Background Physical activity has been implicated in improving symptom management and quality of life in Huntington’s disease (HD). Verifying the role of physical activity in HD requires accurate quantification of exercise metrics.

Aim To assess the reliability and validity of Fitbit® Charge 4 step-count compared to research accelerometers and observer count in HD patients.

Methods 17 manifest HD participants completed two indoor 2-minute walking tests (2MWT) while wearing a Fitbit® charge 4, and an ActiGraph GTX9 on their non-dominant wrist, ActivPAL4™ on both anterior thighs, and ActiGraph GTX9 on both anterior shanks. Steps were manually counted from video recordings of the 2MWTs. Step-counts for devices were obtained from their proprietary software. Intraclass correlation coefficients (ICC) determined Fitbit and observer reliability, whilst Bland-Altman (B-A) analysis demonstrated monitor agreement.

Results Intra-rater reliability of researcher count from repeat trials was excellent (ICC 0.95/n = 17) meanwhile, Fitbit yielded good reliability (ICC 0.81/n = 16). B-A plots displayed greatest agreement between Fitbit and manual count (bias -4.7/SD 36.2). Average monitor count revealed largest consistencies with ActivPAL4 and Fitbit (bias -3.4/SD 37.4) whereas, wrist-worn ActiGraph deviated markedly (bias 4.9/SD 66.9). Motor symptom severity did not influence average Fitbit count although, upper extremity devices occasionally underestimated steps produced.

Conclusions Whilst superior step-count accuracy was seen with devices placed on lower limbs, these data show that Fitbit charge 4 may be a reliable device for monitoring steps in manifest HD.