The psychophysical method of adjustment was used to determine whether slight changes in load position and comfort could have significant effects on the maximum backpack load acceptable to subjects. Four males and four females, who were accustomed to walking with backpacks, were given 15 minutes to adjust the load until it was, in their judgment, neither too heavy nor too light for an 8-hour trek. The variables in this 2³ within-subject experimental design were horizontal and vertical load position, as well as backpack type. The levels of the horizontal load position differed by a distance of 4.0 cm, which corresponds to the difference between the internal and external-frame backpacks used in the experiment. The vertical distance levels were 1/3 and 2/3 of the height of the pack. The pack types (internal and external-frame) were selected on the basis of their harnesses, to provide two distinct levels of comfort.

The results indicated that the horizontal and vertical load positions did not have an appreciable effect on the psychophysically determined maximum acceptable load, although there was a significant interaction between the horizontal position and pack type. There was a significant difference in trunk angle due to Pack type and Horizontal load position. The stride rate was not affected by the treatment conditions, but it decreased significantly with the addition of load. There was a significant difference in comfort ratings between backpacks, with subjects choosing to carry 6% larger loads in the one they rated as more comfortable (the external-frame backpack).