The Effects of backpack weight on the biomechanics of load carriage
ARMY RESEARCH INST OF ENVIRONMENTAL MEDICINE NATICK MA MILITARY PERFORMANCEDIV
Pagination or Media Count:
An analysis of the effects of 4 backpack loads 6, 20, 33, and 44 kg on walking gait was performed on 16 male volunteers using a cinematographic system, force platform, tri-axial accelerometer, and 6 surface electrodes located over the trapezius, spinal erector, quadricep, hamstring, gastrocnemius and tibialis anterior muscles. When the load became very heavy, stride frequency increased. Double-support as percent of stride increased along with the load, effected by a delayed floor push-off. Knee range of motion increased with load during the eccentric knee flexion period from heel-strike until mid-stance. A lower total body center of mass position as the load increased was effected both by greater knee flexion and a more forward leaning trunk. An initial propulsive impulse at heel-strike resulted from flexion at the knee rather than from extension at the hip. A protective gait adjustment when increasing to the heaviest load limited the medial travel of the center of mass. As the load increased, hip extensor torque increased proportionately. Yet knee extensor torque increased more than expected, while ankle plantarflexor torque increased less than expected. Trapezius muscle activity showed that the frame-and-belt system did not prevent the shoulders from supporting considerable load. The spinal erectors produced the largest burst of activity at contralateral heel-strike. The gastrocnemius was largely inactive except for high activity during push-off, which did not increase with very heavy loads. The burden of carrying a very heavy load fell less on the calf muscles than on the muscles around the knee and hip. Trunk forwarddownward excursion and acceleration increased with load. The erector spinae acted eccentrically to decelerate trunk motion as the trunk approached its maximum forward lean. Slack in the straps enabled peak forward acceleration of the pack to occur later and be of lower magnitude than the peak forward acceleration of the trunk.
- Stress Physiology