Optical motion capture (OMC) systems are widely used in rehabilitation, sports, and robotics to obtain accurate segment attitudes. However, OMC marker data can be lost due to occlusions, and reliably recovering data for long-duration missing intervals remains challenging. This study proposes a method for recovering missing markers using inertial measurement unit (IMU) signals and rigid-body constraints. We implemented two recovery methods and validated their performance. The proposed method (M1) combines inter-marker distance constraints with an acceleration constraint, while the comparison method (M2) combines inter-marker distance constraints with a tilt constraint. M1 demonstrated superior performance, with an average root mean squared error that was 3.10 and 3.99 mm lower than that of M2 for the 30 and 180 s missing intervals, respectively. This performance difference arises because M1 directly utilizes measured IMU signals, whereas M2 incurs additional uncertainty due to attitude estimation errors. Furthermore, the proposed method maintained reliable performance even during long-duration missing intervals, as it operates independently of past data, preventing recovery error accumulation. These results demonstrate the feasibility of the proposed IMU-based method for recovering longduration missing marker data in OMC systems.
