REVIEW 3 major objections 4 minor 1 cited by
Machine Learning-based sEMG Signal Classification for Hand Gesture Recognition
T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read 97% hand-gesture accuracy from sEMG with dilated CNN
desk verdict Useful benchmark on two recent sEMG datasets, but the underspecified 80/20 split makes the headline accuracies unverified. 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 load-bearing machinery is the sliding-window feature cascade. Band-pass filtered sEMG (20–450 Hz) is cut into 600 ms windows with 50% overlap, and each window is reduced to one of three descriptor sets: fused time-domain descriptors (fTDD), temporal-spatial descriptors (TSD), or five-level Symlet-8 wavelet features. fTDD combines log-transformed spectral moments with sparsity, irregularity factor, and waveform length ratio; TSD adds coefficient of variation and Teager-Kaiser energy to within-channel and pairwise between-channel features. These descriptors are then fed to the classifier grid, where the 1D dilated CNN uses increasing dilation rates to see multi-scale temporal patterns, and the random forest builds a hundred-tree ensemble. The winner on each dataset is the feature-classifier pair that best exploits the temporal or spatial structure in that dataset's signals.
What would settle it
Re-run the same feature and classifier grid using a person-disjoint split, and for Grabmyo a session-disjoint split, with all windows from any given participant kept out of training when that participant is in the test set. If the dilated CNN's 97% and the random forest's 94.95% fall substantially under such held-out-user evaluation, the paper's central claim about best configurations does not generalize.
Extended reading notes
Core claim
On the paper's own terms, the discovery is empirical: the configuration that wins is different on each dataset. For Grabmyo, a 1D dilated CNN fed with fused time-domain descriptors—power spectral moments, sparsity, irregularity factor, and waveform length ratio computed from 600 ms windows with 50% overlap—achieves 97% accuracy with 0.97 precision, recall, and F1, outperforming every classic model and also edging out the dilated CNN-LSTM at 96.53%. For FORS-EMG, a random forest using temporal-spatial descriptors—time-domain features supplemented by coefficient of variation, Teager-Kaiser energy, and pairwise between-channel statistics—achieves 94.95%, tied by AdaBoost, and clearly above the deep models on that dataset. The paper positions this as a benchmark that compares recent 2024 datasets with the same feature and classifier pipeline, and it reports the winning pairs as the practical choices among the tested alternatives.
Load-bearing premise
The load-bearing assumption is that the 80/20 train/test split keeps each person's recordings entirely on one side; the paper does not say that it separates by subject or session, so the reported accuracies could reflect memorizing individual muscle signatures rather than recognizing gestures for new users.
Editorial extensions
If this is right
- On Grabmyo, fused time-domain descriptors plus the 1D dilated CNN give 97% accuracy, beating SVM by 4.17 points and LDA by 15.13 points on the same feature set.
- On FORS-EMG, temporal-spatial descriptors plus random forest reach 94.95%, beating SVM by 30.74 points and LDA by 49.97 points.
- Ensemble variants—voting, bagging, and AdaBoost—consistently sit at or above their single-model counterparts, which is most pronounced on the smaller FORS-EMG dataset.
- Wavelet-transform features are competitive, reaching roughly 94–96% depending on classifier, but never surpass the winning fTDD and TSD combinations in this comparison.
Reading between the lines
- If the reported evaluation is taken at face value, the practical takeaway is that feature engineering still matters: a deliberately designed descriptor set can let a classical tree ensemble match or beat deep networks, as the random forest does on FORS-EMG.
- A subject-disjoint or session-disjoint split would be a stricter test of generalization to new users; because the paper describes only an 80/20 split, its accuracy numbers may overstate cross-user performance.
- The 50% window overlap means adjacent training windows share muscle activity; testing with non-overlapping windows would separate true gesture discrimination from temporal smoothing.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper benchmarks three feature extraction approaches (fused time-domain descriptors, temporal-spatial descriptors, and wavelet transform-based features) combined with several machine learning classifiers (LDA, SVM, KNN, random forest, voting/bagging/boosting ensembles) and two deep models (1D dilated CNN and 1D dilated CNN-LSTM) for sEMG-based hand gesture recognition on two recent public datasets, Grabmyo and FORS-EMG. The headline results are 97% accuracy for the 1D dilated CNN with fused time-domain descriptors on Grabmyo and 94.95% accuracy for random forest with temporal-spatial descriptors on FORS-EMG; the authors conclude these are the best combinations among those compared. The evaluation uses 600 ms windows with 50% overlap, an 80/20 train/test split, and standard metrics reported on the test set.
