Novel Use of Inertial Measurement Units to Drive Inverse Kinematics Model to Estimate Motion During Military Parachute Tethered Tandem Bundle Operations.

Military free-fall parachute operations subject Service Members to sudden mechanical loads during tether snatch and canopy opening, creating a high risk of musculoskeletal injury. Traditional video-based motion capture is impractical in these environments, limiting the ability to estimate joint-level kinematics in vivo. This study evaluated the feasibility of integrating wearable inertial measurement units (IMUs) with musculoskeletal modeling to capture neck and shoulder kinematics during operational parachute jumps. Experienced parachutists conducted high-altitude low opening (HALO), high-altitude high opening (HAHO), and no-bundle jumps while equipped with seven synchronized IMUs positioned on the head, torso, and arms. Data were processed with the OpenSim OpenSense framework using a modified upper-body model. Model scaling was based on each subject's standing height, generic range of motion limits were applied, and calibration relied on an iterative alignment of the sensor orientations obtained while participants stood in a neutral anatomical posture. These methodological choices enabled analysis under operational conditions but also introduced potential sources of error, including calibration drift due to sensor shifts, lack of segment-specific scaling, and the absence of subject-specific joint range of motion data. Despite these challenges, the framework successfully generated estimates of neck and shoulder kinematics across jump types, demonstrating feasibility in a setting where conventional motion capture systems cannot be used. This work demonstrates the feasibility, promise, and current limitations of IMU-driven inverse kinematics in extreme environments. Lessons learned emphasize the need for individualized segment scaling, subject-specific range of motion measurements, and improved calibration protocols. By addressing these challenges in future research, IMU-based musculoskeletal modeling can become a powerful tool for integrating human biomechanics into parachute system design, protective equipment development, and training strategies aimed at reducing injury risk.