Device Design for Measuring Weight Distribution Performance of Military Backpacks

Weight distribution helps users feel a backpack's weight as little as possible. It is essential for special forces, who carry backpacks for long periods during specialized missions. According to the paper, current South Korean military backpacks are not designed to distribute weight onto the pelvis, which puts special forces at risk of back injuries. Even if new backpacks with weight distribution functions are developed, there is a lack of devices and protocols to objectively measure how much load they place on the shoulders and pelvis. The paper cites an ideal of 80% of the load on the pelvis and 20% on the shoulders.

The authors designed a measuring device and test protocol for weight distribution performance (WDP) and then tested a real backpack.

• Design criteria: average body dimensions of South Korean special forces candidates and the weight of a fully packed military backpack (40 kg). From the 8th Korean Direct Measurement Anthropometric Survey (5092 people), 2024 individuals (988 men, 1026 women) who met the special forces height, weight and BMI criteria were used to compute 15 average body measurements.
• Torso angle: two adult men in their 20s wearing a 40 kg backpack were photographed from the side, and the average upper-torso bending angle (about 20 degrees) was built into the device.
• Device: a CNC-machined, torso-shaped device with four CE-certified load cells (50 kg limit each) on both shoulders and both sides of the pelvis, mounted perpendicular to the backpack contact surface, each connected to its own indicator.
• Protocol: zero the indicators, mount the backpack, fasten the hip belt, shoulder straps, top shoulder straps and chest strap in that order with the shoulder straps fully shortened, check that left–right differences are within 3 kg, then lengthen the shoulder straps in 3 cm steps and record each load cell. A scorecard rates WDP from Low to Superlative by the percentage of weight on the shoulders and pelvis.
• Test: a newly designed military backpack with a weight distribution function, packed to 35.1 kg, was measured six times, with four people running the test.

• Proposed method: a standardized device and test protocol, with a scorecard (Low, Intermediate, Advanced, Superlative), for objectively measuring and rating backpack WDP.
• Empirical result: As the shoulder straps were lengthened, the percentage of weight on the shoulders decreased and the percentage on the pelvis increased.
• Empirical result: Lengthening the left and right straps one after the other sometimes increased the difference between the left and right sides, but it stayed within the protocol's 3 kg tolerance.
• Empirical result / limitation: The total measured weight became greater than the backpack's actual weight (35.1 kg) as the straps were lengthened. The authors attribute this to the device's fixed torso angle, which cannot adjust to the changing rotational force the way a person would.

The authors conclude that the device and protocol adequately and objectively measure macroscopic (overall) changes in the ratio of military backpack weight distribution. They believe the device and protocol could become a standard for measuring the WDP of future military backpacks, but they say some improvements are needed. Future versions could make the torso bending angle controllable, and average bending angles could be calculated with more participants and different backpack weights (e.g., 10 kg, 20 kg and 30 kg) so that the device can measure backpacks of different weights. The authors add that a total measured weight higher than the actual backpack weight is a natural phenomenon and could be accepted.