The trunk is the main part of the body. It does not include the head or the limbs but everything in between (front to back, top to bottom).
The thorax is also known as the chest. It is between the neck and the bottom of the ribs. The thorax includes the ribs, part of the spine, and all of the organs inside the chest.
The pelvis refers to four bones in the lower part of the trunk. These include the sacrum, coccyx (tailbone), and the iliac bones (where you rest your hands when you place them on your "hips").
When walking at slow speeds, these body parts move together in a smooth, coordinated pattern. This is called in-phase coordination. At faster speeds, the body actually switches to a pattern of movement that is out of coordination, sometimes called anti-phase coordination.
For someone with back pain, the muscles around the trunk and pelvis contract and hold, locking the body in a rigid position. This means the body can only use the in-phase coordination pattern, no matter what the walking speed. This can cause problems. A specific exercise program set by a physical therapist can help restore normal movement.