REVIEW 4 major objections 4 minor 1 cited by
Versatile, Robust, and Explosive Locomotion with Rigid and Articulated Compliant Quadrupeds
T0 review · 4 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read Parallel springs let a quadruped jump 25% farther and turn 15% wider
desk verdict Solid modeling/control pipeline for compliant quadrupeds, but the headline performance numbers rest on single trials and a confounded froggy jump; still worth refereeing. 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 TD-aSLIP, or trunk dual actuated spring-loaded inverted pendulum: a reduced-order model with a single rigid trunk, two massless legs, and, on each leg, an actuation force and a parallel spring force acting in parallel. Its load-bearing move is a configuration-aware stiffness mapping that converts joint-space spring constants into an equivalent Cartesian leg stiffness through the leg Jacobian, then captures how that stiffness changes with leg length by sampling the robot's workspace and fitting a cubic polynomial. This makes the model's spring term match the actual hardware configuration rather than assuming a fixed constant. Around this model the paper builds a two-layer planner, first SLIP-based trajectory optimization and then quaternion kinodynamics optimization, and a controller made of a quaternion single-rigid-body MPC feeding a whole-body quadratic program that compensates spring torques and enforces safety constraints.
What would settle it
Run the same 90-degree hop-turn plan under motion capture that reconstructs whole-body angular momentum, then re-plan with a centroidal-dynamics model that includes leg inertia; if the yaw error stays around 20 degrees and the rigid-versus-compliant gap persists, the single-rigid-body assumption is not the limiting factor and the paper's explanation would need revision.
Extended reading notes
Core claim
The central claim is that explosive quadruped motion with large trunk rotation can be planned and tracked using a two-legged, one-trunk template model that separates spring forces from motor forces, and that this explicit separation is what unlocks the benefits of parallel compliance. Concretely, the paper claims that the TD-aSLIP model, where each of two massless legs couples an actuation force with a unidirectional spring force whose stiffness varies with configuration, generates pronking, froggy jumping, and hop-turns without a hand-tuned reference trajectory. A dual-layer trajectory optimizer first solves a coarse SLIP problem and then refines it with quaternion kinematics and dynamics, avoiding Gimbal-lock singularities during large rotations. A quaternion-based model-predictive controller and a whole-body controller with explicit spring compensation track the plan in hardware. Comparison between a rigid quadruped and the same robot with parallel thigh and calf springs shows that the compliant version reaches larger pronks, larger yaw turns, and longer froggy jumps, and tolerates twice the terrain height variation when jumping from unknown uneven surfaces.
Load-bearing premise
The entire pipeline treats the robot as a single rigid body with massless legs, so it assumes leg swing and body-shape changes do not carry significant angular momentum; the authors concede this is why the rigid robot undershoots a 90-degree hop-turn by about 20 degrees.
Editorial extensions
If this is right
- The same dual-layer planner, with only waypoints and contact sequence changed, should generate further flight-phase maneuvers such as sideways jumps or larger rotations without hand-tuned references; the paper demonstrates pronking, froggy jumping, and hop-turn from one formulation.
- If parallel compliance is doing the work, the robustness gain should persist under modest spring mistuning or surface uncertainty, and the paper reports the allowable support-surface height variation doubles from 5 cm to 10 cm for froggy jumping.
- The singularity-free quaternion formulation means the planned motions transfer across platforms without Gimbal-lock issues during large rotation, and the paper reports the same control gains work for both the rigid and spring-equipped versions.
- The modeling principle of decoupling spring force from actuation force in a reduced-order template should extend to other explosive motions, such as consecutive jumps, because the dynamics equations do not depend on a particular contact schedule.
Reading between the lines
- I would expect the configuration-aware stiffness mapping to matter most in deeply crouched stances like froggy jumping, where the leg Jacobian changes substantially; rerunning the planner with a fixed stiffness constant should produce visibly worse landing tracking on those motions.
- The four legs are symmetrically paired into two SLIP legs; I would expect an asymmetric front/rear version with different spring constants or rest lengths to handle the 50 cm froggy jump even better than the authors' workaround of removing the right-rear calf spring, because the rear legs carry much more load during the rear-leg stance phase.
