Accelerometer Measurements Research Articles

Deep Learning-Based DAS to Geophone Data Transformation

Seismic data are the primary way to study the subsurface structure and properties. A conventional seismic sensor like geophone or accelerometer measures particle velocity or acceleration at a local point only, while distributed acoustic sensing (DAS) measures dynamic strain along the fiber optic cable at densely spaced sample points, where strain rate is obtained over certain gauge length interval. Therefore, DAS measures subsurface properties with high sampling resolution and large coverage. When an optical fiber is installed in a well, DAS can provide continuous, dense downhole recording. However, currently, most of the seismic processing, imaging, and inversion techniques are developed for geophone data. These well-established techniques can be readily and properly utilized if DAS data are transformed into geophone measurements, such as particle velocity. In this study, we present a recurrent neural network (RNN) framework to perform this transformation. This effectiveness of the deep learning-based mapping is then demonstrated with a field measurement data, showing that DAS data can be transformed into particle velocity accurately and robustly using the proposed deep-learning approach.

GNSS/accelerometer integrated deformation monitoring algorithm based on sensors adaptive noise modeling

The Global Navigation Satellite System (GNSS) and accelerometer integrated system can obtain more stable and reliable deformation displacement with the full use of the complementary technical characteristics of these two types of sensors. The Kalman Filter (KF) method is often applied to integrate GNSS three-dimensional (3D) coordinates and high-rate accelerometer measurements. The key to obtaining accurate solutions is the reasonable setting of measurement noise and process noise of KF. However, in some complex deformation monitoring scenarios such as landslides, both GNSS and accelerometers are susceptible to interference from factors like vegetation cover and external dynamic disturbance. Traditional adaptive filtering methods is hard to simultaneously and appropriately adjust the two types of noise matrices. To address this problem, an adaptive noise model based on sensors is proposed. The adaptive measurement noise is constructed using the posterior coordinate covariance derived from GNSS-RTK process. The random walk coefficient corresponding to the process noise is estimated online via the standard deviation (STD) of acceleration time-varying information. A series of numerical experiments about simulated three-dimensional (3D) deformation displacement are carried out to validate the proposed algorithm. The results show that: (1) When the GNSS signal is disturbed by complex environment, the adaptive measurement noise model effectively contributes to the accuracy improvement, from decimeter level to centimeter level. (2) In the presence of unstable deformable bodies, the adaptive process noise algorithm significantly suppresses the divergent deformation time series caused by external accelerometer disturbances. (3) Even if suffering the overlying effects of electromagnetic interference and external disturbance, the centimeter-level deformation monitoring accuracy can be achieved by using the joint adaptive measurement and process noise model. The improvement effect is particularly significant in the main deformation direction.

Update and Novel Validation of a Pregnancy Physical Activity Questionnaire.

The aim of this study was to update and validate the Pregnancy Physical Activity Questionnaire (PPAQ), using novel and innovative accelerometer and wearable camera measures in a free-living setting, to improve the measurement performance of this method for self-reporting physical activity. A prospective cohort of 50 eligible pregnant women were enrolled in early pregnancy (mean= 14.9 weeks' gestation). In early, middle, and late pregnancy, participants completed the updated PPAQ and, for 7 days, wore an accelerometer (GT3X-BT; ActiGraph, Pensacola, Florida) on the nondominant wrist and a wearable camera (Autographer; OMG Life (defunct)). At the end of the 7-day period, participants repeated the PPAQ. Spearman correlations between the PPAQ and accelerometer data ranged from 0.37 to 0.44 for total activity, 0.17 to 0.53 for moderate- to vigorous-intensity activity, 0.19 to 0.42 for light-intensity activity, and 0.23 to 0.45 for sedentary behavior. Spearman correlations between the PPAQ and wearable camera data ranged from 0.52 to 0.70 for sports/exercise and from 0.26 to 0.30 for transportation activity. Reproducibility scores ranged from 0.70 to 0.92 for moderate- to vigorous-intensity activity and from 0.79 to 0.91 for sports/exercise, and were comparable across other domains of physical activity. The PPAQ is a reliable instrument and a valid measure of a broad range of physical activities during pregnancy.

