This study examined the effect of sagittal trunk posture on the gait of able-bodied subjects. Understanding the effect of trunk posture on gait is of clinical interest since alterations in trunk posture often occur with age or in the presence of spinal pathologies, such as lumbar flatback. Gait analysis was conducted on 14 adults walking at self-selected slow, normal, and fast walking speeds while maintaining three trunk postures: upright, and with 25+/-7 degrees and 50+/-7 degrees of trunk flexion from the vertical. During trunk-flexed gait, subjects adopted a crouch posture characterized by sustained knee flexion during stance and an increase in ankle dorsiflexion and hip flexion angles. During stance, these kinematic adaptations produced a posterior shift in the positions of the trunk and pelvis, which helped to offset the anterior shift in the trunk mass that occurred with trunk flexion. In this way, kinematic adaptations may have been used to maintain balance by shifting the body's center of mass to a position similar to that of upright walking. These changes in lower limb joint kinematics created a phase lag in the position of the hip joint center relative to that of the ankle joint center in the sagittal plane. Alterations in the sagittal alignment of the hip and ankle joint positions were associated with a phase lag in the vertical position, velocity, and acceleration of the body's center of mass (BCOM) relative to upright walking. Since the vertical ground reaction force (GRF(v)) is proportional to the vertical acceleration of the BCOM, significant changes were also seen in the GRF(v) during trunk-flexed gait. In summary, kinematic adaptations necessary to maintain dynamic balance altered the trajectory and acceleration of the BCOM in the vertical direction, which was reflected in the GRF(v). The results of this study may help clinicians better understand the nature and impact of compensatory mechanisms in patients who exhibit trunk-flexed postures during gait.