HIIT the Gym: The effects of High Intensity Interval Training (HIIT) on improving fatigue in Multiple Sclerosis

The intervention is High-Intensity Interval Training (HIIT) using stationary cycle ergometry conducted in people with multiple sclerosis. Participants are stratified into high-fatigue and low-fatigue groups based on age- and education-adjusted Modified Fatigue Impact Scale (MFIS) cut-off scores; however, all participants receive the same intervention protocol. Materials required for the intervention include stationary cycle ergometers (Fluid E750 Cycle Body Trainer) and Polar H10 heart-rate monitors for continuous heart-rate monitoring. The intervention procedure involves participants attending supervised HIIT cycling sessions for 8 weeks (two 30-minute sessions per week; 16 sessions total). Each session begins with a 5-minute warm-up at low intensity, followed by 10 repeated cycles of 90 seconds of high-intensity cycling and 30 seconds of moderate-intensity cycling for active recovery. Each session concludes with a 5-minute cool-down at low intensity. The session structure, prescribed intensity zones, and cycling cadence are standardised across participants. High-intensity intervals target a Borg Rating of Perceived Exertion (RPE) of 15–19, corresponding to approximately 85–95% of age-predicted maximal heart rate (HRmax), while moderate-intensity intervals target an RPE of 12–14, corresponding to approximately 60–75% HRmax. Cycling cadence is standardised at approximately 80–100 revolutions per minute during high-intensity intervals and 60–80 revolutions per minute during moderate-intensity intervals. Cycling resistance is adjusted individually for each participant to achieve the target intensity. Sessions are delivered face-to-face on a one-to-one basis by tertiary-qualified Exercise Physiologists with experience in neurological and multiple sclerosis rehabilitation. Absolute workload (e.g. cycling resistance) is individualised and adjusted within or between sessions by the Exercise Physiologist based on continuous heart-rate monitoring, RPE ratings, fatigue, and clinical presentation to safely maintain the intended relative intensity of the intervention. All sessions are conducted within an outpatient clinical setting in a rehabilitation gym located in Sydney, Australia. Exercise intensity and intervention fidelity are monitored during each session using continuous heart-rate monitoring (Polar H10) and repeated RPE ratings recorded by the supervising Exercise Physiologist. Adherence is assessed as the proportion of prescribed sessions attended, calculated as the percentage of completed sessions out of the total 16 scheduled sessions. Any protocol deviations, intensity adjustments, or adverse events are documented in standardised session records.