outcome.

The outcome is measured at more than one time point, and the registry indicates that it has several categories or time points rather than presenting a single reduced comparison. This makes the precise estimand important: the estimand is the treatment effect the analysis is intended to estimate, such as a difference at a chosen assessment or a difference in change. The specific estimand and analysis are not reported in the registry record. An interpretation should not select one after seeing results, because choosing among possible contrasts can affect both the estimate and its uncertainty.

Planning assumptions and observed results

The registry states that sample-size planning illustrations use the observed effect, two-sided significance level 0.05, equal allocation between the two arms, and a normal approximation. The illustrations consider an effect that is a stated fraction of the observed effect. These are planning assumptions, not evidence about the trial’s originally intended sample size or its analysis. The record does not report the original sample-size justification or the numerical planning results. A planning calculation based on an observed effect also depends on that effect estimate; if the estimate is uncertain, the resulting planning values inherit that uncertainty.

General explanation: power is a property of a design and an assumed effect, variability, and analysis procedure. It is not a measure of the probability that a reported finding is true. In particular, a calculation made after results are observed should not be described as observed power or used as a substitute for the estimated effect and its confidence interval. The registry’s planning illustrations are explicitly described as planning illustrations, not as the trial’s own design calculation.

Distributional and model assumptions

A normal approximation is one of the stated planning assumptions, but the record does not identify the model used for the primary outcome analysis. General explanation: statistical models make assumptions about how observations vary and how the outcome relates to the treatment comparison. For a continuous measure, the choice of model can affect standard errors and confidence intervals, especially when data are sparse, highly skewed, or depart from other model assumptions. The registry record does not report diagnostic checks or details that would allow these assumptions to be assessed. It would be inappropriate to infer a particular test or model from the endpoint’s continuous scale alone.

The registry lists 4 outcome measures and 2 analyses, as well as a secondary outcome of oxygen consumption at the ventilatory anaerobic threshold. Those entries do not, by themselves, specify how each outcome or analysis was handled, or whether multiplicity was addressed. The statistical plan, including any adjustment, is not reported in the registry record.

In short, the design and endpoint identify a potentially interpretable comparison, but the statistical quantities required to evaluate its result are absent. Means, standard deviations, effect estimates, uncertainty intervals, and analysis details would be needed to judge both the direction and precision of the primary outcome comparison.

10. Multiplicity, Missing Data and Analysis Populations

The primary outcome is maximal oxygen consumption at baseline and 8 weeks. The registry indicates that the primary outcome has several categories or time points and was not reduced to one comparison. It does not report the specific categories, the primary contrast, or which time point or summary was intended to carry the principal inference.

The record lists 4 outcome measures and 2 analyses, and names oxygen consumption at the ventilatory anaerobic threshold as a secondary outcome. These entries show that more than one outcome or analysis is represented in the record, but they do not establish how many statistical tests were conducted or how results were prioritized. The statistical analysis plan and any method for controlling multiplicity are not reported in the registry record. General explanation: when several hypotheses are tested, the chance of at least one apparently unusual result can increase if no adjustment or other prespecified strategy is used. Whether that concern applies, and how it was handled in this trial, cannot be determined from the record.

Missing data and analysis populations also affect interpretation. The registry record does not report the number of participants with primary outcome measurements at each time point, reasons for missing measurements, or how missing values were handled. It does not report whether the primary analysis included all randomized participants, only participants who completed training, or another analysis population. These details matter because exclusions and missingness can affect comparability between randomized groups and the meaning of the estimated effect.

There is not enough information to assess whether the analysis matched a prespecified plan, whether all intended outcomes were reported, or whether sensitivity analyses were conducted. Those procedures are not reported in the registry record.

11. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

The adverse-event entries report 0 serious adverse events among 20 participants at risk in the HIIT group and 0 among 19 participants at risk in the CT group. The entries for other adverse events also report 0 among 20 at risk in HIIT and 0 among 19 at risk in CT. Deaths are recorded as 0 in each group, and no top serious-event terms are listed.

These figures describe what is recorded for the stated risk groups; they are not a complete assessment of safety. The registry record does not report event definitions, how events were collected or adjudicated, the monitoring period, or whether participants had comparable exposure and follow-up. It also does not provide detail on less serious symptoms beyond the recorded other-event counts. The adverse-event denominators should not be added or treated as the primary outcome analysis sizes.

General explanation: observing no events in a study does not establish that an intervention has no risk. The ability to characterize uncommon events depends on factors including the amount of exposure, follow-up, and how events are sought and classified. With no additional safety detail or comparative analysis reported, the entries support only the narrow statement that the registry records zero events in the listed categories for the stated groups.

12. What the Result Does, and Does Not, Mean

The registry describes a randomized comparison of HIIT and CT, with maximal oxygen consumption as the primary endpoint. It establishes the intended treatment contrast and assessment schedule, but it does not report the endpoint values or comparative analysis. As a result, it does not establish whether average maximal oxygen consumption changed in either group, whether the groups differed, or how precise any estimated difference was.

