Table 2:

Recommendations for future research.

Essential		Additional
Participants
Sex/Age/Race	Include male and female subjects as well as subjects with different race and ethnicity.	Investigate adolescents and older subjects, because they are underrepresented in the current literature.
Inclusion criteria	Use pre-specified inclusion criteria based on age, sex, and level of fitness.	Depending on the research question consider including further criteria, such as training history, habitual exercise time or chronotype.
Blinding	Ensure that subjects are unaware of the performance they achieved at a certain time of the day until the end of the study.	Blind the investigator analyzing the data.
Sample size	A priori sample size needs to be performed at least for the primary outcome of the study.	If subgroup analyses are planned (e.g. for different chronotypes) the sample size needs to be adequate for these analyses too.
Measurements
Time points	Measure at least five time points during the day.	Previous studies mainly focused on the time windows from 07:00 to 20:00. Consider investigating broader time windows.
Gold standard	Use gold standard methods for each outcome.	Ensure that the test-retest reliability is sufficient to enable showing the difference anticipated in the sample size calculation.
Outcomes	If two or more outcomes are measured pre-specify the primary one.	For all studies investigating strength performance we recommend to include hand grip strength, countermovement jump and isometric mid-thigh-pull-up to the protocol.
Familiarization	Perform at least one familiarization trial.	Time trials for example might require more than one familiarization trial.
Calibration	Ensure that the devices are calibrated in sufficient intervals according to the recommendation of the manufacturer.
External factors	Ensure equal preparation (i.e. warm-up, preparation before test) and equal external factors (i.e. room temperature, humidity).
Randomization	Randomize the sequence of time points measured.
Verification	Implement methods to ensure that subjects actually reached their maximum (e.g. secondary VO2max criteria).	Measure each time of the day twice to ensure that the pattern of diurnal variation seen is repetitive and to assess the coefficient of variation at each time of the day.
Regeneration	Ensure sufficient regeneration time between tests. The required duration will depend on the task and can be short for tasks like hand grip strength test but may be long for long duration time trials.	Include physiological markers (e.g. blood) to ensure the regeneration.
Monitoring during study period
Sleep	Give instructions to maintain a constant sleeping routine.	Use diaries, questionnaires or ideally objective methods to monitor sleep.
Physical activity	Give instructions how long to refrain from exercise before each test.	Use diaries, questionnaires or ideally objective methods to monitor activity.
Nutrition	Give instructions such as allowed diets, required fasting durations before the tests, or time windows subjects are allowed to eat.	Use diaries, questionnaires or ideally less subjective methods such as recording nutrition and meal timing by taking pictures with a reference sized measure.
Reporting
Study	Report everything that is recommended by CONSORT.
Results	Report effect sizes or mean differences including 95% confidence intervals
Real data	Report data as it is measured (e.g. L/min, N, s, cm – ideally SI-units) and not only as percentage values.	Report absolute and relative data (e.g. L/min and mL/kg/min or N and N/kg) and if possible report different units for the same outcome (e.g. jump height in cm, flight time in s, and peak power N/kg for the countermovement jump).
Times of day	Report all times of the day tested and not only the peak and nadir.
Deviations	Report if the actual test time deviated from intended test time. Report for which outcomes and time points data is missing.
Outcomes	Report all outcomes measured or provide an explanation if some outcomes are not reported.