Characterization of shoulder load for backpack shoulder strap design based on the relationship between interface pressure and shoulder pain

With the increasing use of backpacks in recent years, proper reduction of shoulder load has become essential. We focused on nociceptive pain, which is a physiological warning signal, and performed subjective evaluation as an index to determine appropriate loading. In this paper, we investigated the relationship between a single-point pressure stimulus on the shoulder and pain and characterized pain sensitivity for each region of the shoulder. In the experiment, seven subjects rated their pain level upon stimulation using a pain scale. In data analysis, the rating scores were revised to an equivalence scale, and the relation between the pain score and stimulus intensity was approximated by a sigmoid function. Moreover, pain thresholds were estimated by approximate expression and classified into four sensitivity levels after normalizing. Combining the measurement points, we proposed characterization mapping of pain sensitivity and showed the pain sensitivity level at each measurement point. The results of pain rating for the strongest stimulus in the experiment, characterization mapping, and the results of pain sensitivity at each measurement point were anatomically matched. As such, we clarified the shoulder regions that can actively support load and those that should avoid loading. These results can be used to evaluate and improve backpack strap design.