Effects of load placement on back muscle activity in load carriage.

The effect of two different load placements (just below mid-back or just above shoulder level) on erector spinae EMG, trapezius EMG, and heart rate were investigated during load carriage. The EMG and heart rates were telemetered from 11 subjects while they walked on a smooth level surface at an average velocity of 5.6 km . h-1 carrying a load of 19.5 kg in a specially designed backpack. The average rectified EMG amplitude was calculated digitally for both load placements. The high load placement resulted in significantly higher levels of muscle activity than did the lower placement. Heart rate was not significantly different between the two placements. A qualitative biomechanical analysis suggests that the EMG differences are primarily due to differences in the moments and forces arising from the angular and linear accelerations of the load and trunk. The results indicate that metabolic measures alone are not sufficient to adequately assess tasks which evoke primarily local muscle demands.