Significance. If the empirical ranking is valid, this is a useful benchmark for practitioners selecting feature/classifier combinations for myoelectric control, especially because it uses two recent (2024) public datasets and covers a wider model family than most prior comparisons. The paper also reports precision, recall, and F1 alongside accuracy, which is valuable for class-imbalance assessment. However, the central claim is a purely empirical ranking and is only as strong as the evaluation protocol; the current manuscript does not provide enough detail on how the train/test split was constructed to establish that the reported accuracies reflect generalization to unseen subjects or even unseen recordings.
major comments (3)
- [Section IV-B] The evaluation protocol is under-specified in a way that is load-bearing for every reported accuracy. Section IV-B states only that 'The data sets were divided into 80% for training and 20% for testing' without saying whether the split respects subject, session, or recording boundaries. Since Section III uses 600 ms windows with 50% overlap, a random window-level split can place highly overlapping or near-duplicate windows from the same contraction in both training and test sets, allowing the model to memorize the signal rather than learn gesture-generalizable features. This concern is amplified by the structure of both datasets: Grabmyo has 43 subjects recorded on days 1, 8, and 29, and FORS-EMG has 19 subjects with five repetitions per gesture. Please clarify whether the split was subject-independent and temporally non-overlapping; if it was not, the experiments should be rerun with a subject-independent split, and the abstract/conclusions should be revised accordingly.
- [Tables III-VIII and Section V] No variance or uncertainty is reported for any of the accuracy numbers, and no repeated runs or statistical tests are described. This is particularly important because many of the reported differences are very small: in Table VII, Random Forest and AdaBoost both report 94.95% accuracy, yet the text and abstract single out Random Forest as the best. Without confidence intervals, paired significance tests, or multiple-seed statistics, the claimed ranking is not statistically grounded. Please report mean plus/minus standard deviation over repeated runs (or an equivalent uncertainty measure) and, where the top accuracies are close, a paired test.
- [Section V-B, Table VII] The results narrative in Section V-B is internally inconsistent and should be corrected. The text says 'Among traditional machine learning models, K-Nearest Neighbors (KNN) obtained the highest accuracy of 89.05%,' but Table VII reports KNN at 89.9% and Random Forest at 94.95%, so KNN is not the highest-accuracy traditional model and 89.05 does not match the table. Since the central claim depends on identifying the best-performing model, this inconsistency between the text and the table needs to be resolved, even though the table itself appears to support the Random Forest claim.
minor comments (4)
- [Section V-A] The comparison with existing work [39] is not apples-to-apples: the text states that [39] uses only five basic hand gestures, while the present work uses all gesture classes, in addition to different features and classifiers. The '6.96% improvement' conclusion is therefore not a controlled comparison and should be phrased as indicative only.
- [Section III-A and Table I] Table I lists features such as COV and TKEO as part of the fTDD table, but the text attributes those to TSD; the table header says 'SUMMARY OF FTDD AND TSD FEATURES,' which is confusing. Please separate or clearly label which features belong to fTDD and which to TSD.
- [Section IV-B] The paper does not report the total number of windows, the number of windows per class, or the class balance after feature extraction. This information would help the reader assess whether the high accuracies could be driven by class imbalance or by a very large overlap between adjacent windows.
- [Section V-B] In the third paragraph of Section V-B, the sentence 'KNN showed substantial improvements. Specifically, compared to LDA (63.3%), KNN improved accuracy by 40.66%' appears to use 89.05 as the KNN value even though the table gives 89.9. Please keep the text and tables consistent.
