Multi-objective analysis for assessing the effects of load carriage on the spine

Abstract

Load carriage studies have been conducted for more than a century; however, the possible risk load carriage poses on the human body is still not fully understood. Although it is difficult to determine the best way to carry a load, a backpack has its merits and comparatively more favourable human responses (in terms of subjective, physiological, kinematic, and kinetic measures) to both light- and heavy-weight carriage. However, the validity of symmetric backpack carriage employment to individuals with asymmetric body alignment, such as patients with scoliosis, remains unclear. Most studies have evaluated various human responses to both symmetric and asymmetric load carriage merely with respect to a single objective approach and have not considered simultaneous changes in the human responses of interest. The objective of this study was to investigate the effects of continuous perturbation of an externally carried symmetric backpack carriage for healthy individuals and asymmetric single-strap cross-chest bag carriage for patients with scoliosis in static and dynamic situations. Changes in regional spinal curvature, trunk muscle activation, and lumbar spine loading were evaluated through multi-objective analysis. This was achieved by performing the present study in four interrelated parts. The first part of the study investigated the effect of backpack carriage on the critical change in sagittal spinal curvature from the neutral upright stance in order to identify the heaviness and correctness of backpack use. This was evaluated by assessing the spinal curvature changes along the whole spine simultaneously. Inertia-measuring sensors were used to measure the curvature changes in the cervical, upper thoracic, lower thoracic, and lumbar regions with no-load and a loaded backpack of up to 20% of body weight (BW). A multi-objective goal programming (GP) model was adopted to determine the global critical load of the maximum curvature change of the whole spine in accordance with the maximum curvature changes of the four spinal regions. The results suggested that the most critical backpack load was 13% of BW for healthy male college students. The second part of the study evaluated the effects of carrying a backpack at 0% (no-load), 5%, 10%, 15%, and 20% of BW on the simultaneous changes in trunk muscle activation and lumbar spine loading while walking. This was investigated using an integrated system equipped with a motion analysis, a force platform, and a wireless surface electromyography (EMG) system to measure the trunk muscle EMG amplitudes and lumbar joint forces. A multi-objective GP model was developed to determine the most critical changes in trunk muscle activation and lumbar joint loading. The results suggested that lightweight backpack carriage at approximately 3% of BW might reduce the peak lumbosacral compression force by 3% during walking, compared with the no-load condition. The most critical changes in both trunk muscle activation and lumbosacral joint loading were found for a backpack loaded with 10% of BW for healthy male college students. The third part of the study considered the effect of backpack load and boundary condition of the optimization process on the prediction of lumbar spine loading while walking towards the development of a computational algorithm in order to refine an EMG-assisted optimization (EMGAO) approach. Experimental data collected in the second part of the study were used as input data. The refined approach catered for the least possible number of variables and parameters in the optimization process and was established based on parameterized muscle gains constraining the lower boundary conditions of trunk muscle coactivations. A multi-objective GP model was developed to determine the optimal boundary condition along the backpack load spectrum between 0% and 20% of BW and a specified range of the boundary condition of the optimization process. The validity and reliability of the optimal boundary condition were analyzed using leave-one-out cross-validation and balanced bootstrap resampling methods. The refined approach provided a good estimator in terms of its unbiasedness, consistency, and efficiency for predicting the peak lumbosacral compression force. The fourth part of the study proposed an asymmetric load carriage method for correcting spinal deformity for patients with scoliosis. Scoliosis is both a subject dependent and time-variant condition. This was investigated by employing photogrammetry to measure the simultaneous changes in scoliotic curvature in the thoracic and lumbar regions with no-load and with a properly controlled single strap cross-chest bag loaded with 2.5%, 5%, 7.5%, 10% and 12.5% of BW. Statistical tests and a multi-objective GP programming model were adopted to determine the loading conditions (placement and weight of the bag) with optimal and minimal corrections of the affected and unaffected scoliotic spinal regions, respectively. Significant short-term postural correction of scoliosis could be achieved by applying an asymmetric load on the contralateral side relative to the apex location of the major scoliotic curve. The results suggested that the application of controlled asymmetric load carriage might be a possible pragmatic method for correcting scoliotic spinal curvature. Further study of the long-term effects of subject-specific optimal asymmetric load carriage on scoliotic spinal curvatures is recommended. In conclusion, a protocol for multi-objective analysis model was developed to investigate the effects of load carriage on the simultaneous changes in regional spinal curvature, trunk muscle activation, and lumbar spine loading during human locomotion. Such a protocol might be generalized and applied to the evaluation of other subjective, physiological, kinematic, and kinetic studies in other regions, such as the lower and upper limbs of the human body.