Gait transition mechanism from quadrupedal to bipedal locomotion in the Japanese macaque based on inverted pendulum
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eLife Assessment
This is an important study that advances our understanding of the transition from quadrupedal to bipedal gait in a neuromechanical model of the Japanese macaque. The method and results are solid; the neuromusculoskeletal model successfully reproduces experimental data, and the stability analysis based on an inverted pendulum model effectively explains the effects of different transition strategies. However, the study would benefit from a more comprehensive sensitivity analysis. The findings are highly relevant for researchers in motor control, comparative physiology/biomechanics, and robotics.
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Abstract
The ability of non-human primates to transition from quadrupedal to bipedal locomotion offers critical insights into both the evolution of human bipedalism and the principles of complex motor control. While quadrupedal and bipedal gaits in non-human primates have been studied, the dynamic mechanisms underlying the transition between these gaits remain poorly understood. Japanese macaques trained to walk bipedally have been reported to utilize inverted pendulum dynamics to achieve efficient bipedal locomotion. Given the intrinsic instability of inverted pendulum systems, which can induce large changes in movement with minimal control input, we hypothesized that this mechanism also contributes to the gait transition. To test this, we developed a neuromusculoskeletal model of the Japanese macaque that integrates a detailed musculoskeletal structure with a physiologically inspired motor control system. Through forward dynamics simulations, we generated a variety of movement patterns by systematically parameterizing motor commands, including failed transitions that are difficult to capture experimentally. We then applied dynamical systems analysis using on an inverted pendulum model to examine the underlying principles of the transition process. Our results demonstrate that successful gait transitions depend on generating an inverted pendulum motion through appropriate control of the forward step length of one hindlimb. These findings provide mechanistic insights into how Japanese macaques coordinate their complex musculoskeletal systems to perform skilled, full-body movements in the gait transition, offering a deeper understanding of both advanced motor control and the evolution of human bipedalism.
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eLife Assessment
This is an important study that advances our understanding of the transition from quadrupedal to bipedal gait in a neuromechanical model of the Japanese macaque. The method and results are solid; the neuromusculoskeletal model successfully reproduces experimental data, and the stability analysis based on an inverted pendulum model effectively explains the effects of different transition strategies. However, the study would benefit from a more comprehensive sensitivity analysis. The findings are highly relevant for researchers in motor control, comparative physiology/biomechanics, and robotics.
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Reviewer #1 (Public review):
Summary:
The article investigates how the Japanese macaque makes gait transitions between quadruped and biped gaits. It presents a compelling neuromechanical simulation that replicates the transition and an interesting analysis based on an inverted pendulum that can explain why some transition strategies are successful and others are not.
Strengths:
I enjoyed reading this article. I think it presents an interesting study and elegant modeling approaches (musculoskeletal + inverted pendulum). The study is well conducted, and the results are interesting. I particularly liked how the success of gait transitions could be predicted based on the inverted pendulum and its saddle node stability. I think it makes a useful and interesting contribution to the state of the art.
Weaknesses:
The article is already in great …
Reviewer #1 (Public review):
Summary:
The article investigates how the Japanese macaque makes gait transitions between quadruped and biped gaits. It presents a compelling neuromechanical simulation that replicates the transition and an interesting analysis based on an inverted pendulum that can explain why some transition strategies are successful and others are not.
Strengths:
I enjoyed reading this article. I think it presents an interesting study and elegant modeling approaches (musculoskeletal + inverted pendulum). The study is well conducted, and the results are interesting. I particularly liked how the success of gait transitions could be predicted based on the inverted pendulum and its saddle node stability. I think it makes a useful and interesting contribution to the state of the art.
Weaknesses:
The article is already in great shape, but could be improved a bit by:
(1) Strengthening the comparison to animal data. In particular, videos of the real animal should be included + snapshots of their gaits (quadruped, biped, and transitions).
(2) Exploring and testing a broader range of conditions. I think it would be very interesting to test gaits and gait transitions on up and down slopes (both with the musculoskeletal model and with the inverted pendulum model). This could be used to make predictions on how the real animal adapts to those conditions. Ideally, this should be tested on the animal as well. I think this could increase (even more) the impact of this work.