Circularity Check
No significant circularity: the paper's claims are direct empirical benchmark results, not derived quantities.
full rationale
The paper's central claims are that 1D Dilated CNN with fused time-domain descriptors reaches 97% accuracy on Grabmyo and random forest with temporal-spatial descriptors reaches 94.95% on FORS-EMG (Abstract; Tables III-VIII). These are reported test-set measurements, not predictions derived from fitted parameters or from the authors' own prior results. The feature descriptors (fTDD, TSD, and wavelet features) are sourced from external cited works [29], [30], and [31], and the datasets are external [24], [25]. No equation in the paper defines a target quantity in terms of the inputs used to produce it, and no parameter fitted to a subset is renamed as a prediction of a closely related quantity. The only load-bearing risk is the underspecified 80/20 split in Section IV-B, which could inflate accuracy if windows overlap across train and test sets; that is a validity and reproducibility threat, not a circularity of the derivation. Consequently, no circular step is identified.
Assumptions & free parameters
free parameters (6)
- Sliding window duration and overlap =
600 ms, 50% overlap
- DWT decomposition level and wavelet =
J=5, sym8
- KNN neighbor count =
k=5
- Random forest tree count =
100 estimators
- SVM regularization =
C=1, linear kernel
- Deep learning learning rate =
0.0001, Adam
assumptions (4)
- domain assumption The 600 ms window with 50% overlap captures enough information to discriminate hand gestures.
- domain assumption The 80/20 split is subject-independent or otherwise does not leak same-subject windows into both train and test.
- domain assumption The feature formulas in Tables I and II, taken from Khushaba et al., are implemented correctly as published.
- domain assumption The datasets are representative of the hand gesture recognition task and the selected classes are balanced or appropriately handled.
Cite this review
Pith. "Pith review of Machine Learning-based sEMG Signal Classification for Hand Gesture Recognition." pith.science (2026). https://pith.science/paper/QEBVRFLF
@misc{pith2026241115655,
author = {Pith},
title = {Pith review of: Machine Learning-based sEMG Signal Classification for Hand Gesture Recognition},
year = {2026},
howpublished = {\url{https://pith.science/paper/QEBVRFLF}},
note = {Machine review of arXiv:2411.15655}
}
abstract
EMG-based hand gesture recognition uses electromyographic~(EMG) signals to interpret and classify hand movements by analyzing electrical activity generated by muscle contractions. It has wide applications in prosthesis control, rehabilitation training, and human-computer interaction. Using electrodes placed on the skin, the EMG sensor captures muscle signals, which are processed and filtered to reduce noise. Numerous feature extraction and machine learning algorithms have been proposed to extract and classify muscle signals to distinguish between various hand gestures. This paper aims to benchmark the performance of EMG-based hand gesture recognition using novel feature extraction methods, namely, fused time-domain descriptors, temporal-spatial descriptors, and wavelet transform-based features, combined with the state-of-the-art machine and deep learning models. Experimental investigations on the Grabmyo dataset demonstrate that the 1D Dilated CNN performed the best with an accuracy of $97\%$ using fused time-domain descriptors such as power spectral moments, sparsity, irregularity factor and waveform length ratio. Similarly, on the FORS-EMG dataset, random forest performed the best with an accuracy of $94.95\%$ using temporal-spatial descriptors (which include time domain features along with additional features such as coefficient of variation (COV), and Teager-Kaiser energy operator (TKEO)).
Figures
Forward citations
Cited by 1 Pith paper
-
An Exploratory Study of Single Channel Surface Electromyography for Hand Gesture Classification
A single sEMG channel plus Pearson-filtered time/frequency features and a compact neural network reaches 90% accuracy on 10 hand gestures, though the evaluation likely leaks label information.