- Since the paper's stated limitation is the ignored centroidal angular momentum, I would expect that replacing the single-rigid-body MPC with a momentum-aware or centroidal-dynamics controller would reduce the yaw tracking error in hop-turns, potentially pushing the compliant robot beyond the reported 145-degree limit.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes TD-aSLIP, a reduced-order quadruped template model with two massless legs, a trunk rotation state, and explicitly modeled parallel compliance obtained by decoupling motor actuation from spring forces. A configuration-aware stiffness mapping is introduced, and a dual-layer trajectory optimization (SLIP-level coarse planning followed by quaternion-based kinodynamic refinement) is combined with a quaternion MPC and a whole-body controller with parallel-compliance compensation. The pipeline is evaluated in PyBullet simulation and on hardware: a rigid Unitree Go1 and a new compliant variant E-Go-V2 with thigh and calf springs. The central claims are that the model enables versatile explosive motions (pronking, hop-turn, froggy jumping), that parallel elasticity improves explosive performance (at least 25%, 15%, and 25% gains in maximum pronking distance, hop-turn yaw, and froggy-jump distance), and that it improves robustness, including a 100% increase in allowable support-surface height variation.
Significance. If the central claims hold, this is a useful contribution to model-based dynamic legged locomotion: it extends the authors' prior aSLIP line to include trunk rotation, uses a singularity-free quaternion formulation in both planning and control, provides a reproducible compliant quadruped platform with open-source CAD, and offers an external baseline (springs disengaged on the same robot) rather than only comparing against the authors' previous model. The simulation comparison against dual-aSLIP is a genuine incremental validation, and the statement that all control tasks share the same gains is a strength. The main significance risk is that the headline experimental gains rest on very thin evidence: single trials, no error bars, and a froggy-jump comparison performed with an asymmetric spring configuration.
major comments (4)
- [Section VIII, Table 2 and the 'Robustness' subsection] The headline quantitative claims are not established as reproducible effects. Table 2 reports single maximum values for pronking distance (40 cm vs 50 cm), hop-turn yaw (125 vs 145 degrees), and froggy-jump distance (40 cm vs 50 cm), and the robustness section reports 5 cm vs 10 cm allowable height variation, all without repeated trials, ranges, medians, or success rates. As written, the abstract's 'at least 25%, 15%, 25%, 100%' statements are maxima from single trials and could reflect trial-to-trial variation or favorable conditions rather than a systematic benefit of parallel elasticity. Please provide repeated trials with spread or success counts for each extreme-motion comparison.
- [Section VIII, Remark 4 (Enhanced froggy jumping)] The froggy-jumping gain, one of the three explosive-motion percentages, is confounded. Remark 4 states that for jumps longer than 40 cm the RR calf spring is removed, so the 50 cm compliant result is achieved by an asymmetric compliant robot, not by the symmetric TD-aSLIP configuration used in planning. The Discussion also notes that unidirectional springs require extra flight-phase torque, so removing the RR spring may improve the result by removing a disturbance rather than by demonstrating the modeled parallel compliance. At minimum, the 25% froggy-jump gain in Table 2 should be separated from the symmetric-compliance claims, or the experiment should be repeated in a configuration that matches the planning model.
- [Section IV-B2 (Configuration-aware stiffness mapping)] The configuration-aware stiffness is obtained as a cubic polynomial fitted to sampled data (Fig. 4), but no fit error, cross-validation, or comparison against the direct Jacobian-based stiffness is reported. Since this polynomial is the mechanism by which 'configuration-aware' compliance enters the planner, its accuracy is load-bearing for the claim of improved model accuracy. In addition, Eq. (9) defines the scalar spring constant as the Euclidean norm of the diagonal of the equivalent stiffness matrix; this does not obviously equal the leg-direction stiffness used in Eq. (4), so the mapping needs a derivation or numerical validation.
- [Section I (Contribution 1) and Discussion ('Momentum-aware motion planning and control')] The paper states in Contribution 1 that the model captures 'the varying angular momentum' and uses this as part of its novelty relative to prior SLIP models, but the TD-aSLIP dynamics assume massless legs and only model trunk inertia, so the centroidal angular momentum is not captured. The Discussion explicitly acknowledges that this limits tracking, e.g., the rigid robot lands at about 70 degrees in a 90-degree hop-turn. This does not invalidate the compliant-vs-rigid comparison, but the contribution statement overstates what the model captures; the claim should be reformulated to 'trunk angular momentum' or the model should be extended with leg inertia.
minor comments (4)
- [Eq. (35)] The last row of the matrix T(Q) appears to be [-qz, qx, qw] as printed; for consistency with Eq. (34) and the quaternion derivative it should be [-qy, qx, qw].
- [Fig. 4] The caption refers to blue, green, and purple curves, but the figure appears in grayscale; please use distinguishable line styles or annotate the curves directly.
- [Section VIII (System setting)] The manual landmark-based distance measurement is described, but it is not stated whether measurements were taken by a single observer, whether images were processed automatically, or whether any inter-rater variability was considered; a brief statement would improve reproducibility.
- [Eq. (24)] The use of Euclidean distance with the min over Q and -Q is an acknowledged approximation to the quaternion distance; this is acceptable, but please state explicitly in the text that this is a heuristic rather than the geodesic metric, since it affects the cost landscape.
