This study aimed to determine the role of trunk muscles during single-leg jumping motion using a new link segment model (P-V model). The commonly used model has a virtual joint at the center point between the right and left ribs. In P-V model, however, the pelvic segment and lumbosacral joint were considered. Eight male jumpers performed single-leg drop jump (SLDJ) from a height of 0.3 m. Jumping motions were videotaped using three high-speed VTR cameras (300 Hz) and analyzed by the DLT technique. Frontal and lateral views of each subject were photographed, and the inertia properties of the pelvic segment were calculated using these images. Ground reaction force was recorded using a force platform (1000 Hz). Kinematics, kinetics, and energetics data were calculated. The results of all analyses were as follows:

1. Elevation-depression torque power at the lumbosacral joint in P-V model was greater than lateral flexion torque power at the virtual joint in the commonly used model.

2. Mechanical energy generated by elevation-depression torque at the lumbosacral joint in P-V model was greater than that generated by lateral flexion torque at the virtual joint in the commonly used model.

3. Elevation-depression torque at the lumbosacral joint was greater than abduction torque at the hip.

4. The relative total work performed by the elevation torque at the lumbosacral joint was greater than relative positive work and negative work.

Because of the difference in the position between the lumbosacral joint and the virtual joint, the joint angular velocity changed. Consequently, the torque power and mechanical energy were different in the lumbosacral joint and the virtual joint. These results suggests that (1) in the commonly used model, the mechanical energy generated through elevation-depression movement of the pelvis is underestimated; (2) in SLDJ, elevation-depression movement of the pelvis is mainly produced through elevation torque at the lumbosacral joint, not by abduction torque at the hip; and (3) muscles that elevate the pelvis play a role in generation of mechanical energy.