Sedentary and physical activity time differs between self-reported ATLS-2 physical activity questionnaire and accelerometer measurements in adolescents and young adults in the United Arab Emirates

BackgroundMost young adults and adolescents in the United Arab Emirates (UAE) do not meet the established internationally recommended physical activity levels per day. The Arab Teen Lifestyle Study (ATLS) physical activity questionnaire has been recommended for measuring self-reported physical activity of Arab adolescents and young adults (aged 14 years to mid-twenties). The first version of the ATLS has been validated with accelerometers and pedometers (r ≤ 0.30). The revised version of the questionnaire (ATLS-2, 2021) needs further validation. The aim of this study was to validate the self-reported subjective sedentary and physical activity time of the ATLS-2 (revised version) physical activity questionnaire with that of Fibion accelerometer-measured data.MethodsIn this cross-sectional study, 131 healthy adolescents and young adults (aged 20.47 ± 2.16 [mean ± SD] years (range 14–25 years), body mass index 23.09 ± 4.45 (kg/m2) completed the ATLS-2 and wore the Fibion accelerometer for a maximum of 7 days. Participants (n = 131; 81% non-UAE Arabs (n = 106), 13% Asians (n = 17) and 6% Emiratis (n = 8)) with valid ATLS-2 data without missing scores and Fibion data of minimum 10 h/day for at least 3 weekdays and 1 weekend day were analyzed. Concurrent validity between the two methods was assessed by the Spearman rho correlation and Bland-Altman plots.ResultsThe questionnaire underestimated sedentary and physical activity time compared to the accelerometer data. Only negligible to weak correlations (r ≤ 0.12; p > 0.05) were found for sitting, walking, cycling, moderate intensity activity, high intensity activity and total activity time. In addition, a proportional/systematic bias was evident in the plots for all but two (walking and moderate intensity activity time) of the outcome measures of interest.ConclusionsOverall, self-reported ATLS-2 sedentary and physical activity time had low correlation and agreement with objective Fibion accelerometer measurements in adolescents and young adults in the UAE. Therefore, sedentary and physical activity assessment for these groups should not be limited to self-reported measures.

Technology assisted physical activity among hospitalised medical patients a cluster randomised clinical trial

Abstract Funding Acknowledgements Type of funding sources: Public hospital(s). Main funding source(s): Bispebjeg University Hospital (public hospital) Danish Physiotherapy Foundation. Background To investigate whether patients, with independent basic mobility, admitted to medical wards, by simple feedback on physical activity (the integration of accelerometer measurements and tablet feedback) can be nudged to increase the level of activity during hospitalization. The aim is to investigate if medical patients with intact basic mobility, can increase the level of physical activity by simple feedback from a new technology solution. Methods Design: A cluster randomised clinical trial. Primary outcome: time spend out of bed (min/day). Power calculation: 320 patients. All included patients had their physical activity level measured. The intervention group received feedback by an app on a tablet. The patients participated for a minimum of 24 hours and up to 7 days. Inclusion of patients from the Department of Carardiology, Geriatric Department and Department of Pulmonary and Infectious disease. Primary efficacy measure: Time out of bed (min). Results 354 patients were included. Difference between those who received feedback (intervention) and those without feedback (control) was 23.6 min/day more out of bed (95%CI 13.75 – 33.50); P = <.0001. Conclusion Patients admitted to medical wards, with independent basic mobility, were on average 23.6 minutes more physically active per day (21%) when they received feedback about their physical activity. Implication This technological solution with feedback can be a tool to nudge the patients with independent basic mobility to stay physically active during hospitalization. The solution may also allocate resources for those patients who have a greater need for help and support for mobilization.

A System to Track Stent Location in the Human Body by Fusing Magnetometer and Accelerometer Measurements.

This paper will introduce a simple locating system to track a stent when it is deployed into a human artery. The stent is proposed to achieve hemostasis for bleeding soldiers on the battlefield, where common surgical imaging equipment such as fluoroscopy systems are not available. In the application of interest, the stent must be guided to the right location to avoid serious complications. The most important features are its relative accuracy and the ease by which it may be quickly set up and used in a trauma situation. The locating approach in this paper utilizes a magnet outside the human body as the reference and a magnetometer that will be deployed inside the artery with the stent. The sensor can detect its location in a coordinate system centered with the reference magnet. In practice, the main challenge is that the locating accuracy will be deteriorated by external magnetic interference, rotation of the sensor, and random noise. These causes of error are addressed in the paper to improve the locating accuracy and repeatability under various conditions. Finally, the system's locating performance will be validated in benchtop experiments, where the effects of the disturbance-eliminating procedures will be addressed.

