REVIEW 4 major objections 4 minor 21 references
Personalised 3D Human Digital Twin with Soft-Body Feet for Walking Simulation
T0 review · 4 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The paper claims that adding personalised soft-body feet to a skeletal human model lets a walking controller trained only on kinematic motion capture produce ground reaction forces comparable to measured data.
desk verdict Useful MuJoCo integration for personalised soft-body feet, but the main GRF result is partly an artifact of tuning and offsets applied to the same data used for evaluation. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the MuJoCo flex object: a deformable mesh simulated with hyperelastic material properties, here a 15-vertex simplified foot generated from an SMPL body shape and attached to the skeletal model by hard pins, chosen because welds and connects are soft contacts that caused oscillation. The mechanism is that the flex foot deforms under ground contact and generates the ground reaction force; tuning its stiffness (2000 to 12000) and damping (100) plus shifting foot position lets the deformation produce a smooth, M-shaped force curve without the skeleton itself touching the floor.
What would settle it
Run model E with its fixed stiffness, damping, and foot position on a second subject's motion capture without re-tuning any parameters; if the vertical ground-reaction-force experimental match falls below the bare-skeleton baseline, the claim that personalised soft feet generate realistic forces from kinematic-only training would be refuted. A simpler check is to re-evaluate model E on the same subject without applying the reported phase offset and record how much the experimental match drops.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that a uniform, 15-vertex hyperelastic foot mesh pinned to a skeleton is sufficient for a kinematic-only trained walking policy to reproduce the measured double-bump vertical ground reaction force profile. By stiffening the foot material from model A through model E and adjusting foot position, the authors eliminate skeleton-floor contact spikes and bring joint angles closer to the reference, with model E outperforming the bare skeleton on the experimental-match metric. The paper attributes residual phase differences between simulated and measured forces to shape simplification of the foot, and applies offsets when comparing curves.
Load-bearing premise
The load-bearing premise is that a 15-vertex, uniformly elastic foot pinned to the skeleton behaves like a real human foot in contact, and that the stiffness increases and phase offsets used during evaluation are legitimate corrections rather than tuning to the measured force data.
Editorial extensions
If this is right
- If the claim is correct, personalised digital twins can be generated from motion capture alone and used to predict ground reaction forces in scenarios where force plates are not available.
- The same soft-foot approach could be transferred to other skeletal or musculoskeletal models in the same physics engine, since only the foot contact needs to be replaced.
- Stiffness tuning of the soft foot becomes a practical lever for matching contact dynamics: too soft feet cause intermittent skeleton-floor contact and force spikes, while too stiff feet approximate the bare skeleton.
- The reported improvement in experimental match from 0.558 to 0.628 suggests that soft-body contact can add dynamic realism on top of kinematic fidelity without changing the control objective.
Reading between the lines
- The paper leaves open whether the phase offsets applied when comparing simulated to measured ground reaction forces are legitimate gait-phase alignment or a post-hoc correction; removing the offsets and reporting the drop in experimental match would clarify how much of the result comes from the model itself.
- The stiffness and foot position were tuned on the same subject's force data used for evaluation, so a held-out subject test would show whether the personalisation procedure generalises or merely fits.
- Because only the left foot had enough complete recordings to form a reliable standard deviation, the force comparison is one-footed; extending the capture protocol to the right foot would verify the claim for both limbs.
- The 15-vertex simplification is likely the main source of the phase offset and the initial gradual rise in force, so increasing mesh resolution while keeping the material model fixed could test whether the offset shrinks.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a personalised 3D human digital twin in MuJoCo that augments an existing skeletal (SK) model with soft-body feet represented as simplified flex objects. The feet are generated from motion-captured SMPL body shapes, reduced to roughly 15 vertices, attached to the skeleton with hard pins, and assigned hyperelastic material properties. A walking control policy is trained for several variants: the original skeleton (SK), two softer-foot models (A and B), and a final stiffened/repositioned model (E). The models are evaluated against recorded ground reaction forces (GRF) using an experimental-match (EM) metric and against joint angles using a linear-fit method (LFM). The paper reports that model E achieves the best GRF match (average EM 0.628 vs 0.558 for SK) and joint-angle results close to the SK model and reference motion.