Reference graph
Works this paper leans on
-
[1]
Manfredo Atzori, Matteo Cognolato, and Henning M ¨uller, “Deep learn- ing with convolutional neural networks applied to electromyography data: A resource for the classification of movements for prosthetic hands,” Frontiers in neurorobotics, vol. 10, pp. 9, 2016
work page 2016
-
[2]
Dinesh K Kumar, Beth Jelfs, Xiaohong Sui, and Sridhar P Arju- nan, “Prosthetic hand control: A multidisciplinary review to identify strengths, shortcomings, and the future,” Biomedical Signal Processing and Control, vol. 53, pp. 101588, 2019
work page 2019
-
[3]
A multi-sensor data fusion enabled ensemble approach for medical data from body sensor networks,
Muhammad Muzammal, Romana Talat, Ali Hassan Sodhro, and Sandeep Pirbhulal, “A multi-sensor data fusion enabled ensemble approach for medical data from body sensor networks,” Information Fusion, vol. 53, pp. 155–164, 2020
work page 2020
-
[4]
Paras Gulati, Qin Hu, and S Farokh Atashzar, “Toward deep gener- alization of peripheral emg-based human-robot interfacing: A hybrid explainable solution for neurorobotic systems,” IEEE Robotics and Automation Letters, vol. 6, no. 2, pp. 2650–2657, 2021
work page 2021
-
[5]
Toward improved control of prosthetic fingers using surface electromyogram (emg) signals,
Rami N Khushaba, Sarath Kodagoda, Maen Takruri, and Gamini Dis- sanayake, “Toward improved control of prosthetic fingers using surface electromyogram (emg) signals,” Expert Systems with Applications , vol. 39, no. 12, pp. 10731–10738, 2012
work page 2012
-
[6]
Turker Tuncer, Sengul Dogan, and Abdulhamit Subasi, “Novel finger movement classification method based on multi-centered binary pattern using surface electromyogram signals,” Biomedical Signal Processing and Control, vol. 71, pp. 103153, 2022
work page 2022
-
[7]
Rami N Khushaba and Sarath Kodagoda, “Electromyogram (emg) feature reduction using mutual components analysis for multifunction prosthetic fingers control,” in 2012 12th International Conference on Control Automation Robotics & Vision (ICARCV) . IEEE, 2012, pp. 1534–1539
work page 2012
-
[8]
Features selection for estimating hand gestures based on electromyography sig- nals,
Raghad R Essa, Hanadi Abbas Jaber, and Abbas A Jasim, “Features selection for estimating hand gestures based on electromyography sig- nals,” Bulletin of Electrical Engineering and Informatics , vol. 12, no. 4, pp. 2087–2094, 2023
work page 2023
Show all 39 references
-
[9]
Portable emg data acquisition module for upper limb prosthesis application,
Sidharth Pancholi and Amit M Joshi, “Portable emg data acquisition module for upper limb prosthesis application,” IEEE Sensors Journal , vol. 18, no. 8, pp. 3436–3443, 2018
2018
-
[10]
Efficient strategies for finger movement classification using surface electromyogram signals,
Sunil Kumar Prabhakar and Dong-Ok Won, “Efficient strategies for finger movement classification using surface electromyogram signals,” Frontiers in Neuroscience, vol. 17, pp. 1168112, 2023
2023
-
[11]
Hand gesture classification framework leveraging the entropy features from semg signals and vmd augmented multi-class svm,
T Prabhavathy, Vinodh Kumar Elumalai, and E Balaji, “Hand gesture classification framework leveraging the entropy features from semg signals and vmd augmented multi-class svm,” Expert Systems with Applications, vol. 238, pp. 121972, 2024
2024
-
[12]
Finger move- ments classification based on fractional fourier transform coefficients extracted from surface emg signals,
Zahra Taghizadeh, Saeid Rashidi, and Ahmad Shalbaf, “Finger move- ments classification based on fractional fourier transform coefficients extracted from surface emg signals,” Biomedical Signal Processing and Control, vol. 68, pp. 102573, 2021
2021
-
[13]
Electromyogram- based classification of hand and finger gestures using artificial neural networks,
Kyung Hyun Lee, Ji Young Min, and Sangwon Byun, “Electromyogram- based classification of hand and finger gestures using artificial neural networks,” Sensors, vol. 22, no. 1, pp. 225, 2021
2021
-
[14]
Performance evaluation of convolutional neural network for hand gesture recognition using emg,
Ali Raza Asif, Asim Waris, Syed Omer Gilani, Mohsin Jamil, Hassan Ashraf, Muhammad Shafique, and Imran Khan Niazi, “Performance evaluation of convolutional neural network for hand gesture recognition using emg,” Sensors, vol. 20, no. 6, pp. 1642, 2020
2020
-
[15]
Hand gesture recognition using compact cnn via surface electromyography signals,
Lin Chen, Jianting Fu, Yuheng Wu, Haochen Li, and Bin Zheng, “Hand gesture recognition using compact cnn via surface electromyography signals,” Sensors, vol. 20, no. 3, pp. 672, 2020
2020
-
[16]
Hand gesture recognition using smooth wavelet packet transformation and hybrid cnn based on surface emg and accelerometer signal,
Le Wang, Jianting Fu, Hui Chen, and Bin Zheng, “Hand gesture recognition using smooth wavelet packet transformation and hybrid cnn based on surface emg and accelerometer signal,” Biomedical Signal Processing and Control, vol. 86, pp. 105141, 2023
2023
-
[17]
Emg gesture recognition algorithm based on parallel multi-scale cnn,
Wenli Zhang and Jiantong Zhang, “Emg gesture recognition algorithm based on parallel multi-scale cnn,” in 2022 2nd International Conference on Frontiers of Electronics, Information and Computation Technologies (ICFEICT). IEEE, 2022, pp. 562–568
2022
-
[18]
An emg gesture recognition model based on batchnorm2d and incremental broad learning system,
Tianyu Wang, Wenli Zhang, Enqi Zhou, Sirui Zheng, Yufei Wang, Gongpeng Pang, and Yifan Li, “An emg gesture recognition model based on batchnorm2d and incremental broad learning system,” in 2023 3rd International Conference on Frontiers of Electronics, Information and Computati...
