Low-back pain and injury from lifting is a significant problem in the workplace. Many of these lifting injuries are due to the person being in an awkward position or position of relative unfamiliarity. It is anticipated that lifting a box from the floor to standing knuckle height from a sloped surface is a task that might place a person at higher risk of injury due to these factors. In order to gain a better understanding of the effects of floor slope on lifting kinematics (technique) and kinetics (forces), as they relate to increased risk for injury, two studies were performed over a two year period. The specific aim of the first study was to determine sub-maximal lifting capacities while lifting a box from an inclined surface (facing both up and down a 10 degree and 20 degree slope and the level) to knuckle height. The task was constructed such that both the person and box were on the inclined surface. Thirty five healthy college aged (18 - 24 years) men (n=17) and women (n=18) successfully completed the protocol. For each of the five lifting conditions a ten minute bout of lifting was required using the psychophysical method. A standard milk crate was lifted and lowered every 15 seconds with the opportunity to add or remove weight between lifts, so that a maximum non-fatiguing load was determined. In addition to recording the weight selected, reflective markers were placed on the right side of the subject in order to assess lifting technique. For the weights selected, there was no difference across lifting condition for the men or women. However, the men lifted significantly more than the women (25 kg vs. 15 kg, respectively). Significant differences were found in work done and lifting technique, with foot placement relative to the box and joint angle minimums (torso, hip, knee, and ankle) altered at start of the lifting phase. These findings suggest that even though submaximal lifting capacity is not affected by slope, risk for injury may. The specific aim of the second study was to examine the low-back forces when lifting from a sloped floor surface. Forty three subjects (21 men and 22 women), similar to those of the first study, successfully completed a single session of six lifts in each condition while erector spinae muscle activity, spinal curvature, and sagittal plane kinematics were measured. The men lifted 25 kg while the women lifted 15 kg. Kinematics were combined with anthropometric estimates of body segment parameters into an inverse-dynamics model that calculated forces in the erector spinae muscles and lumbar disc at L5/S 1. Consistent with the first year, foot placement relative to the box and joint ranges of motion were significantly affected by lifting condition. Also consistent with the lifting capacity results of the first year was the finding that maximum low-back forces were not affected by lifting condition. Confirming the results of the model was the finding that erector spinae muscle activity was not affected by lifting condition. However, it was found that the natural lordosis of the low back spinal curvature was lost at the time of maximum forces in the down hill lifting conditions. Based on lifting capacity and low-back force estimates, the results suggest that floor slope does not affect risk for injury when lifting from the floor to knuckle height. However, when spinal curvature and lift distance (work) are factored in, there may be an increased risk for injury in the down hill conditions. Additionally, horizontal foot distance to the box is increased in the up hill lifting conditions, possibly increasing risk for injury in a different manner relative to the down hill slope. In conclusion, care should be taken in both down hill and up hill sloped lifting conditions to minimize the potential for low-back pain and injury.
Colorado State University, Fort, Collins, CO 80523-1582