PM-EKF: A Physiological Model-Based Extended Kalman Filter for Daily-Life Physical Activity Energy Expenditure Estimation
Pith reviewed 2026-05-08 03:06 UTC · model grok-4.3
The pith
A simplified physiological model inside an extended Kalman filter estimates daily physical activity energy expenditure from IMU motion and heart rate measurements.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that a simplified physiological model explicitly linking IMU-captured body-center-of-mass motion to time-varying cardiac output and metabolic gas-exchange processes, when formulated as a nonlinear state-space system and embedded in an EKF, produces PAEE estimates with median R² of 0.72 (range 0.60-0.87) against COSMED K5 reference data and outperforms linear regression (median R² 0.52) and CNN-LSTM (median R² 0.65) on the same nine-subject dataset while remaining interpretable.
What carries the argument
The simplified physiological model that links three-IMU body-center-of-mass motion to measured heart-rate cardiac output and underlying metabolic gas-exchange processes, cast as a nonlinear state-space system for EKF state estimation.
If this is right
- PAEE estimates become available in real time with explicit handling of sensor noise and model mismatch.
- IMU mechanical workload dominates accuracy, so heart-rate data may be dropped without major loss in similar protocols.
- Estimates remain personalized and physiologically interpretable rather than opaque.
- The framework supports continuous daily-life monitoring without laboratory equipment.
- Performance exceeds both linear and recurrent neural baselines on the tested dataset.
Where Pith is reading between the lines
- Wearable devices could adopt this approach to give users ongoing, explainable feedback on energy expenditure during normal routines.
- Extending the model to include additional signals such as skin temperature or breathing rate might further improve robustness outside controlled settings.
- The dominance of motion data suggests prioritizing improvements in IMU placement and signal quality for broader applicability.
- Similar state-space physiological modeling could be tested for related quantities such as fatigue or recovery estimation.
Load-bearing premise
The simplified physiological model accurately represents the relationship between measured body movements and actual metabolic energy expenditure across the range of daily activities.
What would settle it
A larger free-living study collecting simultaneous IMU, heart-rate, and indirect-calorimetry data over full days of varied activities where the model's median R² falls substantially below 0.6 would falsify the reliability claim.
Figures
read the original abstract
Monitoring physical activity energy expenditure (PAEE) in daily life is essential for characterizing individual health and metabolic status. Although indirect calorimetry provides gold-standard PAEE measurements, it is impractical for continuous daily-life monitoring. Consequently, wearable sensing approaches using inertial measurement units (IMUs) and heart rate (HR) sensors have attracted substantial interest. However, most existing IMU- and HR-based methods are purely data-driven and offer limited physiological interpretability. In this work, we propose a simplified physiological model that explicitly links body movement during activities of daily living to the underlying metabolic gas-exchange processes governing PAEE. The model is formulated as a nonlinear state-space system and embedded within an Extended Kalman Filter (EKF), enabling principled handling of measurement noise, model uncertainty, and system nonlinearities. The proposed framework provides personalized, interpretable PAEE estimates without employing black-box models. Our model was validated using a dataset, including 9 subjects with around 50 minutes of measurements per subject, collected in our lab simulating a free-living condition. Using the respiratory data measured by COSMED K5 as reference and explained variance (R^2) as evaluation metric, our model's predicted PAEE yielded median (min-max) R^2 = 0.72 (0.60--0.87), using three IMUs (pelvis and two thighs) for capturing the body-center-of-mass motion and measured HR for the time-varying cardiac output. Our model outperformed a linear regression (LR) model (R^2 = 0.52 (0.23--0.92)) and CNN-LSTM model (R^2 = 0.65 (0.46--0.78)) on the same dataset. Notably, excluding the sensory HR measurement did not significantly degrade PAEE estimation of all three models, indicating that IMU-captured mechanical workload dominated PAEE estimation performance in our protocol.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes PM-EKF, a nonlinear state-space physiological model of physical activity energy expenditure (PAEE) embedded in an Extended Kalman Filter. The model explicitly links IMU-derived body center-of-mass kinematics (pelvis + bilateral thighs) and heart-rate measurements to time-varying cardiac output and oxygen uptake. On a 9-subject dataset of ~50 min lab-simulated ADL per subject, the filter yields median R² = 0.72 (range 0.60–0.87) against COSMED K5 reference, outperforming linear regression (0.52) and CNN-LSTM (0.65). The authors note that removing HR input does not significantly degrade performance.
