With the advancement of information technologies such as the Internet of Things and artificial intelligence, cyber-physical systems are being introduced into society. At the core of these systems is the digital twin, a computer model (twin) of a physical entity built in cyberspace for simulation-based prediction.

Currently, the digital twin mainly targets artificial objects, such as aircraft engines and factories. However, if it can be extended to humans, it could lead to the realization of human-machine cooperative systems and health promotion services, thereby solving social issues stemming from the aging of society. However, humans are the weakest link in the system, and many technical problems remain to be solved, such as realizing the measurement, modeling, and prediction of human behavior, if the human digital twin is to become reality.

This special issue contains 9 papers on developing essential technologies for the human digital twin and constructing human-machine systems for specific applications. The topics covered include learning algorithms, motion measurement and analysis techniques, human perception, system development, and platform software for system development. These clearly show that cross-disciplinary efforts are essential to the realization of the human digital twin.

We thank the authors of the papers submitted for this special issue. We are confident that the information provided by the authors is suggestive and informative for both specialists and non-specialists alike. We also sincerely appreciate the efforts of the reviewers. Their contributions helped to make this special issue possible. We hope that this special issue will catalyze sharing across the boundaries of research fields along the path to realizing a human digital twin.