During a field observation of professional movers, it was noticed that some experienced movers carried loads by placing them posterior to their pelvis and holding them against their back. There is ample load carriage research in biomechanics and ergonomics regarding backpack usage in both civilian and military populations. However, there is a void of literature that investigates hand-held load carriage. The purpose of this study was two-fold: (1) to assess the biomechanical differences between hand-held load carriage anterior and posterior to the pelvis; and, (2) to determine if an assistive load carriage device could reduce muscle effort while carrying loads either anterior or posterior to the pelvis.

To compare the biomechanical differences between anterior load carriage (AC) and posterior load carriage (PC) postures, an electromyographic (EMG) analysis was conducted on each carrying posture while participants carried a load on a treadmill. A specialized box, loaded with 20% of the subject’s body weight, was carried by ten male volunteer subjects who had no previous back injury or moving experience. Isometric maximum voluntary exertions (iMVE) were measured for each muscle tested. The subjects then conducted three trials of AC and PC techniques, while EMG data were being collected. All trial data were normalized to their respective iMVE values. An amplitude probability distribution function (APDF) was used to compare EMG amplitudes at the 10 th , 50 th , and 90 th percentiles.

Results indicated that PC significantly reduced EMG activity of the erector spinae (>50% reduction), trapezius, and anterior deltoid (p<0.05) as well as increasing EMG activity in the posterior deltoid (p<0.05). Such large reductions in the erector spinae muscle activity may lead to substantial reductions in spinal compression forces. Although there were significant reductions in erector spinae activity, 80% of the subjects reported that the PC method felt awkward and cumbersome. Due to its awkwardness, many individuals may not use the PC technique, even though it may be beneficial to back health. Based on this subjective response, the second purpose of this thesis was to design an assistive movers’ pack that would not only aid in AC and PC techniques, but also make the PC method easier to perform.

The second study in this thesis involved 10 male subjects with no previous back injury and no prior moving experience. Subjects were asked to walk unloaded while EMG was recorded. The subjects then performed the AC and PC methods with and without the assistive device. All EMG signals were normalized to unloaded gait followed by EMG APDF analyses of the testing conditions.

Results confirmed the findings from the first study, in that PC significantly reduced erector spinae activity (p<0.05) and moved the shoulder load from the trapezius and anterior deltoid and focused it on the posterior deltoid. The assistive device effectively reduced flexor digitorum activity (>40% reduction, p<0.03) and anterior deltoid activity (>75% reduction, p=0.5) in both AC and PC. Only small increases in external oblique activity occurred with device use in AC. Erector spinae EMG remained similar to the respective unassisted condition. These results provide evidence that the assistive load carriage device used in this study can be an effective ergonomic tool to alleviate grip effort and shoulder activity in both AC and PC conditions. Additionally, subjective surveys indicate that the assistive device decreased the awkwardness and difficulty in performing the PC technique among less experienced movers.

Категория: Научные статьи | Нет комментариев »

Комментарии