Significance. A correctly specified physiological state-space model would supply an interpretable, parameter-light alternative to black-box regressors for wearable PAEE monitoring and could enable principled uncertainty propagation. The reported dominance of IMU kinematics over HR is a practically useful observation for sensor-minimization studies.
major comments (2)
- [§3 and §4] §3 (Physiological Model) and §4 (EKF Formulation): the central justification for using an EKF rather than a generic regressor is that the state equations correctly encode the IMU-to-metabolic linkage. No ablation that disables individual physiological terms (e.g., the cardiac-output or mechanical-work sub-models) or direct comparison of internal filter states (predicted VO₂, stroke volume) against the COSMED reference is presented; only end-to-end PAEE R² is reported. This leaves the load-bearing modeling assumption untested.
- [§5] §5 (Experiments): the evaluation uses only 9 subjects and laboratory-simulated ADL. No subject-independent cross-validation statistics, no hold-out free-living recordings, and no power analysis for the reported median R² difference are provided. With such a small cohort the performance gap versus CNN-LSTM could be driven by overfitting or protocol-specific artifacts rather than model fidelity.
minor comments (2)
- [Table 1 and §5.2] Table 1 and §5.2: clarify whether the linear-regression and CNN-LSTM baselines received identical IMU + HR feature sets and whether their hyperparameters were tuned on the same cross-validation folds as the EKF.
- [§2] §2 (Related Work): several recent IMU-only PAEE papers using biomechanical energy models are cited only in passing; a brief quantitative comparison of their reported R² ranges would strengthen the positioning.
Simulated Author's Rebuttal
We thank the referee for the constructive comments and the positive evaluation of the work's significance. We address each major comment point by point below, indicating planned revisions where appropriate.
read point-by-point responses
-
Referee: [§3 and §4] §3 (Physiological Model) and §4 (EKF Formulation): the central justification for using an EKF rather than a generic regressor is that the state equations correctly encode the IMU-to-metabolic linkage. No ablation that disables individual physiological terms (e.g., the cardiac-output or mechanical-work sub-models) or direct comparison of internal filter states (predicted VO₂, stroke volume) against the COSMED reference is presented; only end-to-end PAEE R² is reported. This leaves the load-bearing modeling assumption untested.
Authors: We agree that targeted ablations would strengthen the justification for the physiological structure. In the revised manuscript we will add results from two ablations: (i) disabling the cardiac-output sub-model (setting cardiac output to a constant) and (ii) disabling the mechanical-work term, reporting the resulting degradation in PAEE R² for each case. For internal states, COSMED K5 directly measures VO₂, so we will add a comparison of the filter's predicted VO₂ time series against the reference VO₂ in a new figure and subsection of §4. Stroke volume and cardiac output are not measured by the reference system; we will therefore report the estimated cardiac-output trajectories and discuss their physiological plausibility against literature values for ADL rather than claiming direct validation. These changes will be incorporated as a partial revision. revision: partial
-
Referee: [§5] §5 (Experiments): the evaluation uses only 9 subjects and laboratory-simulated ADL. No subject-independent cross-validation statistics, no hold-out free-living recordings, and no power analysis for the reported median R² difference are provided. With such a small cohort the performance gap versus CNN-LSTM could be driven by overfitting or protocol-specific artifacts rather than model fidelity.
Authors: We accept that the current evaluation is limited by cohort size and protocol. We will re-run the experiments with leave-one-subject-out cross-validation, reporting mean ± std R² across the nine folds for PM-EKF, LR, and CNN-LSTM. A post-hoc power analysis will be added to quantify the statistical reliability of the median R² differences. The dataset contains only laboratory-simulated ADL and no free-living recordings, so we cannot supply hold-out free-living results; this limitation will be stated more explicitly in §5 and the conclusions. revision: partial
- The study does not contain free-living recordings, preventing any hold-out validation on unconstrained daily-life data.
Circularity Check
No significant circularity detected; derivation remains self-contained against external reference
full rationale
The paper constructs a simplified physiological model linking IMU-captured body-center-of-mass motion and HR to metabolic gas-exchange processes, casts it as a nonlinear state-space system, and applies EKF for state estimation. PAEE predictions are evaluated via R^2 against independent COSMED K5 indirect calorimetry measurements on held-out lab-simulated ADL data from 9 subjects; this reference is external to the model inputs and not used for fitting the core physiological equations. The model also outperforms purely data-driven LR and CNN-LSTM baselines on identical data, and the note that HR removal does not degrade performance is reported as an empirical observation rather than a definitional necessity. No self-citations, parameter-fitting loops that rename fits as predictions, ansatzes smuggled via prior work, or uniqueness theorems appear in the derivation chain. The approach is therefore grounded in first-principles physiological relations plus external validation rather than reducing to its own inputs by construction.
