Research Article: Mechanical Predictors of Discomfort during Load Carriage

Date Published: November 3, 2015

Publisher: Public Library of Science

Author(s): Patrick D. Wettenschwiler, Silvio Lorenzetti, Rolf Stämpfli, René M. Rossi, Stephen J. Ferguson, Simon Annaheim, Christophe Egles.

http://doi.org/10.1371/journal.pone.0142004

Abstract

Discomfort during load carriage is a major issue for activities using backpacks (e.g. infantry maneuvers, children carrying school supplies, or outdoor sports). It is currently unclear which mechanical parameters are responsible for subjectively perceived discomfort. The aim of this study was to identify objectively measured mechanical predictors of discomfort during load carriage. We compared twelve different configurations of a typical load carriage system, a commercially available backpack with a hip belt. The pressure distribution under the hip belt and the shoulder strap, as well as the tensile force in the strap and the relative motion of the backpack were measured. Multiple linear regression analyses were conducted to investigate possible predictors of discomfort. The results demonstrate that static peak pressure, or alternatively, static strap force is a significant (p<0.001) predictor of discomfort during load carriage in the shoulder and hip region, accounting for 85% or more of the variation in discomfort. As an additional finding, we discovered that the regression coefficients of these predictors are significantly smaller for the hip than for the shoulder region. As static peak pressure is measured directly on the body, it is less dependent on the type of load carriage system than static strap force. Therefore, static peak pressure is well suited as a generally applicable, objective mechanical parameter for the optimization of load carriage system design. Alternatively, when limited to load carriage systems of the type backpack with hip belt, static strap force is the most valuable predictor of discomfort. The regionally differing regression coefficients of both predictors imply that the hip region is significantly more tolerant than the shoulder region. In order to minimize discomfort, users should be encouraged to shift load from the shoulders to the hip region wherever possible, at the same time likely decreasing the risk of low back pain or injury.

Partial Text

Discomfort during load carriage is a major issue for activities using backpacks (e.g. infantry maneuvers, children carrying school supplies, or outdoor sports). According to Sheir-Neiss et al. [1], 74% of adolescent backpack wearers suffer from neck or back pain, validated by significantly poorer general health, more limited physical functioning, and more bodily pain. As loads have increased during the last decades [2, 3], discomfort during load carriage will become an even more important topic. With increasing loads, discomfort is more likely to be accompanied by injuries like the impairment of the brachial plexus [4–8] or low back pain or injury [9–11]. Any efforts to reduce the occurrence of these medical issues, ranging from discomfort to severe injury, are thus most welcome. A potential improvement could be achieved by optimizing load carriage system design, e.g. system structure or the material properties at the interface between the system and the body. Partial success towards this goal has already been achieved through the use of load carriage systems with a hip belt or a comparable structure, supporting a load shift from the shoulders to the hip [12]. Manufacturers of load carriage systems have an additional motivation to improve their design, as discomfort is known to have considerable influence on user acceptance [13, 14]. However, the variety of currently available load carriage systems suggests that the optimum has not yet been reached. The optimization is hindered by the fact that discomfort is a subjective perception and also depends on the length of time the system is worn [15]. In addition, the personal mood of the subjects and their physical constitution may influence the subjective perception of discomfort [16, 17]. Therefore, objective measurement of discomfort during load carriage is currently very challenging.

Ten male subjects without any history of back pain and with the following anthropometrical characteristics were tested (mean ± standard deviation): age 28.0 ± 3.7 years, weight 73.1 ± 10.4 kg, height 178.7 ± 5.7 cm. All subjects provided written informed consent.

The absolute values of the discomfort and the objectively measured mechanical parameters are not the primary interest in this study. Nevertheless, they are provided as a mean of all subjects for all configurations as supporting information (S1–S3 Tables).

The aim of this study was to identify objectively measured mechanical predictors of discomfort during load carriage. We compared twelve configurations of a typical load carriage system, evaluating average pressure, peak pressure, strap forces and relative motion between the bulk of the system and the body in the shoulder and hip region for static and dynamic conditions. We conducted the main multiple linear regression analyses twice: once using only the pressure parameters as independent variables and once using all mechanical parameters as independent variables. For both versions, we used regional discomfort as dependent variables.

In this study, we found that static peak pressure, or alternatively, static strap force, is a significant (p<0.001) predictor of discomfort during load carriage in the shoulder and hip region, accounting for 85% or more of the variation in perceived discomfort. As an additional finding, we discovered that the regression coefficients of these predictors are significantly smaller for the hip than for the shoulder region. For a quantitative comparison, the linear relationship of static peak pressure and static strap force between both regions at equal discomfort was evaluated. This revealed the shoulder region to be more than twice as sensitive as the hip region to an increase in static peak pressure or static strap force. In order to minimize discomfort, users should be encouraged to shift load from the shoulders to the hip region wherever possible, at the same time likely decreasing the risk of low back pain or injury. However, the main outcome of this study is the successful identification of objectively measured mechanical predictors of discomfort, which represent a valuable tool for the optimization of load carriage system design.   Source: http://doi.org/10.1371/journal.pone.0142004