Table 2 Exclusion criteria

Performance assessed during recreational, open entry events (e.g. marathon)

Lack of controlled conditions

Performance only measured as an indicator of recovery (e.g. hours or days following an exercise stimulus designed to induce damage/stress) rather than being assessed at maximum performance capacity or more closely following a standardised preload

To reduce the influence of post-exercise recovery, which reflects a different mechanism

Treatment comparator was a food rich in nitric oxide precursors (unless differences in content are quantified)

Difficult to infer how much treatment increases nitric oxide precursors relative to control

Supplement contained >1mg caffeine per kg of body mass, as calculated from participant mean mass

Avoid ergogenic effects seen with higher intakes of caffeine [32]

Performance test intensity was acutely regulated by heart rate or oxygen consumption.

Nitric oxide supplements may alter these internal measures and thereby confound the ability to identify effects on endurance exercise performance.