ReviewMachine learning in human movement biomechanics: Best practices, common pitfalls, and new opportunities
Introduction
For most of the 20th century, inference in biomedical research was predominantly based on hypothesis testing using parametric tests, such as the Student’s t test. The current surge of data, however, presents new challenges and opportunities that are shifting the data analytics landscape in many biomedical disciplines, including human movement biomechanics. Data characterizing human movement are high-dimensional, heterogeneous, and growing in volume with wearable sensing; often, they do not satisfy assumptions associated with parametric tests. Advanced analytical techniques to extract informative features from these data and model underlying relationships that cannot be modeled with traditional statistical tools could transform biomechanics research, as they have autonomous driving, speech recognition, and automated cancer detection.
Efforts to modernize biomechanical data analysis are exemplified by the use of feature extraction algorithms such as principal component analysis (PCA). The literature reflects an evolving awareness about the drawbacks of using only summary metrics (e.g., mean acceleration) or salient features (e.g., the peak knee adduction moment) to describe gait data, as summary metrics are not always the most informative with respect to outcomes of interest (e.g., disease status). PCA, which preserves the variability of multivariate datasets while reducing dimensionality to make analyses more tractable, has been used as an alternative (Deluzio and Astephen, 2007, Donoghue et al., 2008, Duhamel et al., 2006, Ryan et al., 2006). Although most biomechanics studies that employ these methods for dimensionality reduction continue to analyze the reduced data with traditional statistical tools, biomechanists are now also considering new problem formulations in which features extracted using PCA are used as inputs in machine learning models.
Two machine learning approaches, predictive modeling and data mining, serve different purposes than traditional inferential statistics. Predictive modeling is concerned with finding a function that optimally maps input data (e.g., kinematic waveforms) to a given output (e.g., disease status) with the goal of making accurate predictions in the future. One example of predictive modeling in biomechanics is myoelectric control of prostheses, where models are trained to recognize an individual’s intention based on myoelectric signals and the predicted intention is used to control the prosthesis (Oskoei and Hu, 2008). More recent efforts have centered around diagnostic and prognostic predictive models for neuromuscular and musculoskeletal pathologies (e.g., Schwartz et al., 2013), fall prediction (e.g., Wei et al., 2017), activity recognition to facilitate out-of-clinic patient monitoring (e.g., Biswas et al., 2015), and event detection to guide interventions such as deep brain stimulation (e.g., Pérez-López et al., 2016). The goal of data mining, on the other hand, is to discover new patterns in the data. Using clustering methods to identify subpopulations that exhibit different types of pathological gaits is an example of data mining (e.g., Rozumalski and Schwartz, 2009).
While applications of machine learning methods are expanding in movement biomechanics, critical evaluation of studies that apply them remains difficult. Machine learning approaches differ from the traditional statistical tools that biomechanists are trained to apply and interpret based on established reporting standards (e.g., p value for statistical significance). As the field becomes more data-intense and the use of machine learning continues to increase, good practices for conducting and reporting research at the intersection of biomechanics and machine learning are needed to ensure that conclusions are valid and reproducible. A discussion of this topic will also enable researchers to develop an intuition for the types of problems that machine learning can address more successfully than traditional statistics. Accordingly, the goal of this survey is to make machine learning efforts more visible and propose standards to increase the quality and impact of future research in this exciting area. To achieve this goal, we first review applications of machine learning that focus on neuromuscular and musculoskeletal diseases. We outline best practices for reporting the results of these analyses and common pitfalls we encountered in the literature. Finally, we offer suggestions for overcoming some of the challenges facing biomechanical data analytics and highlight opportunities where emerging techniques are likely to have great impact in upcoming years. Key terms are defined in Appendix A and our most important recommendations are summarized in the Conclusions section.
Section snippets
Literature search approach
We carried out a search for original research articles published up to December 31, 2017 using the PubMed/Medline database (1946-). Our search identified articles that used machine learning methods to study human movement biomechanics and was limited to studies of common musculoskeletal and neuromuscular diseases affecting mobility. We used search terms from three different categories to identify relevant studies: (1) movement biomechanics terms, such as gait, kinematics, and kinetics; (2)
Results
Our search yielded 3193 research articles, out of which 129, dating from 1996 to 2017, satisfied the inclusion criteria (Fig. 1A; Supplementary Table 1). The majority of studies focused on predictive tasks—classification (80.6%) and regression (11.6%)—while a few focused on data mining, in particular clustering tasks (7.8%). The most used algorithms were support vector machines, artificial neural networks, and generalized linear models (linear or logistic regression) for predictive modeling and
Discussion
The use of machine learning methods in movement biomechanics research is on the rise (Fig. 1A). From passively monitoring post-stroke patients with wearable devices to predicting outcomes of interventions in children with cerebral palsy, the range of applications where advanced analytics can improve rehabilitation research will continue to expand, particularly as wearable sensing generates vast amounts of data. The aim of this review is to bring to light machine learning efforts in movement
Acknowledgements
This work was funded by the National Institutes of Health (NIH) Grant U54EB020405. The authors would like to thank Jessica Selinger, Rachel Jackson, Łukasz Kidziński, Wolf Thomsen, and Jennifer Yong for their insightful feedback.
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