Effect of load placement on select trunk, shoulder, and leg muscle activity using two backpack designs

The muscle activity of the trapezius, pectoralis major, rectus abdominus, and rectus femoris was investigated while walking on a treadmill using: a day pack with two shoulder straps, placing the load in the middle of the back; a lumbar pack with waist straps, placing the load on the lower back and hips; and no load. Fifteen participants walked at 1.3 m/s at 1% grade while electromyographic activity was recorded from the aforementioned muscles. Maximal voluntary contractions were performed and used to normalize the root mean square (RMS) of the EMG data. Univariate statistics revealed that mean rectus femoris EMG values for the lumbar pack condition were significantly different than values for the day pack (p = .01) and no-load conditions (p = .001). The rectus femoris exhibited equal or less EMG activity during the day pack load carriage condition than the lumbar pack for all four muscles.