Aim: To determine the criterion validity of the Activ8a and VitaMoveb activity monitors (AM) in measuring body postures and movements (P&M) in adults with spastic cerebral palsy (CP) with different gross motor functional abilities in a simulated free-living environment. Methods: In this observational validation study participants were included between March and June 2016. Participants were divided into two groups: ambulatory and non-ambulatory adults (wheelchair users). For ambulatory adults, three Activ8 monitors (attached to the frontal thigh, lateral thigh, and in the pant pocket) and three synchronized VitaMove monitors (attached to the chest and each leg) were worn while participants were video recorded when performing a standardized activity protocol. For non-ambulatory adults, two synchronized Activ8 monitors (attached to the dominant wrist and wheelchair wheel) and three VitaMove monitors (attached to the chest and each wrist) were worn while participants were video recorded when performing a standardized wheelchair protocol. A native Dutch student provided standardized instructions while the author video recorded participants. Activities were performed in a movement laboratory and simulated free-living environment. Output data of the Activ8 and the VitaMove monitors were compared with video observation as a criterion measure. Absolute and relative time differences were calculated for each detected body P&M and for total measurement time for each participant. The following body P&M categories were detected: sitting, standing, walking, running, cycling, wheelchair driving, active upper body (stable wheelchair), and assisted driving. Agreement between the Activ8 and VitaMove during activity measurement was determined for each activity using Spearman Rho correlation coefficients, which were then compared with Meng’s test. Adapted Bland-Altman plots were completed to determine agreement at an individual level and Wilcoxon Signed-Rank tests were calculated to evaluate differences between detected P&M time across measures. Relative time differences of <10% between measures were indicative of acceptable validity. Results: Fifteen adults with spastic CP [10 men; mean(SD) age, 35.7(13) years; Gross Motor Function Classification System distribution: level I (n=6), level II (n=5), level III (n=3), level IV (n=1)] were included. For ambulatory adults (n=14), criterion validity of the Activ8 at the lateral thigh location during basic and complex daily life activities was moderate to good (average relative time differences: 0.25% for sitting, 4.69% for standing, 2.46% for walking, 3.19% for cycling, and 1.96% for light-to-moderate upright activity), except for running (34.6% average difference) The criterion validity of the VitaMove was acceptable, with the exception of standing and running (average relative time differences: 1.69% for sitting, 12.3% for standing, 4.84% for walking, 9.70% for cycling, 23.8% for running, and 0.91% for light-to-moderate upright activity). Spearman Rho correlation coefficients were significantly greater between video/lateral thigh Activ8 than video/frontal thigh Activ8 and video/pant pocket Activ8 for P&M categories sitting, basic standing, basic walking, and light-to-moderate upright activity (p<0.01 for all). Moreover, Spearman Rho correlation coefficients were significantly greater between video/Activ8 than video/VitaMove for basic walking and light-to-moderate upright activity P&M categories (p<0.01). In non-ambulatory, wheelchair users (n=3), the Wheelchair Activ8 demonstrated greater validity than the VitaMove monitor, when compared to video observation, for the detection of sedentary upper body and active upper body behaviours. Conclusion: The Activ8, positioned on the lateral thigh, demonstrates adequate validity as a tool for monitoring of body P&M during free-living activities in ambulatory adults with spastic CP. The Activ8 demonstrated superior validity as a direct measure of body P&M compared to the VitaMove AM. The sample size of non-ambulatory adults was small (n=3) and allowed for descriptive and exploratory analysis only; therefore, validity of the Wheelchair Activ8 for has to be further examined in a larger number of participants. Clinical relevance: The Activ8 and Wheelchair Activ8 shows promise as a clinical measurement tool of physical behaviour for adults with CP across GMFCS levels I-IV. The Activ8 could be used in the future development of health promotion initiatives, due to the features of goal setting and user feedback, while providing researchers greater insight in activity data than consumer-grade AMs. Suppliers: a. Activ8, trademark of Remedy Distribution Ltd. (original equipment manufacturer is 2M Engineering Ltd.), John F Kennedylaan 3, 5555XC, Valkenswaard, The Netherlands b. VitaMove, 2M Engineering Ltd., John F Kennedylaan 3, 5555XC, Valkenswaard, The Netherlands / Thesis / Master of Science (MSc)