This is the first study to quantify the proportion of force supported by the lower back during load carriage, and for the MOLLE pack, this proportion was not affected by backpack mass. One limitation of this study is that data were collected only on male subjects. Females typically have a broader sacrum resulting in a wider pelvis and more prominent iliac crest (11). The difference in the structure of the pelvis between males and females may influence the proportion of the backpack’s weight supported by the lower back, which is made up of the sacrum and iliac crest. Consequently, the current data set is limited in that it may not accurately illustrate the proportion of force carried on the lower back in female subjects. Work is currently underway replicating this study using female subjects.
The notion that balancing torques results in the consistent anterior force exerted on the lower back and consistent posterior force on the upper back, suggests changing the location of the COM of the load may influence the anterior/posterior forces exerted on the upper and lower back. For instance, a double (back and front) pack shifts the COM of the load anteriorly compared with a backpack and, consequently, may result in a decrease in the peak and mean anterior/posterior forces exerted by the pack on the lower back. This, in turn, may decrease the occurrence of low-back pain associated with load carriage. Previous research comparing ratings of pain, soreness, and discomfort between a backpack and a double pack substantiate this claim. The use of a double pack results in a decrease in low-back pain while carrying heavy loads (8).
The current study used a novel approach to measuring the distribution of forces between the upper and lower back during load carriage. The results clearly demonstrated that the use of an external frame backpack with a hip belt transfers approximately 30% of the vertical force generated by the backpack to the lower back (iliac crest and sacrum). Although it appears certain that transferring some of the load from the upper back to the lower back is advantageous, information is lacking as to what percentage of load distribution between the upper and lower back is optimal in terms of reducing the potential for injury resulting from high forces, and how this proportion may influence metabolic cost. Future research should focus on how variation in the proportion of weight supported by the upper and lower back during load carriage affects comfort, injury risk, and metabolic cost, so as to determine an optimal load distribution between upper and lower back.
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