Axiom & Free-Parameter Ledger
Reference graph
Works this paper leans on
-
[1]
Ekelund, U., Brage, S., Franks, P.W., Hennings, S., Emms, S. and Wareham, N.J., 2005. Physical activity energy expenditure predicts progression toward the metabolic syndrome independently of aerobic fitness in middle-aged healthy Caucasians: the Medical Research Council Ely Study. Diabetes care, 28(5), pp.1195-1200
work page 2005
-
[2]
Hills, A.P., Mokhtar, N. and Byrne, N.M., 2014. Assessment of physical activity and energy expenditure: an overview of objective measures. Frontiers in nutrition, 1, p.5
work page 2014
-
[3]
New methods for calculating metabolic rate with special reference to protein metabolism
de V Weir, J.B., 1949. New methods for calculating metabolic rate with special reference to protein metabolism. The Journal of physiology, 109(1-2), p.1
work page 1949
-
[4]
and Murphy- Alford, A.J., 2019
Speakman, J.R., Pontzer, H., Rood, J., Sagayama, H., Schoeller, D.A., Westerterp, K.R., Wong, W.W., Yamada, Y ., Loechl, C. and Murphy- Alford, A.J., 2019. The international atomic energy agency interna- tional doubly labelled water database: aims, scope and procedures. Annals of Nutrition and metabolism, 75(2), pp.114-118
work page 2019
-
[5]
Jeran, S., Steinbrecher, A. and Pischon, T., 2016. Prediction of activity- related energy expenditure using accelerometer-derived physical ac- tivity under free-living conditions: a systematic review. International journal of obesity, 40(8), pp.1187-1197
work page 2016
-
[6]
Hedegaard, M., Anvari-Moghaddam, A., Jensen, B.K., Jensen, C.B., Pedersen, M.K. and Samani, A., 2020. Prediction of energy expen- diture during activities of daily living by a wearable set of inertial sensors. Medical engineering & physics, 75, pp.13-22
work page 2020
-
[7]
Santos, D.A., Silva, A.M., Matias, C.N., Magalh ˜aes, J.P., Fields, D.A., Minderico, C.S., Ekelund, U. and Sardinha, L.B., 2014. Validity of a combined heart rate and motion sensor for the measurement of free- living energy expenditure in very active individuals. Journal of Science and Medicine in Sport, 17(4), pp.387-393
work page 2014
-
[8]
Assah, F.K., Ekelund, U., Brage, S., Wright, A., Mbanya, J.C. and Wareham, N.J., 2011. Accuracy and validity of a combined heart rate and motion sensor for the measurement of free-living physical activity energy expenditure in adults in Cameroon. International journal of epidemiology, 40(1), pp.112-120
work page 2011
-
[9]
Montoye, A.H., Vusich, J., Mitrzyk, J. and Wiersma, M., 2018. Heart rate alters, but does not improve, calorie predictions in Fitbit activity monitors. Journal for the Measurement of Physical Behaviour, 1(1), pp.9-17
work page 2018
-
[10]
Alvarez-Garcia, J.A., Cvetkovi ´c, B. and Lu ˇstrek, M., 2020. A sur- vey on energy expenditure estimation using wearable devices. ACM Computing Surveys (CSUR), 53(5), pp.1-35
work page 2020
- [11]
-
[12]
Scientific Reports, 15(1), p.36277
Interpretable deep learning for personalized energy expenditure prediction using ECG and acceleration signals in incremental exercise. Scientific Reports, 15(1), p.36277
-
[13]
Jung, J., Lim, H., Jeong, H., Upadhye, S., Kim, J.H. and Park, S., 2025. Estimation of Walking Energy Expenditure using a Single Sacrum-Mounted IMU Based on Biomechanically-Informed Machine Learning
work page 2025
-
[14]
Optimal state estimation: Kalman, H infinity, and nonlinear approaches
Simon, D., 2006. Optimal state estimation: Kalman, H infinity, and nonlinear approaches. John Wiley & Sons
work page 2006
-
[15]
and Wang, Y ., 2022, September
Thoonen, M., Veltink, P., Halfwerk, F., Van Delden, R. and Wang, Y ., 2022, September. A movement-artefact-free heart-rate prediction system. In 2022 Computing in Cardiology (CinC) (V ol. 498, pp. 1-4). IEEE
