REVIEW 3 major objections 5 minor 14 references
Motion Analysis of Upper Limb and Hand in a Haptic Rotation Task
T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read Blindfolded people systematically overshoot when rotating a knob to a target angle, and this paper traces that error to the wrist and to sideways and distal finger joint motions.
desk verdict Novel decomposition of haptic overshoot into joint excursions, but unsigned predictors and pooled trials make the mechanistic claims conditional. 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 machinery is a motion-capture-to-regression pipeline. Reflective markers on the hand and upper limb are converted, via a 16-joint, 26-DoF kinematic hand model and a 7-DoF shoulder-elbow-wrist model, into joint-angle trajectories, synchronized with a rotary-knob apparatus that records absolute orientation with 0.022-degree resolution. Each trial is compressed to one predictor per joint — the range (maximum minus minimum angle) over the rotation segment — and to the criterion, the signed error from the 90-degree target. Stepwise ordinary least squares regression with backward elimination, using min-max normalization per participant per joint, then selects the joints whose ranges significantly predict the error, which is the step that localizes the overshoot.
What would settle it
Refit the same model with a random intercept per participant (a linear mixed-effects model) and check whether the wrist and finger coefficients remain significant; if they do not, the claimed localization is an artifact of pooled trials. Alternatively, measure fingertip contact-point displacement during rotation directly: if overshoot occurs without any rolling of the fingertips over the knob surface, the paper's rolling mechanism is falsified.
Extended reading notes
Core claim
The central discovery is a statistical localization of the haptic-rotation overshoot. Regressing the signed error (actual rotation minus 90 degrees, normalized per participant and per joint to [0,1]) on each joint's range of motion yields a final model with $R^2 = 0.691$, in which the three wrist axes (coefficients 30.61, 21.45, 20.13), the thumb's TMC flexion/extension (24.05), the index MCP abduction/adduction (8.55), and the index DIP flexion/extension (34.67) all contribute positively and significantly to overshoot. Elbow flexion/extension (-13.62), thumb MCP flexion/extension (-25.96), and thumb TMC abduction/adduction (-17.46) contribute negatively, i.e., they limit the overshoot. The paper interprets the wrist effect as evidence for a hand-centered egocentric reference frame and the finger effects as evidence that the fingertips roll over the knob surface, changing the contact point, without the participant compensating.
Load-bearing premise
The result depends on treating every trial from every participant as an independent observation in a single regression; if within-participant correlations are large, the reported p-values and the list of significant joints could be spurious.
Editorial extensions
If this is right
- The overshoot in blindfolded haptic rotation is a local, joint-specific phenomenon: the wrist and particular finger joints carry the error, while the elbow and some thumb motions actively compensate.
- The wrist's significant positive contribution supports a hand-centered egocentric reference frame, meaning the brain systematically under-weights the hand's own rotation.
- Fingertip rolling over the knob surface, driven by sideways proximal-joint motion and distal flexion, adds rotary movement that the blindfolded participant does not compensate.
- Grasping orientation changes the joint usage pattern: the shoulder contributes in all five orientations, the wrist drops out at the 90-degree grasp, and the index finger shifts from MCP abduction/adduction to PIP and DIP flexion as the grasp angle increases.
- Future experiments can test the two mechanisms directly by restricting hand movement or preventing rolling, and the recorded data can be re-analyzed to explain why using more fingers reduces overshoot.
Reading between the lines
- If the egocentric reference-frame account is right, then providing participants with visual or haptic feedback about hand orientation during the movement should reduce the wrist's contribution to overshoot, a testable prediction the paper does not make.
- The per-orientation regressions imply that the set of significant predictors is not universal: a joint can contribute positively in one grasp and negatively in another, so the overshoot mechanism likely shifts with task geometry rather than being a fixed motor error.
- Compressing each joint trajectory to a single range discards timing and phase; using velocity or temporal-alignment features might reveal that the order of joint recruitment matters for the size of the overshoot.
