With the increasing adoption of Zero-Gravity Seats in intelligent cockpits, there is a growing concern over the safety of occupants in reclined postures during collisions. The newly released anthropomorphic test device (ATD), THOR-AV, has modified the neck, spine, and pelvis structures to better match reclined postures. This study aims to investigate the changes in kinematic response and injury metrics for occupants in reclined postures, through high-speed frontal sled tests utilizing the THOR-AV. The tests were conducted using an adjustable rigid seat with a zero-gravity characteristic and an integrated three-point seat belt. Six tests were performed across four seat configurations: Standard, Semi-Reclined, Reclined, and Zero-gravity postures. The input acceleration pulse for these tests was derived from the equivalent double trapezoidal waveform of the Mobile Progressive Deformable Barrier (MPDB) test. Data from sensors and high-speed video were collected for analysis. The results indicated that with an increasing seat back angle, the degree of head flexion relative to the torso and neck load increased, with abnormal contact between the shoulder belt and neck. After posture reclining, the forward displacement of the ATD's torso increased, with a concomitant increase in lower chest compression, a decrease in thoracic forces, and a significant rise in lumbar axial forces. The zero-gravity posture exhibited submarining, as inferred from the iliac force reduction rate and video analysis. These findings provide critical insights for optimizing occupant restraint systems in reclined postures. Furthermore, the simplified rigid seat sled test environment demonstrated in this study is conducive to modeling and validation, suggesting the potential for further simulation-based investigations.