Continuous bridge displacement estimation using millimeter-wave radar, strain gauge and accelerometer

Monitoring of bridge displacement is essential for providing critical information regarding the health of bridge structures. However, continuous and accurate monitoring of structural displacement remains challenging. This study proposes a bridge displacement estimation technique that combines an accelerometer, a strain gauge, and a millimeter-wave radar considering intermittent radar target occlusion common in long-term displacement monitoring. The technique primarily estimates displacement using radar and accelerometer measurements but switches to using strain and acceleration measurements when radar targets are occluded. A radar target occlusion detection algorithm is developed to automatically achieve this switching. Automated initial calibration is performed to select suitable targets from all radar-detected targets and estimate the conversion factor for converting radar-based displacement from the line-of-sight direction to the actual movement direction. An artificial neural network model is automatically trained for strain–displacement transformation without the need for any pre-knowledge of a target structure. The proposed technique was validated via a laboratory test on a 10-m-long beam-type structure and a field test on a pedestrian steel box-girder bridge. The proposed technique correctly detected the intermittent radar target occlusion, and continuously estimated displacements for up to 75 min. The intermittent radar target occlusion leads to errors of a few millimeters in the displacements estimated using radar and accelerometers. However, the proposed method accurately estimates displacements with errors of less than 0.1 mm.

Adaptive Precise Attitude Estimation Using Unscented Kalman Filter in High Dynamics Environments

In this paper, a novel adaptive scaled unscented Kalman filter (ASUKF) algorithm is developed using low-cost micro-electro-mechanical system (MEMS) triaxial gyroscope and accelerometer. The body non-gravitational accelerations are estimated and used to compensate the accelerometers measurements. The estimated external acceleration is used to adapt the acceleration measurement noise covariance matrix to achieve robustness during harsh environments. The proposed ASUKF uses the adapted covariance matrix and the compensated accelerometer measurements to precisely estimate the body attitude angles. The achieved accuracies for the proposed model are discussed and compared with other state-of-the-art algorithms through a laboratory and field tests. The results show that the proposed algorithm achieves an outstanding level accuracy in high dynamics environments in comparison to other attitude estimators.

Attitude in Motion: Constraints Aided Accurate Vehicle Orientation Tracking in Harsh Environment

Accurate vehicle orientation tracking in 3 dimensions or 3D attitude tracking, is an essential requirement for various vehicle stability and safety applications. In this paper, we consider the problem of vehicle attitude tracking along with its external acceleration estimation using low cost inertial measurement units. The current state of the art approaches are unable to estimate orientation when the body is in motion for a prolonged period, as the accelerometer measurements are corrupted by external accelerations that are produced due to body movements. In this paper, a novel filtering framework is proposed to eliminate the acceleration induced uncertainty for body observing severe and prolonged external acceleration, which is based on linear Kalman filter. We show that the sensor constraints allow the removal of external acceleration from all all of its axes. This approach is unique and superior from current state of the art as it estimates the acceleration without using any information from additional sensors such as wheel encoder, GPS, and/or camera. A unique state formulation is proposed which incorporate magnetometer to compliment accelerometer in rotation matrix frame work. Simulations and real world experimental tests are conducted to verify the performance of the proposed algorithm in various dynamic conditions.

Study on the Thermospheric Density Distribution Pattern during Geomagnetic Activity

The atmospheric density of the thermosphere is a fundamental parameter for spacecraft launch and orbit control. Under magnetic storm conditions, the thermospheric atmospheric density experiences significant fluctuations, which have a negative impact on spacecraft control. Exploring thermospheric density during geomagnetic storms can help to mitigate the effects of such events. Research on the inversion of accelerometer measurements for different satellites and the variations of atmospheric density under extreme conditions is still in its infancy. In this paper, the distribution of atmospheric density during three geomagnetic storms is investigated from the inversion results of the Swarm-C accelerometer. Three major geomagnetic storms and their recovery phases are selected as case studies. The thermospheric density obtained by Swarm-C is separated into day and night regions. The empirical orthogonal function analysis method is used to study the spatiotemporal distribution of thermospheric density during geomagnetic storms. The results indicate that storms have a more significant impact on nighttime thermospheric density. The impact of magnetic storms on the temporal distribution of thermospheric density is considerable. The first-order empirical orthogonal function (EOF) time coefficient value on the day after the storm is the largest, reaching 2–3 times that before the magnetic storm. The impact of magnetic storms on atmospheric density is mainly reflected in the time distribution. The spatial distribution of atmospheric density is less affected by magnetic storms and is relatively stable in the short term. The impact of magnetic storms on the spatial distribution of nighttime thermospheric density is more significant than that of daytime regions, and the response of daytime regions to magnetic storms is slower.