2023
-
[19]
A novel autonomous learning framework to enhance semg-based hand gesture recognition using depth information,
Salih Ertug Ovur, Xuanyi Zhou, Wen Qi, Longbin Zhang, Yingbai Hu, Hang Su, Giancarlo Ferrigno, and Elena De Momi, “A novel autonomous learning framework to enhance semg-based hand gesture recognition using depth information,” Biomedical Signal Processing and Control , vol. 66,...
2021
-
[20]
Recurrent neural network for electromyographic gesture recognition in transhumeral amputees,
Olivier Barron, Maxime Raison, Guillaume Gaudet, and Sofiane Achiche, “Recurrent neural network for electromyographic gesture recognition in transhumeral amputees,” Applied Soft Computing , vol. 96, pp. 106616, 2020
2020
-
[21]
Emg map image processing for recognition of fingers movement,
Ivan Topalovi ´c, Stevica Graovac, and Dejan B Popovi ´c, “Emg map image processing for recognition of fingers movement,” Journal of Electromyography and Kinesiology, vol. 49, pp. 102364, 2019
2019
-
[22]
Novel wearable hd-emg sensor with shift-robust gesture recognition using deep learning,
F ´elix Chamberland, ´Etienne Buteau, Simon Tam, Evan Campbell, Ali Mortazavi, Erik Scheme, Paul Fortier, Mounir Boukadoum, Alexandre Campeau-Lecours, and Benoit Gosselin, “Novel wearable hd-emg sensor with shift-robust gesture recognition using deep learning,” IEEE Transactio...
2023
-
[23]
Long short-term memory (lstm) recurrent neural network for muscle activity detection,
Marco Ghislieri, Giacinto Luigi Cerone, Marco Knaflitz, and Valentina Agostini, “Long short-term memory (lstm) recurrent neural network for muscle activity detection,” Journal of NeuroEngineering and Rehabilitation, vol. 18, pp. 1–15, 2021
2021
-
[24]
Multi-day dataset of forearm and wrist electromyogram for hand gesture recognition and biometrics,
Ashirbad Pradhan, Jiayuan He, and Ning Jiang, “Multi-day dataset of forearm and wrist electromyogram for hand gesture recognition and biometrics,” Scientific data, vol. 9, no. 1, pp. 733, 2022
2022
-
[25]
Fors- emg: A novel semg dataset for hand gesture recognition across multiple forearm orientations,
Umme Rumman, Arifa Ferdousi, Md Sazzad Hossain, Md Johirul Islam, Shamim Ahmad, Mamun Bin Ibne Reaz, and Md Rezaul Islam, “Fors- emg: A novel semg dataset for hand gesture recognition across multiple forearm orientations,” arXiv preprint arXiv:2409.07484 , 2024
2024 arXiv
-
[26]
Emg pattern recognition using convolutional neural network with different scale signal/spectra input,
Wei Yang, Dapeng Yang, Yu Liu, and Hong Liu, “Emg pattern recognition using convolutional neural network with different scale signal/spectra input,” International Journal of Humanoid Robotics , vol. 16, no. 04, pp. 1950013, 2019
2019
-
[27]
Electromyography gesture iden- tification using cnn-rnn neural network for controlling quadcopters,
Ray Antonius and Hendra Tjahyadi, “Electromyography gesture iden- tification using cnn-rnn neural network for controlling quadcopters,” in Journal of Physics: Conference Series. IOP Publishing, 2021, vol. 1858, p. 012075
2021
-
[28]
Cnn-lstm and post-processing for emg-based hand gesture recognition,