Circularity Check
No significant circularity: the central claims are validated against an external spring-disengaged baseline, and the model equations are derived in this paper.
full rationale
The paper's derivation chain is self-contained. The TD-aSLIP dynamics (Eqs. 1-5) are derived explicitly, with spring forces defined by Eq. 4 and the configuration-aware stiffness mapping obtained by equating joint-space and Cartesian-space potential energies (Eqs. 6-9), then fitting a cubic polynomial to sampled workspace data (Section IV-B2). This calibration is an input to motion generation, not a fitted stand-in for the reported outcomes. The signature claims - pronking distance, hop-turn yaw, froggy jumping distance, and allowable terrain height variation - come from hardware comparisons on the same E-Go-V2 robot with springs engaged versus disengaged (Section VIII, Table 2), which is an external baseline rather than a quantity reconstructed from the model. Self-citations to Ding et al. (2024a,b) are contextual lineage references for the aSLIP family and the earlier E-Go platform; no uniqueness theorem or ansatz is imported from them to force the present results. The acknowledged limitations (Discussion: momentum-aware planning; Remark 4: RR calf spring removed for froggy jumps above 40 cm) are experimental-design confounds and modeling gaps, not circular reductions; they affect the reliability and interpretation of the headline percentages, but they do not make any prediction equivalent to its inputs by construction.
Assumptions & free parameters
free parameters (5)
- Stiffness polynomial coefficients =
not reported
- TO cost weights =
not reported
- Waypoint slack variable xi =
not reported
- Spring rest lengths =
0.38 m for hardware
- Contact phase durations =
not specified
assumptions (5)
- domain assumption Lumped mass, massless legs for TD-aSLIP and SRB models.
- domain assumption Small joint angle variations in the equivalent stiffness mapping.
- domain assumption Unidirectional spring force.
- domain assumption Leg pairing symmetry.
- ad hoc to paper Quaternion distance approximated by minimum Euclidean distance.
Cite this review
Pith. "Pith review of Versatile, Robust, and Explosive Locomotion with Rigid and Articulated Compliant Quadrupeds." pith.science (2026). https://pith.science/paper/A34ROZ6D
@misc{pith2026250412854,
author = {Pith},
title = {Pith review of: Versatile, Robust, and Explosive Locomotion with Rigid and Articulated Compliant Quadrupeds},
year = {2026},
howpublished = {\url{https://pith.science/paper/A34ROZ6D}},
note = {Machine review of arXiv:2504.12854}
}
read the original abstract
Achieving versatile and explosive motion with robustness against dynamic uncertainties is a challenging task. Introducing parallel compliance in quadrupedal design is deemed to enhance locomotion performance, which, however, makes the control task even harder. This work aims to address this challenge by proposing a general template model and establishing an efficient motion planning and control pipeline. To start, we propose a reduced-order template model-the dual-legged actuated spring-loaded inverted pendulum with trunk rotation-which explicitly models parallel compliance by decoupling spring effects from active motor actuation. With this template model, versatile acrobatic motions, such as pronking, froggy jumping, and hop-turn, are generated by a dual-layer trajectory optimization, where the singularity-free body rotation representation is taken into consideration. Integrated with a linear singularity-free tracking controller, enhanced quadrupedal locomotion is achieved. Comparisons with the existing template model reveal the improved accuracy and generalization of our model. Hardware experiments with a rigid quadruped and a newly designed compliant quadruped demonstrate that i) the template model enables generating versatile dynamic motion; ii) parallel elasticity enhances explosive motion. For example, the maximal pronking distance, hop-turn yaw angle, and froggy jumping distance increase at least by 25%, 15% and 25%, respectively; iii) parallel elasticity improves the robustness against dynamic uncertainties, including modelling errors and external disturbances. For example, the allowable support surface height variation increases by 100% for robust froggy jumping.
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Forward citations
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Reference graph
Works this paper leans on
-
[1]
In: 2019 18th European control conference (ECC)
Ames AD, Coogan S, Egerstedt M, Notomista G, Sreenath K and Tabuada P (2019) Control barrier functions: Theory and applications. In: 2019 18th European control conference (ECC). IEEE, pp. 3420–3431. Andersson JA, Gillis J, Horn G, Rawlings JB and Diehl M (2019) Casadi: a software framework for nonlinear optimization and optimal control. Mathematical Progr...