Significance. If the evaluation were unbiased, the paper would make a useful engineering contribution: it demonstrates a low-cost way to add personalised soft-body contact to an existing skeletal model while training the control policy only on kinematic motion capture. The pipeline is concrete and reproducible in principle, and the use of real subject data, multiple stiffness variants, EM, and LFM metrics are strengths. However, the central empirical claim is not currently supported because the material properties, foot position, and phase offsets are tuned against the same recordings used for evaluation, and no held-out validation is performed. The kinematic results also show that model E is generally worse than the bare skeleton, so the soft-foot benefit rests almost entirely on the confounded GRF comparison. The contribution is therefore promising but requires a substantially strengthened evaluation before the reported claims can be accepted.
major comments (4)
- [§3.2 and Table 4] The central GRF comparison is confounded because the EM values for models A, B, and E are computed only after hand-applied phase offsets were used to 'better match the GRF profiles,' and the offset values are not reported. Because these offsets are chosen on the four gait cycles from the same recordings on which EM is then evaluated, the EM values in Table 4 are fit statistics rather than unbiased predictions; consequently, the improvement of model E over model SK (0.628 vs 0.558) does not by itself support the abstract's claim that the soft-body feet generate GRF comparable to real measured data. Please report the offsets, justify them independently of the evaluation data (e.g., from a calibration gait cycle), and recompute EM without data-dependent alignment.
- [§2.3 and §3.1] The material stiffness and foot position were fine-tuned using the GRF and joint-angle results of the same recordings used for the evaluation, as stated in Section 3.1: 'the material properties of the flex and the position of the flex foot were fine-tuned to improve the GRF and joint angle results.' This creates a direct circularity for the main empirical claim: the reported match of model E may largely reflect fitting to the test data. Please describe the tuning procedure (search grid, number of configurations tried, stopping criterion), and validate the final model on held-out gait cycles or subjects not used during tuning.
- [§3.3 and Table 5] The joint-angle results do not support the claim that model E 'closely follow[s] joint angle results of the bare skeletal model.' In Table 5, the mean R² of model SK is higher than that of model E for five of the six joints (e.g., L Ankle 0.950 vs 0.838; R Knee 0.928 vs 0.918), and model E only exceeds SK on R Ankle (0.945 vs 0.893). The paper should either soften the kinematic claim or provide an analysis showing that the differences are within measurement noise and not systematically worse.
- [§3.2] The GRF evaluation is limited to one foot and a small number of recordings: six complete left-foot recordings, with the right foot excluded because only three complete recordings were available. With n=6, the mean and standard deviation of the reference GRF are noisy, and no statistical test is provided for the EM differences across models; the per-cycle EM ranges for SK (0.54–0.58) and E (0.60–0.67) do not overlap, but it is unclear whether this is significant given the small sample. At a minimum, report per-recording values, the number of subjects, and a confidence interval or test for the EM comparison.
minor comments (4)
- [Table 1] The connective tissue Young's modulus range is printed as '1.5 × 10^6 – 2.25 × 10^5 kPa', which is in descending order and uses units that appear inconsistent with the muscle and fat rows; please verify the values and units.
- [§3.2] The sentence 'The higher EM values of model A over model SK confirm this' contradicts Table 4, where model A has average EM 0.463 ± 0.013 and model SK 0.558 ± 0.022; please correct the text or the table.
- [§3.1] The statement that 'the model took on average 11.7 seconds to train and around 4200 iterations to converge' alongside 'the base skeletal model required 5.3 seconds per iteration' suggests a missing 'per iteration' in the first quantity; please clarify.
- [General] Please add a data/code availability statement or clarify whether the models, motion-capture recordings, and evaluation scripts are available for replication.
Circularity Check
Reported GRF match is a tuned fit: stiffness, foot position, and phase offsets are adjusted on the evaluation recordings before computing the EM metric that supports the abstract's claim.
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fitted input called prediction
[Section 3.1 (and Section 2.3); Table 3 and Table 4]
"From the initial results of the training, the material properties of the flex and the position of the flex foot were fine-tuned to improve the GRF and joint angle results. This was done by progressively increasing the stiffness of the flex material and evaluating the GRF results based on the approaches detailed below. Material properties that produced better GRF results were deemed to be more representative properties."
