Questions: Can circuit class therapy provide sufficient exercise dosage (at least 20 minutes at ≥. 50% heart rate reserve or total caloric expenditure ≥. 300 kilocalories) to induce a cardiorespiratory fitness effect in adults with traumatic brain injury? Can feedback from heart rate monitors influence exercise intensity? Design: Randomised controlled trial within an observational study. Participants: Fifty-three people with severe traumatic brain injury, of whom 40 progressed into the trial. Intervention: All participants undertook circuit class therapy. Participants allocated to the experimental group received exercise intensity feedback from a heart rate monitor and the control group received no feedback. Outcome measures: Proportion of participants exercising at ≥ 50% heart rate reserve for at least 20 minutes or expending ≥ 300 kilocalories during circuit class therapy. The primary outcome measure for the trial was the time spent in the heart rate training zone (ie, at ≥ 50% heart rate reserve) during the intervention and re-assessment periods. Results: Circuit class therapy provided sufficient cardiorespiratory exercise dosage for 28% (95% CI 18 to 42) of the cohort according to the heart rate reserve criteria and 62% (95% CI 49 to 74) according to the caloric criteria. Feedback did not increase the time in the training zone during the intervention (mean difference 4.8 minutes, 95% CI -1.4 to 10.9) or re-assessment (1.9 minutes, -4.4 to 8.3) periods. Conclusion: The low intensity, long duration structure of circuit class therapy can provide sufficient exercise dosage for a fitness training effect for 62% of people with traumatic brain injury. Feedback from heart rate monitors does not necessarily influence exercise intensity. Trial registration: ACTRN12607000522415.