work page 2022
-
[16]
IMU Data Processing to Recognize Activities of Daily Living with a Smart Headset
Aranburu, A., 2018. IMU Data Processing to Recognize Activities of Daily Living with a Smart Headset. University of California, Santa Cruz
work page 2018
-
[17]
Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters
De Leva, P., 1996. Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters. Journal of biomechanics, 29(9), pp.1223-1230
work page 1996
-
[18]
Bar-Shalom, Y ., Li, X.R. and Kirubarajan, T., 2001. Estimation with applications to tracking and navigation: theory algorithms and soft- ware. John Wiley & Sons
work page 2001
-
[19]
Que, S., van Dartel, D., Heeringa, I., Hegeman, H., V ollenbroek- Hutten, M. and Wang, Y ., 2026. Synthetic Data Guided Feature Selection for Robust Activity Recognition in Older Adults. arXiv preprint arXiv:2601.17053
-
[20]
Misconceptions about aerobic and anaerobic energy expenditure
Scott, C., 2005. Misconceptions about aerobic and anaerobic energy expenditure. Journal of the International Society of Sports Nutrition, 2(2), p.32
work page 2005
-
[21]
Chiari, L., Avanzolini, G. and Ursino, M., 1997. A comprehensive sim- ulator of the human respiratory system: validation with experimental and simulated data. Annals of biomedical engineering, 25, pp.985-999
work page 1997
-
[22]
McArdle, W.D., Katch, F.I. and Katch, V .L., 2010. Exercise physiol- ogy: nutrition, energy, and human performance. Lippincott Williams & Wilkins
work page 2010
-
[23]
Hardman, J.G. and Aitkenhead, A.R., 2003. Estimating alveolar dead space from the arterial to end-tidal CO2 gradient: a modeling analysis. Anesthesia & Analgesia, 97(6), pp.1846-1851
work page 2003
-
[24]
Grodins, F.S., Buell, J. and Bart, A.J., 1967. Mathematical analysis and digital simulation of the respiratory control system. Journal of applied physiology, 22(2), pp.260-276
work page 1967
-
[25]
Fincham, W.F. and Tehrani, F.T., 1983. A mathematical model of the human respiratory system. Journal of biomedical engineering, 5(2), pp.125-133
work page 1983
- [26]
-
[27]
Journal of Applied Physiology, 18(3), pp.447-456
Effect of exercise on pulmonary diffusing capacity. Journal of Applied Physiology, 18(3), pp.447-456
-
[28]
O’regan, R.G. and Majcherczyk, S., 1982. Role of peripheral chemore- ceptors and central chemosensitivity in the regulation of respiration and circulation. Journal of experimental biology, 100(1), pp.23-40
work page 1982
-
[29]
Iturriaga, R., Alcayaga, J., Chapleau, M.W. and Somers, V .K., 2021. Carotid body chemoreceptors: physiology, pathology, and implications for health and disease. Physiological reviews, 101(3), pp.1177-1235
work page 2021
- [30]
-
[31]
Muscle blood flow is reduced with maintenance of arterial perfusion pressure
Relation between central and peripheral hemodynamics during exercise in patients with chronic heart failure. Muscle blood flow is reduced with maintenance of arterial perfusion pressure. Circulation, 80(4), pp.769-781
-
[32]
Zhang, Y ., Fresiello, L., Veltink, P.H., Donker, D.W. and Wang, Y ., 2025. Physiological-Model-Based Neural Network for Heart Rate Estimation during Daily Physical Activities. arXiv preprint arXiv:2506.10144
-
[33]
Wijayasiri, L. and McCombe, K., 2017. The Primary FRCA Structured Oral Exam Guide 1. CRC Press
work page 2017
-
[34]
(2016) Regulation of Tissue Oxygenation
Pittman, R.N. (2016) Regulation of Tissue Oxygenation. Chapter 3: Gas Transport by the Blood. San Rafael (CA): Morgan & Claypool Life Sciences
work page 2016
-
[35]
Pontzer, H., Yamada, Y ., Sagayama, H., Ainslie, P.N., Andersen, L.F., Anderson, L.J., Arab, L., Baddou, I., Bedu-Addo, K., Blaak, E.E. and Blanc, S., 2021. Daily energy expenditure through the human life course. Science, 373(6556), pp.808-812