Boosted Convolutional Neural Network Algorithm for the Classification of the Bearing Fault form 1-D Raw Sensor Data.

Renewable energy sources are a growing branch of industry. One such source is wind farms, which have significantly increased their number over recent years. Alongside the increased number of turbines, maintenance problems are growing. There is a need for newer and less intrusive predictive maintenance methods. About 40% of all turbine failures are due to bearing failure. This paper presents a modified neural direct classifier method using raw accelerometer measurements as input. This proprietary platform allows for better damage prediction results than convolutional networks in vibration spectrum image analysis. It operates in real time and without signal processing methods converting the signal to a time-frequency spectrogram. Image processing methods can extract features from a set of preset features and based on their importance. The proposed method is not based on feature extraction from image data but on automatically finding a set of features from raw tabular data. This fact significantly reduces the computational cost of detection and improves the failure detection accuracy compared to the classical methods. The model achieved a precision of 99.32% on the validation set, and 96.3% during bench testing. These results were an improvement over the method that classifies time-frequency spectrograms of 97.76% for the validation set and 90.8% for the real-world tests, respectively.

Supine MDS-UPDRS-III Assessment: An Explorative Study.

The Movement Disorder Society Unified Parkinson's Disease Rating Scale-part III (MDS-UPDRS-III) is designed to be applied in the sitting position. However, to evaluate the clinical effect during stereotactic neurosurgery or to assess bedridden patients with Parkinson's disease (PD), the MDS-UPDRS-III is often used in a supine position. This explorative study evaluates the agreement of the MDS-UPDRS-III in the sitting and the supine positions. In 23 PD patients, the MDS-UPDRS-III was applied in both positions while accelerometric measurements were performed. Video recordings of the assessments were evaluated by two certified raters. Agreement between the sitting and supine MDS-UPDRS-III was studied using Cohen's kappa coefficient. Relationships between the MDS-UPDRS-III tremor scores and accelerometric amplitudes were calculated for both positions with linear regression. A fair to substantial agreement was found for MDS-UPDRS-III scores of individual items in the sitting and supine positions, while combining all tests resulted in a substantial agreement. The inter-rater reliability was fair to moderate for both positions. A logarithmic relationship between tremor scores and accelerometric amplitude was revealed for both the sitting and supine positions. Nevertheless, these data are insufficient to fully support the supine application of the MDS-UPDRS-III. Several recommendations are made to address the sensitivity of the scale to inter-rater variability. In conclusion, although an overall substantial agreement between sitting and supine MDS-UPDRS-III is confirmed, its application in the supine position is not endorsed for the whole range of its individual items. Caution is warranted in interpreting the supine MDS-UPDRS-III, pending additional research.

Child perceived motor competence as a moderator between physical activity parenting and child objectively measured physical activity

This longitudinal study aimed to examine how physical activity parenting (PAP) directly predicted objectively measured children's moderate-to-vigorous physical activity (MVPA) and sedentariness over a three-year transitional period from early to middle childhood, and second, whether the children's perception of motor competence (PMC) mediated or moderated the influence of PAP to children's MVPA or sedentariness. At time 1 (T1), PAP and children's (N=396, mean age 5.80, SD 1.04) PA were assessed by parental questionnaire. Three years later, at time 2 (T2), children's (N=396, mean age 8.80, SD 1.04) PMC was measured by a validated pictorial scale, and MVPA and sedentariness were measured by accelerometers. All the analyses were conducted using the Mplus statistical package (Version 8.4). The models were adjusted for the following covariates: children's PA (T1), gender (T1), age (T1), mean accelerometer measurement in hours per day (T2), and parents' education level (T1). Results showed that PAP at T1 did not significantly predict level of MVPA or sedentary time at T2 and, therefore, PMC did not mediate the PAP-children's MVPA or sedentary time relationship either. However, PMC significantly moderated the relationship between PAP and MVPA but not between PAP and sedentary time. The results suggested that parental support positively predicts children's MVPA among children with low PMC but not among children with high PMC. This unique finding proposes that family-based PA interventions could benefit from screening of children with low PMC and provision of PA counselling to their parents.

