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THE QUANTIFIED MEASUREMENT OF BACKPACK INSTABILITY USING IMPROPERLY FIT STRAPS

Abstract

V. Wimer, A. Higgins, T. Cherry, S. LaVigne, B. Higginson

Gonzaga University, Spokane, WA

PURPOSE: The purpose of this study was to quantify the amount of stability contributed by individual straps when using a military-style backpack. METHODS: Ten college-aged males and females at Gonzaga University were recruited to participate in the current study. The subjects were required to be able to walk with a 27.3 kg backpack for approximately 15 minutes. Each subject had one visit to the lab to complete the study. During this visit, subjects walked on a treadmill under 5 different conditions: 1) All backpack straps properly fit (A), 2) Waist belt unbuckled (W), 3) Shoulder straps improperly fit (S), 4) Load tensioners improperly fit (T), and 5) All the straps improperly fit (N). Subjects walked at a 6% incline (U), 0% grade (L), and -6% decline (D) for one minute for each condition, resulting in 15 different trials total. Inertial sensors were placed on the chest of the subject and on the backpack to measure movement in the vertical (V), mediolateral (ML), and anterior/posterior (AP) directions. Video analysis was used to quantify and analyze motion of the pack vs. the motion of the subject using frontal view and sagittal view cameras. A 2-way (grade x strap) Repeated Measures ANOVA was performed to detect difference in outcome measures of motion between conditions. RESULTS: There was a significant difference between strap conditions in the horizontal (p=0.000) and vertical (p=0.008) directions in the frontal plane. There was a significant difference in pack motion between grade conditions from the sagittal view in the horizontal direction (D: 2.44 ± 0.73 cm, L: 2.71 ± 0.88 cm, U: 2.60 ± 0.82 cm; p=0.013). AP acceleration during condition A was significantly less than the acceleration during condition T (4.11 ± 1.27 m/s2 and 4.17 ± 1.73 m/s2; p=0.035). AP acceleration in the W condition was significantly less than the acceleration in the N condition (4.05 ± 1.12 m/s2, 5.73 ± 5.26 m/s2; p=0.031) or T condition (4.05 ± 1.12 m/s2, 5.42 ± 1.73 m/s2; p=0.022). The AP velocity in the N condition was significantly greater than in A condition (0.66 ± 0.19 rad/s, 0.30 ± 0.14 rad/s, p=0.000) and the T condition (0.52 ± 0.76 rad/s, 0.30 ± 0.14 rad/s, p=0.001). There was no significant difference between the use of a properly strapped pack and one with the discontinued use of the waist belt in any condition. CONCLUSION: Loosening of the load tensioners caused the most instability of any condition, while the waist belt contributed the least to overall pack stability. The most pack motion occurred in the anterior/posterior and mediolateral directions.

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