work page 2021
-
[36]
Control of energy expenditure in humans
Westerterp, K.R., 2017. Control of energy expenditure in humans. European journal of clinical nutrition, 71(3), pp.340-344
work page 2017
-
[37]
and H ¨ormann-Wallner, M., 2021
N ¨osslinger, H., Mair, E., Toplak, H. and H ¨ormann-Wallner, M., 2021. Underestimation of resting metabolic rate using equations compared to indirect calorimetry in normal-weight subjects: Consideration of resting metabolic rate as a function of body composition. Clinical Nutrition Open Science, 35, pp.48-66
work page 2021
-
[38]
Ainsworth, B.E., Haskell, W.L., Whitt, M.C., Irwin, M.L., Swartz, A.M., Strath, S.J., O’brien, W.L., Bassett, D.R., Schmitz, K.H., Emplaincourt, P.O. and Jacobs, D.R., 2000. Compendium of physical activities: an update of activity codes and MET intensities. Medicine & science in sports & exercise, 32(9), pp.S498-S516
work page 2000
-
[39]
Paraschiakos, S., de S ´a, C.R., Okai, J., Slagboom, P.E., Beekman, M. and Knobbe, A., 2022. A recurrent neural network architecture to model physical activity energy expenditure in older people. Data Mining and Knowledge Discovery, 36(1), pp.477-512
work page 2022
-
[40]
Available from:https://www.movella.com/ products/wearables/movella-dot
Movella DOT;. Available from:https://www.movella.com/ products/wearables/movella-dot
-
[41]
CardioBAN kit;. Available from:https://www. pluxbiosignals.com/products/cardioban
-
[42]
DeBlois, J.P., White, L.E. and Barreira, T.V ., 2021. Reliability and va- lidity of the COSMED K5 portable metabolic system during walking. European journal of applied physiology, 121, pp.209-217
work page 2021
-
[43]
and Evi- dence Analysis Working Group, 2006
Compher, C., Frankenfield, D., Keim, N., Roth-Yousey, L. and Evi- dence Analysis Working Group, 2006. Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review. Journal of the American Dietetic Association, 106(6), pp.881-903
work page 2006
-
[44]
Hermann, R. and Krener, A., 2003. Nonlinear controllability and observability. IEEE Transactions on automatic control, 22(5), pp.728- 740
work page 2003
-
[45]
Guyton, A.C. and Hall, J.E., 2006. Medical physiology. G ¨okhan N, C ¸ avus ¸o˘glu H (C ¸ eviren), 3
work page 2006
-
[46]
Lee, C.J. and Lee, J.K., 2024. IMU-Based Energy Expenditure Esti- mation for Various Walking Conditions Using a Hybrid CNN–LSTM Model. Sensors, 24(2), p.414
work page 2024
-
[47]
Que, S., Poelarends, R., Veltink, P., V ollenbroek-Hutten, M. and Wang, Y ., 2025. Accelerometry-based Energy Expenditure Estimation During Activities of Daily Living: A Comparison Among Different Accelerometer Compositions. arXiv preprint arXiv:2502.10112
-
[48]
Bouten, C.V ., Westerterp, K.R., Verduin, M. and Janssen, J.D., 1994. Assessment of energy expenditure for physical activity using a triaxial accelerometer. Medicine and science in sports and exercise, 26(12), pp.1516-1523
work page 1994
-
[49]
Individual comparisons by ranking methods
Wilcoxon, F., 1992. Individual comparisons by ranking methods. In Breakthroughs in statistics: Methodology and distribution (pp. 196- 202). New York, NY: Springer New York
work page 1992
-
[50]
Teoria statistica delle classi e calcolo delle prob- abilita
Bonferroni, C., 1936. Teoria statistica delle classi e calcolo delle prob- abilita. Pubblicazioni del R istituto superiore di scienze economiche e commericiali di firenze, 8, pp.3-62
work page 1936
- [51]
-
[52]
Medicine and science in sports and exercise, 33(6), pp.939-945
Estimating energy expenditure by heart-rate monitoring without individual calibration. Medicine and science in sports and exercise, 33(6), pp.939-945
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.