AbstractThe determination of the location of the horizontal axis in the frontal plane for movements in the sagittal plane between L4 and L5 is based on the observation that the erectores spinae muscles and the rectus abdominis muscles in the upright standing position are acting as antagonists and antigravity muscles. X‐ray pictures were taken in the sagittal plane with the subjects standing a) with slight activity in the erector spinae muscles and b) with slight activity in the rectus abdominis muscles. The location of the line of gravity for the parts of the body above the level of L5 was then determined with the subjects standing in the same positions. These determinations revealed that the activity in the erector spinae muscles ceases and is replaced by activity in the rectus abdominis muscles, when the line of gravity passes a point about 3 cm dorsally to the center of the inter‐vertebrate disc between L4 and L5. Similarly the location of the line of gravity in the frontal plane was determined in relation to L5. The results are discussed in view of previous findi