- A direct non-invasive check is feasible: track the fingertips with small markers or high-speed video during rotation to confirm the rolling motion and quantify its angular contribution.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes motion capture data from a haptic rotation task (90° counterclockwise, blindfolded, two-finger grasp) to identify which upper limb and hand joint angle ranges predict the signed overshoot (δτ − 90°). Using stepwise ordinary least squares regression with backward elimination, the authors report that the wrist joint, the sideways movement in proximal finger joints, and the distal finger joints contribute significantly to overshooting. They also run per-grasping-orientation regressions to examine which joint ranges contribute to the achieved rotation. The results are interpreted as evidence for two mechanisms: a hand-centered egocentric reference frame bias (wrist contribution) and fingertip rolling over the knob surface (finger joint contributions).
Significance. The paper addresses a clearly relevant question in haptics and human motor control: where in the kinematic chain the systematic overshoot in blindfolded rotary knob rotations originates. The experimental setup (motion capture with 18 participants, five grasping orientations, repeated trials) and the joint-level analysis framework are potentially valuable for future work in rehabilitation, robotics, and human-robot interaction. The paper is also transparent in reporting coefficients and p-values. However, the current analysis has serious statistical and conceptual limitations that prevent the central claim from being established. The proposed framework may be salvageable with a different predictor definition and a mixed-model approach, but as presented the evidence does not support the conclusions.
major comments (3)
- [Section 2.6 and Table 1 (left)] The predictors are unsigned max-min ranges δ_j of each joint angle over the rotation segment, while the criterion is the signed overshoot δτ − 90°. A positive regression coefficient for a range variable only indicates that larger total joint excursion is associated with more overshoot; it cannot distinguish movement in the direction that adds to the rotation from movement in the opposite (compensatory) direction. For example, a wrist rotation that opposes the knob movement and a wrist rotation that reinforces it can produce the same range value. The central claim that the wrist and finger joints 'contributed significantly to overshooting' (Abstract and Section 4) is therefore not supported by the regression as specified. The two proposed mechanisms are directional (egocentric reference frame bias and fingertip rolling), so the analysis needs a signed measure of joint displacement (e.g., net angle change over the rotation segment) or a decomposition into positive and negative contributions.
- [Section 3, Tables 1 and G (Tables 2–7)] The overshoot regression pools all trials from 18 participants, with each trial treated as an independent observation. Since each participant contributes multiple trials under each grasping orientation, within-participant correlations are likely substantial, and the reported p-values and standard errors are accordingly unreliable. Additionally, the stepwise backward elimination procedure performs many significance tests without any correction for multiple comparisons, so the final model (reported R² = 0.691) is likely overfitted and the list of 'significant' joints may be spurious. The lack of random effects for participants is a load-bearing problem for the central claim because the identification of contributing joints depends on the validity of these p-values.
- [Section 3, Table 1 (right) and Tables 3–7] The per-orientation regression analyses of the rotation angle on joint ranges suffer from the same unsigned-predictor problem. A negative coefficient is interpreted as the joint 'compensating' for the rotation (Section 4), but a negative association between a range variable and the rotation angle does not imply that the joint moved in the opposite direction to the knob rotation. Moreover, these regressions also pool trials without participant random effects, and the constant term (ranging from 28.36 to 70.50) is treated as a catch-all for 'additional body movements' without a clear mechanistic interpretation. The reported p-values are conditional on the stepwise selection and are not corrected for multiplicity.
minor comments (5)
- [Section 1] The phrase 'the mean difference between the target and the encoded positions' is ambiguous; it should be clarified whether the signed error is computed as encoded minus target or vice versa.
- [Section 3] There is a typo: 'shown in shown in Table 1' should be 'shown in Table 1'.
- [Appendix G, Table 7] In the regression results for the free grasping condition, the predictor labeled 'δf/e i,MCP' appears twice with different coefficients (41.50 and 26.80). One of these is likely meant to be a different joint (e.g., PIP); please correct the labeling.
- [Introduction/Appendix access] The appendix is hosted on a password-protected website with the German word 'Passwort'; for an international readership and for review, the access information should be in English and ideally the supplementary material should be included with the submission.