Calibration and validation of a space electrostatic accelerometer onboard Tianzhou-1 cargo spacecraft using GNSS and attitude data

Calibration and validation of a space electrostatic accelerometer onboard Tianzhou-1 cargo spacecraft using GNSS and attitude data

The substantiation of the application of the integral criterion at the data processing

The problem of online calibration of navigational instruments is considered. Calibration of devices implies minimizing the deviations between the real values and the measurement data. In the problem under investigation, the trajectory of motion is traced with GPS, which is taken as the real values of the trajectory coordinates. The accelerometer and gyroscope measurement data can also be used to calculate the trajectory. It is required by calibrating the sensors to achieve matching of these values, with the proximity of trajectories implying a uniform metric. It is proved in the article that the integral metric can be used in solving the problem of sensor calibration.

Influence of Viscous Lubricant on Nucleation and Propagation of Frictional Ruptures

AbstractFluids are pervasive in the Earth's crust and saturate fractures and faults. The combination of fluids and gouge layers developing along faults can generate fluids of different viscosities. Such viscous fluids were found to influence the reactivation, frictional stability of faults, and eventually the dynamics of propagating earthquake ruptures. We reproduced laboratory earthquakes on analog material (PMMA) to study the influence of viscous lubricant on fault frictional stability, rupture nucleation, and propagation under mixed lubrication conditions. Experiments were conducted in dry conditions, and with fluids presenting a viscosity ranging from 1 to 1,000 mPa.s. Through photoelasticity, high‐frequency strain gauge sensors, and accelerometer measurements, we obtained new insights about the influence of lubricant on a characteristic nucleation length, rupture propagation velocity, and local slip and slip rate evolution during the reproduced frictional ruptures. Our experiments show that the presence of a lubricant generating mixed lubricated conditions along the fault induces, (a) a reduction of the frictional resistance, (b) an increase in nucleation length, (c) a decrease in the fracture energy. In addition, the larger the viscosity of the fluids, the larger the reduction of frictional strength and the increase in the nucleation length. Moreover, ruptures occurring under mixed lubricated conditions showed a pulse‐like rather than crack‐like behavior, suggesting that viscous lubrication can induce the transition from crack‐like to pulse‐like rupture along natural faults. We demonstrate, supported by existing theory, that this transition is mainly governed by the stress acting on the fault at the onset of nucleation, which is drastically reduced in presence of a lubricant.

MAVEN Accelerometer Observations of Thermospheric Densities During Aerobraking and Deep Dip 2: Wave Features and Connections to Upward Propagating Thermal Tides

AbstractIn early 2019, the Mars Atmosphere and Volatile Evolution (MAVEN) mission underwent an ∼2‐month aerobraking campaign during which time the spacecraft periapsis altitude was lowered from its nominal altitude range of 140–160 km to as low as ∼123 km. Excluding spacecraft walk‐in/out maneuvers, accelerometer measurements were made along 272 orbits with coverage spanning Ls 340–3°, latitudes ∼5°–54°S, longitudes 0–360°, and local solar time ∼22–17 hr. In this study, we perform a diagnostic analysis of the full aerobraking data set by fitting 4‐harmonic waves to mass densities. We then study the variations of these waves as a function of latitude with an emphasis on those observed previously in Mars' thermosphere by MAVEN and other missions. Additionally, we utilize data collected during the same time period from the Mars Reconnaissance Orbiter's Mars Climate Sounder to study the vertical propagation of waves originating from the middle atmosphere. Key results indicate that normalized wave amplitudes decrease with latitude, and this is consistent with the latitudinal structure of a diurnal Kelvin mode. We also observe that waves imprinted from the middle atmosphere show normalized amplitude growth with increasing altitude. A complete summary of data sets, analysis methodology, and scientific results is given. The purpose of this study is to add to the body of knowledge surrounding Martian atmospheric wave features and to provide further constraints for future numerical modeling and subsequent tidal mode identification.

