ABSTRACT
Objective To investigate the effects of unevenly-distributed backpack loads on human physiological parameters as compared to the traditional centrally-placed load at the backpack bottom. Methods For the unevenly-distributed load mode, the backpack was divided into four spaces including top right, top left, bottom right and bottom left. The loads were divided into two parts proportionally (1/2∶1/2, 1/4∶3/4 and 3/4∶1/4) and placed at the bottom left and top right of the backpack, respectively. The remaining two spaces were filled with the plastic foam. Ten healthy volunteers performed 30-min walking trials on the treadmill at the speed of 1.1 m/s with the backpack load equal to 10% body weight (BW). The changes of subjects’ body posture, muscle fatigue, heart rate, blood pressure before and after the trial, as well as the electromyography (EMG) changes at 0, 5, 10, 15 min after the trial were measured by the Bortec AMT-8 and the NDI Optotrak Certus. Results The muscle fatigue of right upper trapezius and forward-leaning angle increased as the loads at top right of the backpack increased. However, the muscle fatigue in the new mode of backpack load distribution showed no significant differences as compared to the traditional mode under the same backpack loads. The new mode with the load ratio of 3/4∶1/4 had no significant effects on posture. The new mode with the load ratio of 1/2∶1/2 could contribute to the decrease of heart rate and blood pressure as compared to the traditional mode. Conclusions Adopting a new and more reasonable load distribution mode can guarantee the body posture unaffected by the backpack loads, and decrease heart rate and blood pressure. Therefore, the cross and evenly distributed load mode is recommended for the backpack design.
ABSTRACT
Objective To study the effect of passive motion from lower extremity on electromyography (EMG) activity of major muscles when the back under head down tilting state. Methods Thirty healthy subjects were selected. The EMGs of erector spinal and trapezius muscles were recorded and analyzed when human body was under head-down tilting at angel of 0°, 10°, 20° and 27° in static mode, respectively, in sway mode (along the axis of Z with frequencies of 120 and 140/min， respectively) and in vibration mode (along the axis of Y with frequency of 680/min) with the help of multifunctional test bed. Results In the static mode, the median frequency (MF) decreased with the increase of head-down titling angle; in the sway mode, the MF at each of 4 head-down tilting angles was smaller than that in the static mode. When the frequency was 120/min, the MF decreased with head-down tilting angle increasing, but when the frequency reached 140/min, the variation of MF became irregular. In the vibration mode, the MF increased with the increase of head-down tilting angle and was larger than that in the static mode. Conclusions In the static mode, the muscle fatigue increased with the head down tilting angle increasing; in the vibration mode, smaller head-down tilting angle should be selected to achieve better treatment effect; but in the sway mode, larger head-down tilting angle would be better, and the sway frequency should be set lower than 120/min to avoid potential damage due to excessive muscle fatigue.