The biomechanical role lifting belts play in the prevention of low-back injury has been the subject of considerable debate. Recent studies have shown that lifting belts restrict trunk motion in the frontal and transverse planes during passive motion tests and when lifting. It is theorized that through this restriction of trunk motion, or in other words the stiffening of the torso, that lifting belts may protect the back from injury, particularly when the body is subjected to sudden unexpected loads. Epidemiological studies have indicated that sudden unexpected events, whether they consist of a sudden load imposed upon the body or a rapid trunk motion during a slip, are frequently related to the onset of low-back disorder (LBD's). Biomechanically, these events create large internal loadings on the spine and it's supporting structures as the muscles attempt to null the perturbation. It was hypothesized that the stiffening effects of the lifting belt may protect workers' backs from the extreme loads encountered during unanticipated loading events. Thus, the objective of the two studies was to determine whether lifting belts protect individuals exposed to sudden loading of the torso. The approach taken in both studies was to simulate sudden loading by rapidly applying a load either directly to the torso (experiment 1) or via a container held in the hands (experiment 2). In the "unexpected" trials within each study, the subjects were blindfolded and auditory cues were masked so that the temporal onset of the loading could not be determined. In half the trials, a lifting was belt was tensioned versus the remaining half of the trials where the same belt was extremely loose. Half of the trials were symmetric about the torso's mid-sagittal plane, and in half it was asymmetric (45 degrees). In each study, 8 trunk muscles were sampled with surface electromyography (EMG) prior to and during the sudden loading. In the first experiment, the subject's pelvis was fixed to a reference frame structure and the loads were applied directly to the torso. This allowed for the isolation of the trunk response to sudden loading independent of other body segments. In this study, 20 subjects, 10 male and 10 female, experienced 24 sudden loads (3 trials of each combination of the belt, expectancy, and asymmetry conditions). When the unexpected trials were examined, the benefits of the lifting belt were only apparent during the asymmetric loading conditions. On average, the normalized left and right erector spinae EMG decreased from 31 to 28 percent of the maximum voluntary contraction level (MVC), respectively. Kinematic changes measured with a Lumbar Motion Monitor (LMM), with the exception of a slight decrease in lateral bending (p<.05), were not existent when the belt was tensioned. No changes were observed in the EMG or the trunk kinematics prior to the unexpected loading with the lifting belt tensioned. In the second unexpected loading study, free-standing subjects were loaded by rapidly applying a force to a container held in the hands. The 10 male and 8 female subjects participated in 12 loading trials that investigated all combination of the three independent variables: belt use, preload, and load symmetry. The applied force was normalized to each subject's isokinetic trunk strength. Kinetic and kinematic data were obtained using two forceplates combined with an electro-magnetic motion measurement system (The Motion Monitors). These data allowed for the determination of postural changes and the computation of moments using a bottom-up model. EMG data were obtained from eight trunk muscles following the onset of the sudden unexpected load. In this study, the belt reduced forward bending of the spine during symmetric unexpected loadings. In females, the belt slightly reduced the lateral bending of the spine. In males, the belt reduced the flexion moment acting on the spine. Little change was found in the peak EMG signals from the posterior muscles during the symmetric loadings; however, there was a reduction in peak response from two of the anterior muscles. With asymmetric unexpected loadings, the peak EMG values increased in the posterior contra-lateral muscles and in the contra-lateral External Oblique. Peak activity in the ipsilateral Erector Spinae was reduced with the lifting belt. In sum, these results suggest the benefits of the lifting belt may be limited to unexpected loadings that are sagittally symmetric. Even though the flexion moment benefit continues to persist with the asymmetric unexpected loads, the additional contra-lateral muscle recruitment associated with the belt may increase the risk of muscle overexertion injury, thereby offsetting the potential benefit of reduced muscle recruitment ipsilaterally. Given that unexpected loads are unpredictable by definition, and will likely involve some degree of asymmetry, the data reported here suggest that a lifting belt may be of little help.
Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, 1653 West Congress Parkway, Chicago, IL 60612