Vertical channel stabilization of barometer‐aided inertial navigation systems by optimal control

AbstractThis study investigates the problem of inertial navigation system's (INS's) vertical channel instability, which primarily results from the positive gravity feedback that takes place in the numerical integration step of the accelerometer measurements. Traditional solutions to the problem include using information from auxiliary sensors, typically barometers and global navigation satellite systems (GNSSs), integrated into INSs through feedback control loops (referred to as mechanizations) and/or Kalman filters (KFs). Most studies on this subject lack clarity regarding the methodology used for the mechanization tuning (by showing excessive empiricism) and/or disregard the benefits of using optimal control techniques for the purpose of the latter. This study, therefore, reviews the main mechanization used for vertical channel stabilization of baro‐inertial integrations and presents, as its main contribution, the performance analysis of new tuning techniques based on optimal control theory, namely, by linear quadratic regulator (LQR) and by minimization of performance indices such as the integral absolute error (IAE), integral squared error (ISE), integral of time multiplied absolute error (ITAE), and integral of time multiplied squared error (ITSE). To validate the proposed tunings, the integrated altitude and vertical velocity mean errors (MEs) and standard deviations (SDs) are used as metrics, which are compared with the corresponding MEs/SDs of the empirical tuning with highest performance/acceptance in the literature. We also evaluate the ability of the mechanizations to fulfill their ultimate goal, namely, to track the reference barometer input, by also attenuating its noise. Based on simulated and experimental results, the present study demonstrates and validates the effectiveness/robustness of the proposed optimal tunings, particularly those based on LQR and ITAE minimization.

Effect of nationwide school policy on device-measured physical activity in Danish children and adolescents: a natural experiment.

A new Danish school policy with a requirement for 45min physical activity daily during school hours was introduced in 2014. The objective of this natural experiment was to evaluate the effect of this nationwide school policy on physical activity in Danish children and adolescents. Four historical studies completed between 2009 and 2012 comprised the pre-policy study population. Post-policy data were collected in 2017/18. All post-policy schools were represented in the four pre-policy studies. Age-groups and seasons were matched. In total, 4816 children and adolescents aged 6-17 were included in the analyses (2346 pre-policy and 2470 post-policy). Children and adolescents were eligible if they had accelerometer measurements and did not have any physical disabilities preventing activity. Physical activity was measured by accelerometry. Main outcome was any bodily movement. Secondary outcomes were moderate to vigorous physical activity and overall movement volume (mean counts per minute). The school policy interrupted a linear decreasing pre-policy trend in physical activity during school hours. All activity outcomes increased post-policy during a standardized school day (8:10 am-1 pm). Increases were more pronounced in the youngest children. Specifically, we observed a daily increase during a standardized school day in 2017/2018 of 14.2min of movement (95% CI: 11.4-17.0, p<0.001), 6.5min of moderate to vigorous physical activity (95% CI: 4.7-8.3, P<0.001), and 141.8counts per minute (95% CI: 108.5-175.2, P<0.001). A national school policy may be an important strategy to increase physical activity during school hours among children and adolescents. The Danish Foundation TrygFonden has funded the PHASAR project (ID 115606).

Abstract MP05: Prospective Associations of Midlife Physical Activity and Sedentary Behavior With Brain Structure: The Cardia Study

Introduction: Strong evidence suggests physical activity may prevent cognitive decline. Less research has explored potential associations of physical activity with brain structure. This is an important research gap given that age related alterations in brain structure are thought to occur prior to signs of cognitive decline. Objective: To determine if accelerometer measured sedentary time (SED), light-intensity physical activity (LPA), and moderate-to-vigorous intensity physical activity (MVPA) at ages 38-50 years are prospectively associated with brain structure. Methods: We studied 562 Black and White men and women enrolled in CARDIA who participated in the 2005-06 exam (baseline) and exams 5 (2010-11) and/or 10 years later (2015-16). SED, LPA, and MVPA were measured by the ActiGraph 7164 accelerometer at baseline. Magnetic resonance imaging (MRI) was used to quantify whole brain, white matter, gray matter, and abnormal tissue volume of white matter at the 5- and/or 10-year follow-up. Compositional isotemporal substitution analysis examined associations of SED, LPA, and MVPA at baseline with repeated measures (unstructured covariance) of MRI measures at follow-up after accounting for intracranial volume and adjusting for confounders ( Table ). Results: Consistent with our hypothesis, substituting 30 minutes of SED to LPA was associated with 0.044% greater total white matter volume (95% CI: 0.001, 0.086). The effect size was larger when substituting SED to MVPA, but this association was not statistically significant. There were no associations observed between accelerometer measures and total brain, total gray matter, or abnormal white matter tissue volumes. Associations did not differ by sex or age. Conclusions: Statistical substitution of time from SED to higher intensity physical activity (LPA or MVPA) was associated with greater total white matter volume. It is unclear why this association was observed in white matter only. Additional research with longer follow-up is needed to confirm these findings.