Three-dimensional MR motion estimation on a single image plane based on tagged MRI and HARP processing. A plurality of maximum-likelihood dates associated with text pages within the book is accessed. The engaging member includes a plurality of elongated arcuate projecting members extending about a rotational center of the engaging member and projecting from an end surface of the engaging member to positively engage the connecting element in the receiver. That is, the trolley can travel forward and backward or leftward and rightward, turning vertically in a narrow space. The extension members are rotatably coupled to the main backpack and rigidly coupled to the roll control rod, such that when a person wearing the backpack hikes, and the person’s hips move up and down, the combination of the roll control rod and interconnected extension sections compensate for the up-and-down movement of the hips to stabilize the load and prevent side-to-side swaying.