Abstract Munguía-Izquierdo D, Santalla A, Legaz-Arrese A. Evaluation of a wearable body monitoring device during treadmill walking and jogging in patients with fibromyalgia syndrome. Objective To evaluate the reliability and validity of a body monitoring device against measures obtained from indirect calorimetry (IC) in patients with fibromyalgia syndrome (FMS) during various incremental exercise intensities. Design Cross-sectional reliability and validity study. Setting Testing was completed in a university exercise physiology laboratory. Participants Women (N=25) with FMS, with a mean age ± SD of 48.6±8.4 years and a median symptom duration of 15 years (25th–75th percentiles, 10–23y), were recruited to the study. Interventions Not applicable. Main Outcome Measures Patients walked and jogged on a treadmill at 4 intensities (50m·min –1, 0% grade [n=25]; 83.3m·min –1, 0% grade [n=25]; 116.7m·min –1, 0% grade [n=21]; 116.7m·min –1, 2.5% grade [n=13]) during 2 measurement conditions, while IC and a multiple-sensor body monitor measured energy expenditure (EE). The differences between the readings (test 1 – test 2) and the SD of the differences, intraclass correlation coefficient (ICC), 95% confidence interval (CI) for the ICC, coefficient of repeatability, intrapatient SD, standard error of mean (SEM), minimal detectable change, Wilcoxon signed-rank test, and Bland-Altman graphs were used to examine reliability. The magnitude of the associations between IC and the body monitoring device, ICC, 95% CI for the ICC, paired t tests, and Bland-Altman graphs were used to examine the validity of the body monitoring device versus the IC. Results Moderate to excellent test-retest reliability was found for the 4 bouts of exercise (ICC=.73–.76). The SEM and minimal detectable change were satisfactory for the 4 bouts of exercise (.54–1.18kcal·min –1 and 1.51–3.28kcal·min –1, respectively). The differences mean between test and retest were lower than the SEM for the 4 bouts of exercise, varying from –.17 to .14kcal·min –1. No significant differences were found between test and retest for any bout. The Bland-Altman plots and the coefficients of repeatability indicated that the differences between repeated tests would lie within 2 SDs in 95% of the cases for the 4 bouts of exercise. Significant associations were found between the body monitoring device and IC measurements of EE for the 4 bouts of exercise ( r=.87–.99). The differences for all bouts between the 2 methods were nonsignificant, except for the second bout ( P<.001). The ICCs and Bland-Altman plots of EE for the 4 bouts showed high agreement (ICCs=.84–.99) and sufficient accuracy for quantifying EE during exercise in patients with FMS. Conclusions The body monitoring device provided a valid and reliable estimate of EE in patients with FMS during walking on horizontal and inclined surfaces in a laboratory setting across various exercise intensities.