The stiffness and foot position of the flex model are selected by comparing simulated GRF with the force-plate recordings, and then those same recordings are used to compute the EM metric in Table 4 that supports the claim 'comparable to real measured data.' Model E's higher EM (0.628 vs SK's 0.558) is therefore a measure of how well the model was tuned to the evaluation data, not an independent prediction of the measured GRF. The SK baseline received no such tuning.
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fitted input called prediction
[Section 3.2, Figure 3 caption and EM calculation]
"Note that for models A, B, and E, offsets were applied to better match the GRF profiles."
The EM metric is defined as the percentage of the gait cycle in which the simulation GRF lies within the standard deviation of recorded data (Section 3.2). For models A, B, and E, a phase offset is applied 'to better match the GRF profiles' before these EM values are computed. Because the offset is chosen on the same recorded curves being evaluated and is not reported, the EM values in Table 4 are post-alignment fit statistics. The SK model was evaluated without an offset, making the comparison inconsistent; the reported advantage of the soft-foot model is confounded.
full rationale
The paper's derivation of the soft-foot model is self-contained: SMPL shape, MuJoCo flex, pin attachment, and RL policy are all described, and none of the results are obtained by definitional identity or by renaming. However, the central GRF evaluation is circular in the statistical sense: the quantities that determine the match—material stiffness, foot position, and per-model phase offsets—are chosen by comparing to the very force-plate recordings used to compute the headline EM metric. The paper is transparent about this tuning and even about the offsets, but transparency does not make the match predictive. The joint-angle analysis is less confounded; however, it does not support an improvement over the bare SK model (Table 5 shows SK has higher mean R2 on five of six joints), so the only benefit claimed for soft feet is the GRF match, which is partly a fit. No self-citation chain or uniqueness theorem is load-bearing: references [10]-[12] describe the prior SK/control pipeline that is an input here, not a justification for the soft-foot result. The score reflects partial circularity in the evaluation protocol rather than a definitional identity.
Assumptions & free parameters
free parameters (4)
- Flex foot material stiffness (stiffness/damping or Young's modulus) =
Models A/B/E: stiffness 2000, 5000, 12000; damping 100; equivalent Young's modulus 57, 110, 500 kPa
- Damping coefficient for elasticity plugin =
0.1
- Flex foot position offset relative to skeletal foot =
Adjusted for model E; exact distance not reported
- GRF phase offsets applied to models A, B, E =
Not reported; described as 'a slight offset'
assumptions (4)
- domain assumption MuJoCo flex hyperelastic model with uniform material properties adequately represents human foot soft tissue
- domain assumption SMPL-based foot shape from motion capture of the subject is a sufficient geometric representation after simplification to 15 vertices
- domain assumption Hard pin connections between flex feet and skeleton preserve walking dynamics without unmodelled compliance
- domain assumption The walking control policy trained only on kinematic observations transfers to the flex-foot model without otherwise retuning dynamics
Cite this review
Pith. "Pith review of Personalised 3D Human Digital Twin with Soft-Body Feet for Walking Simulation." pith.science (2026). https://pith.science/paper/CX4QHSTD
@misc{pith2026241114701,
author = {Pith},
title = {Pith review of: Personalised 3D Human Digital Twin with Soft-Body Feet for Walking Simulation},
year = {2026},
howpublished = {\url{https://pith.science/paper/CX4QHSTD}},
note = {Machine review of arXiv:2411.14701}
}
read the original abstract
With the increasing use of assistive robots in rehabilitation and assisted mobility of human patients, there has been a need for a deeper understanding of human-robot interactions particularly through simulations, allowing an understanding of these interactions in a digital environment. There is an emphasis on accurately modelling personalised 3D human digital twins in these simulations, to glean more insights on human-robot interactions. In this paper, we propose to integrate personalised soft-body feet, generated using the motion capture data of real human subjects, into a skeletal model and train it with a walking control policy. Through evaluation using ground reaction force and joint angle results, the soft-body feet were able to generate ground reaction force results comparable to real measured data and closely follow joint angle results of the bare skeletal model and the reference motion. This presents an interesting avenue to produce a dynamically accurate human model in simulation driven by their own control policy while only seeing kinematic information during training.
Figures
Figures from the paper (1 more)
Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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