That distinction matters. A trial’s design and measurement plan describe what investigators set out to compare; the results are needed to estimate what was observed. The enrollment of 42 does not reveal the number contributing data to the primary outcome, the distribution of outcomes, or the size of an effect. The serious and other adverse-event entries are separate from the fitness endpoint and cannot fill that gap.

Even if a difference were reported, statistical interpretation would require its scale, uncertainty, and the analysis used. A point estimate alone would not describe precision, and a p-value alone would not convey how large or practically important the difference might be. The registry record does not report the information needed for those judgments. It also does not report participant characteristics or sufficient training-completion information to understand how broadly any result might apply.

The absence of comparative results should not be interpreted as proof that HIIT and CT have the same effect. It also does not support a claim that either approach is superior. It means that the comparative primary outcome cannot be evaluated from the information recorded. The trial’s stated rationale and endpoint description explain why maximal oxygen consumption was chosen, but they are not evidence of an observed benefit.

For the same reason, planning illustrations based on an observed effect cannot establish the strength of the trial’s findings. Planning calculations concern hypothetical design performance under assumptions; they are not a replacement for the observed estimate, its uncertainty, and a transparent account of the analysis. The record supports a description of the design and recorded adverse-event counts, but not a conclusion about comparative effectiveness.

13. Why This Trial Matters Statistically

This trial is statistically interesting because it directly compares two exercise-training approaches using a continuous measure of cardiorespiratory fitness. Direct comparisons can address a different question from studies that assess each approach separately: whether outcomes differ between the approaches under the conditions studied. The registry’s rationale notes that direct comparisons of HIIT and CT are limited.

The design is recorded as randomized and parallel, which identifies the intended basis for a group comparison. The primary outcome is maximal oxygen consumption, with assessments at baseline and 8 weeks. These features make clear what kind of evidence the trial was designed to collect. However, a design label and endpoint definition are not enough to evaluate an effect; the group results, analysis, and uncertainty are essential.

The missing comparison also illustrates why specifying the estimand matters. With a baseline and a later assessment, a report needs to make clear whether it estimates a difference at follow-up, a difference in change, or another contrast. The registry indicates multiple categories or time points but does not reduce the outcome to one comparison. Without an explicit contrast and its result, readers cannot tell what effect an estimate would represent.

The record’s planning assumptions offer another statistical distinction. A sample-size illustration based on an observed effect is not observed power and does not demonstrate that a study had adequate precision for its primary question. The estimate and its confidence interval remain the central quantities for interpreting a comparison. In this record, the primary outcome results needed for that interpretation are not reported.

Thus, the trial matters as a clearly framed direct comparison, while its registry entry also demonstrates a reporting limitation: intended design information does not substitute for analyzable outcome results. The current record allows readers to identify the question and the stated methods, but not to quantify the answer.

14. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The main trial-specific limitation for interpretation is that the registry record does not report the primary outcome values or the between-group result. Means, standard deviations, changes from baseline, confidence intervals, and a primary-outcome p-value are not reported in the registry record. The direction, size, and precision of the comparison therefore remain unknown.

The outcome is specified at baseline and 8 weeks, but the registry says it has several categories or time points and does not reduce it to one comparison. The exact primary contrast and its analysis are not reported in the registry record. Without that information, readers cannot determine which treatment effect the intended analysis was meant to estimate.

The participant population and eligibility criteria are not described in detail. The registry record does not report participant characteristics, group-specific primary outcome analysis sizes, training attendance, adherence, withdrawals, or reasons for missing outcome measurements. These omissions limit assessment of how the randomized groups were followed and how applicable any result might be.

Intervention details are also incomplete. HIIT is described on cycle ergometry, while the CT description gives its stations and intensity and says duration gradually increased after an initial 20 minutes. The later duration, session frequency, and complete schedule are not reported in the registry record. These omissions make it difficult to characterize the delivered training dose.

Finally, allocation concealment, masking procedures beyond the record’s designation of none, the statistical analysis plan, multiplicity handling, and missing-data methods are not reported in the registry record. These gaps restrict evaluation of bias and analytical choices. The adverse-event counts are recorded, but event definitions and collection procedures are not described. These limitations concern the available record and do not establish that the procedures were absent from the conduct of the trial.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: High Intensity Interval Training Versus Circuit Training — NCT03700671.
• Registry record last updated 2020-02-17; status last verified 2020-02.
• Results first posted 2020-02-17.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

18. Record Summary

NCT03700671 is a completed randomized, parallel-group comparison of high intensity interval training and circuit training, sponsored by the University of Central Lancashire. Enrollment is recorded as 42. The study start date is 2016-05-11, and the primary completion date is 2017-05-11. Masking is recorded as none, and phase is listed as NA.

The primary endpoint is maximal oxygen consumption in ml/kg/min, assessed at baseline and 8 weeks. The registry record does not report the group outcome values or the comparative analysis, so the effect and its precision cannot be evaluated. It also does not report the detailed analysis plan, primary outcome missing-data handling, or participant flow.

For safety, the record lists 0 serious and 0 other adverse events in each group, with deaths recorded as 0 in each group. These entries do not establish the absence of risk. They are limited to the recorded categories and denominators.