- [Section 2.5] The description of the hand model states that the thumb has joints TMC, MCP, and IP, while the index finger has MCP, PIP, and DIP; this is clear, but the notation in Table 1 (e.g., δf/e_t,MCP) could be confusing because the subscript 't' is used for the thumb and 'i' for the index finger; a brief reminder in the caption of Table 1 would help.
Circularity Check
No significant circularity: the joint-contribution analysis is an independent regression on measured kinematics and does not reduce to its inputs or to prior conclusions.
full rationale
The paper's central claim is that wrist, proximal sideways finger, and distal finger joint ranges contribute significantly to overshoot in a blindfolded 90-degree haptic rotation. This claim is obtained by stepwise OLS regression in Section 3, where the criterion is the measured signed error (delta_tau - 90 degrees) and the predictors are measured joint-angle ranges delta_j. The criterion and predictors are distinct physical measurements: the knob rotation comes from the Twister encoder, and the joint ranges come from Vicon marker trajectories processed through an inverse-kinematics hand model. No equation defines one in terms of the other by construction, and no fitted parameter is renamed as a prediction. The paper does reuse data and experimental apparatus from the authors' prior work [4], but that reuse supplies the dataset and the known overshoot phenomenon, not the joint-level attribution; the regression analysis is new and its significant-joint results are not assumed in the preprocessing or model setup. Statistical concerns about pooling trials across participants, unsigned range predictors, and multiple testing are real threats to the inference but are not circularity: they concern whether the regression estimates are unbiased and interpretable, not whether the conclusion is equivalent to the input data or to a self-citation. The self-citations to [3], [4], [7], and [8] are used for apparatus, motion-tracking software, and prior experimental context, not as an unverified authority that forces the paper's central result. No step in the derivation chain reduces to its own inputs, so no circular step can be identified with a specific quotation.
Assumptions & free parameters
free parameters (2)
- Stepwise elimination significance threshold =
p < 0.05
- Min-max normalization bounds =
[0,1] per participant per joint
assumptions (3)
- domain assumption Vicon marker trajectories and the model-based inverse kinematics software produce accurate joint angle estimates without ground-truth validation in this dataset.
- domain assumption Each trial is an independent observation in the OLS regressions.
- domain assumption The max-minus-min joint angle range over the rotation segment is a sufficient summary of a joint's contribution to the final rotation and overshoot.
Cite this review
Pith. "Pith review of Motion Analysis of Upper Limb and Hand in a Haptic Rotation Task." pith.science (2026). https://pith.science/paper/QD2E4A7A
@misc{pith2026241112765,
author = {Pith},
title = {Pith review of: Motion Analysis of Upper Limb and Hand in a Haptic Rotation Task},
year = {2026},
howpublished = {\url{https://pith.science/paper/QD2E4A7A}},
note = {Machine review of arXiv:2411.12765}
}
read the original abstract
Humans seem to have a bias to overshoot when rotating a rotary knob blindfolded around a specified target angle (i.e. during haptic rotation). Whereas some influence factors that strengthen or weaken such an effect are already known, the underlying reasons for the overshoot are still unknown. This work approaches the topic of haptic rotations by analyzing a detailed recording of the movement. We propose an experimental framework and an approach to investigate which upper limb and hand joint movements contribute significantly to a haptic rotation task and to the angle overshoot based on the acquired data. With stepwise regression with backward elimination, we analyze a rotation around 90 degrees counterclockwise with two fingers under different grasping orientations. Our results showed that the wrist joint, the sideways finger movement in the proximal joints, and the distal finger joints contributed significantly to overshooting. This suggests that two phenomena are behind the overshooting: 1) The significant contribution of the wrist joint indicates a bias of a hand-centered egocentric reference frame. 2) Significant contribution of the finger joints indicates a rolling of the fingertips over the rotary knob surface and, thus, a change of contact point for which probably the human does not compensate.
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
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Reviewed August 12, 2026 · model on record in the stance chip above.
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