work page 2019
-
[2]
Online Omnidirectional Jumping Trajectory Planning for Quadrupedal Robots on Uneven Terrains
Wensing PM and Orin DE (2013) High-speed humanoid running through control with a 3d-slip model. In: 2013 IEEE/RSJ International Conference on Intelligent Robots and Systems . IEEE, pp. 5134–5140. Wensing PM, Posa M, Hu Y , Escande A, Mansard N and Del Prete A (2023) Optimization-based control for dynamic legged robots. IEEE Transactions on Robotics 40: 43...
work page Pith review arXiv 2013
-
[57]
Frontiers in Robotics and AI 9:
Wang K, Fei H and Kormushev P (2022) Fast online optimization for terrain-blind bipedal robot walking with a decoupled actuated slip model. Frontiers in Robotics and AI 9:
work page 2022
-
[67]
Mathematical Programming Computation 12(4): 637–672
Stellato B, Banjac G, Goulart P, Bemporad A and Boyd S (2020) OSQP: an operator splitting solver for quadratic programs. Mathematical Programming Computation 12(4): 637–672. Vezzi F, Ding J, Raffin A, Kober J and Della Santina C (2024) Two-stage learning of highly dynamic motions with rigid and articulated soft quadrupeds. In: 2024 IEEE International Conf...
work page 2020
-
[489]
In: 2018 IEEE/RSJ international conference on intelligent robots and systems (IROS)
Di Carlo J, Wensing PM, Katz B, Bledt G and Kim S (2018) Dynamic locomotion in the mit cheetah 3 through convex model-predictive control. In: 2018 IEEE/RSJ international conference on intelligent robots and systems (IROS). IEEE, pp. 1–9. Ding J, Atanassov V , Panichi E, Kober J and Della Santina C (2024a) Robust quadrupedal jumping with impact-aware landi...
work page 2018
-
[513]
IEEE Robotics and Automation Letters 8(4): 2285–2292
Calzolari D, Della Santina C, Giordano AM, Schmidt A and Albu- Sch¨affer A (2023) Embodying quasi-passive modal trotting and pronking in a sagittal elastic quadruped. IEEE Robotics and Automation Letters 8(4): 2285–2292. Chignoli M and Kim S (2021) Online trajectory optimization for dynamic aerial motions of a quadruped robot. In: 2021 IEEE International ...
work page 2023
-
[782]
IEEE/ASME Transactions on Mechatronics 22(2): 898–908
Mazumdar A, Spencer SJ, Hobart C, Salton J, Quigley M, Wu T, Bertrand S, Pratt J and Buerger SP (2016) Parallel elastic elements improve energy efficiency on the steppr bipedal walking robot. IEEE/ASME Transactions on Mechatronics 22(2): 898–908. Neunert M, St ¨auble M, Giftthaler M, Bellicoso CD, Carius J, Gehring C, Hutter M and Buchli J (2018) Whole- b...
work page 2016
-
[924]
In: 2018 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)
Bledt G, Powell MJ, Katz B, Di Carlo J, Wensing PM and Kim S (2018) Mit cheetah 3: Design and control of a robust, dynamic quadruped robot. In: 2018 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) . IEEE, pp. 2245–
work page 2018
Show all 12 references
-
[1080]
In: 2020 IEEE International Conference on Robotics and Automation (ICRA)
Ma WL and Ames AD (2020) From bipedal walking to quadrupedal locomotion: Full-body dynamics decomposition for rapid gait generation. In: 2020 IEEE International Conference on Robotics and Automation (ICRA). IEEE, pp. 4491–4497. Margolis GB, Yang G, Paigwar K, Chen T and Agrawa...
2020
-
[1619]
In: 2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)
Hong J, Yeo C, Bae S, Hong J and Oh S (2024) Slip embodied robust quadruped robot control. In: 2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). IEEE, pp. 14219–14224. Hutter M, Gehring C, Jud D, Lauber A, Bellicoso CD, Tsounis V , Hwangbo J, Bod...
2024 arXiv
-
[2252]
Journal of biomechanics 22(11-12): 1217–1227
Blickhan R (1989) The spring mass model for running and hopping. Journal of biomechanics 22(11-12): 1217–1227. Bravo-Palacios G, Li H and Wensing PM (2024) Engineering compliance in legged robots via robust co-design. IEEE/ASME Transactions on Mechatronics29(6): 4711–4722. Buc...
1989
-
[5892]
In: 2012 4th IEEE RAS & EMBS International Conference on Biomedical Robotics and Biomechatronics (BioRob)
H¨aufle DF, Taylor M, Schmitt S and Geyer H (2012) A clutched parallel elastic actuator concept: Towards energy efficient powered legs in prosthetics and robotics. In: 2012 4th IEEE RAS & EMBS International Conference on Biomedical Robotics and Biomechatronics (BioRob) . IEEE,...
2012
Reviewed August 16, 2026 · model on record in the stance chip above.
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