(3) Better explaining several aspects of the PSO optimization.
(4) (Ideally) performing a sensitivity analysis on the optimized parameters (e.g. variations of +-5, 10, 20%) in order to determine their respective importance and how much their instantiated values have influenced the results.
(5) Running a spell checker, as there are quite a few typos.
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Reviewer #2 (Public review):
Summary:
This article presents a neuromusculoskeletal (NMS) model of the Japanese Macaque. This model is added with a neural feedforward controller based on CPG and synergy that allows for reproducing quadrupedal and bipedal gait as well as the transition between quadrupedal and bipedal gait. The model and controller were validated using experimental data. Results were also compared to an inverted pendulum model to show that the transition between quadrupedal and bipedal in macaque is using this kind of representation for transition and stability. Overall, the article is very interesting, but it sometimes lacks clarity.
Strengths:
The results of the model present impressive results for quadrupedal, bipedal, and transition, validated by experimental data. NMS controllers based on feedforward controllers are …
Reviewer #2 (Public review):
Summary:
This article presents a neuromusculoskeletal (NMS) model of the Japanese Macaque. This model is added with a neural feedforward controller based on CPG and synergy that allows for reproducing quadrupedal and bipedal gait as well as the transition between quadrupedal and bipedal gait. The model and controller were validated using experimental data. Results were also compared to an inverted pendulum model to show that the transition between quadrupedal and bipedal in macaque is using this kind of representation for transition and stability. Overall, the article is very interesting, but it sometimes lacks clarity.
Strengths:
The results of the model present impressive results for quadrupedal, bipedal, and transition, validated by experimental data. NMS controllers based on feedforward controllers are very difficult to fine-tune.
Weaknesses:
(1) The movement regulator is not clear and should be better explained. At first, it seems that it is just a new CPG/synergy (feedforward) added, but in the methods, it seems to be a feedback controller.
(2) It is also not clear what is meant by discretizing the weight for the trigger limb from 0 to 1 (page 8).
(3) The controller is mainly using a feedforward controller, allowing only anticipatory movement. Animals are also using a reflex-based feedback controller. A controller with feedback/reflex could reduce failed attempts in training and better represent the transition.
(4) There are small typos throughout the article that should be corrected.
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Reviewer #3 (Public review):
Summary:
The purpose of this study was to test the hypothesis that the inverted pendulum mechanism contributes to the gait transition from quadrupedal to bipedal gait in Japanese macaques. The author uses a neuromusculoskeletal model to generate different motor tasks by varying motor command parameters during forward dynamics simulations. After simulations were done, the authors used dynamical system analysis of the inverted pendulum model to reveal the underlying principles of gait transition control. The authors showed that successful gait transition from quadrupedal to bipedal gait mostly depends on increased step length of a hindlimb.
Strengths:
This study is important not only for understanding gait transition, but also to understand stability control of bipedal gaits. Another advantage of this study is …
Reviewer #3 (Public review):
Summary:
The purpose of this study was to test the hypothesis that the inverted pendulum mechanism contributes to the gait transition from quadrupedal to bipedal gait in Japanese macaques. The author uses a neuromusculoskeletal model to generate different motor tasks by varying motor command parameters during forward dynamics simulations. After simulations were done, the authors used dynamical system analysis of the inverted pendulum model to reveal the underlying principles of gait transition control. The authors showed that successful gait transition from quadrupedal to bipedal gait mostly depends on increased step length of a hindlimb.
Strengths:
This study is important not only for understanding gait transition, but also to understand stability control of bipedal gaits. Another advantage of this study is that it allows us to estimate the effect of one control mechanism and find its effect and limits. In animal studies, we also have a combination of compensatory stability control mechanisms.
Weaknesses:
Any simulation is not perfect, so discrepancies from experimental data are expected. A 2D model is used, but the advantage of using a 3D model is not clear, and it is much more complicated.
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