Lorena Isabel Barona L ´opez, Francis M Ferri, Jonathan Zea, ´Angel Leonardo Valdivieso Caraguay, and Marco E Benalc ´azar, “Cnn-lstm and post-processing for emg-based hand gesture recognition,” Intelligent Systems with Applications , vol. 22, pp. 200352, 2024
2024
-
[29]
A fusion of time-domain descriptors for improved myoelectric hand control,
Rami N Khushaba, Ahmed Al-Ani, Ali Al-Timemy, and Adel Al- Jumaily, “A fusion of time-domain descriptors for improved myoelectric hand control,” in 2016 IEEE Symposium Series on Computational Intelligence (SSCI). IEEE, 2016, pp. 1–6
2016
-
[30]
A framework of temporal-spatial descriptors-based feature extraction for improved myoelectric pattern recognition,
Rami N Khushaba, Ali H Al-Timemy, Ahmed Al-Ani, and Adel Al- Jumaily, “A framework of temporal-spatial descriptors-based feature extraction for improved myoelectric pattern recognition,” IEEE Trans- actions on Neural Systems and Rehabilitation Engineering , vol. 25, no. 10, pp...
2017
-
[31]
getmswtfeat,
Rami N. Khushaba, “getmswtfeat,” 2020
2020
-
[32]
Automatic screening of diabetic retinopathy using fundus images and machine learning algorithms,
KK Mujeeb Rahman, Mohamed Nasor, and Ahmed Imran, “Automatic screening of diabetic retinopathy using fundus images and machine learning algorithms,” Diagnostics, vol. 12, no. 9, pp. 2262, 2022
2022
-
[33]
Support vector machines and kernels for computational biology,
AOC Soon, S S ¨oren, S Bernhard, and RG Ben-Hur, “Support vector machines and kernels for computational biology,” PLoS Comput Biol , vol. 4, no. 10, 2008
2008
-
[34]
2, Springer, 2009
Trevor Hastie, Robert Tibshirani, Jerome H Friedman, and Jerome H Friedman, The elements of statistical learning: data mining, inference, and prediction, vol. 2, Springer, 2009
2009
-
[35]
Learning k for knn classification,
Shichao Zhang, Xuelong Li, Ming Zong, Xiaofeng Zhu, and Debo Cheng, “Learning k for knn classification,” ACM Transactions on Intelligent Systems and Technology (TIST), vol. 8, no. 3, pp. 1–19, 2017
2017
-
[36]
Practical machine learning tools and techniques,
Ian H Witten, Eibe Frank, Mark A Hall, Christopher J Pal, and Mining Data, “Practical machine learning tools and techniques,” in Data mining. Elsevier Amsterdam, The Netherlands, 2005, vol. 2, pp. 403–413
2005
-
[37]
Surface emg signal classification using tqwt, bagging and boosting for hand movement recognition,
Abdulhamit Subasi and Saeed Mian Qaisar, “Surface emg signal classification using tqwt, bagging and boosting for hand movement recognition,” Journal of Ambient Intelligence and Humanized Com- puting, vol. 13, no. 7, pp. 3539–3554, 2022
2022
-
[38]
An introduction to roc analysis,
Tom Fawcett, “An introduction to roc analysis,” Pattern recognition letters, vol. 27, no. 8, pp. 861–874, 2006
2006
-
[39]
Machine learning-based feature extraction and classification of emg signals for intuitive prosthetic control,
Chiang Liang Kok, Chee Kit Ho, Fu Kai Tan, and Yit Yan Koh, “Machine learning-based feature extraction and classification of emg signals for intuitive prosthetic control,” Applied Sciences , vol. 14, no. 13, pp. 5784, 2024
2024
Reviewed August 12, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.