Report on the impact of the exoskeleton on the L4L5 disk during handling movements — Musculoskeletal simulation
Back pain, often referred to as the “pain of the century”, is becoming increasingly important in industrialized countries. In fact, according to data from the National Institute for Research and Safety (INRS), more than two out of three employees have had, have or will have low back pain in France (INRS, 2018). Low back pain appears to be a multifactorial pathology with which several risk factors are associated, including manual handling, falls, demanding postures, major efforts, etc. So many factors can be found in the professional environment, all sectors combined. Low back pain accounts for 20% of work accidents and 7% of occupational diseases, accounting for nearly 11.5 million days not worked per year.
To limit the risks of low back pain at work, some companies carry out ergonomic studies in order to best organize workstations. When it is impossible to adapt the station, new solutions such as exoskeletons can be considered. The objective of this study is to analyze the impact of the Japet exoskeleton on the L4L5 intervertebral disc, relating to frequent movements during manual handling.
Model used
This musculoskeletal simulation was carried out using the AnyBody Modeling System software, using a whole-body model (full-body, AnyBody Managed Model Repository, AMMR). This model is widespread in the world of research, more than fifty scientific publications refer to it (AnyBody Technology). Many researchers have used this software to study the efforts in the various joints, and in particular in the vertebral column (Zee, 2007) (Rajaee, 2005) (Rasmussen, 2009) (Bassani, 2017), taking as a reference the in vivo measurements carried out by Wilke et al. (Wilke, 2001).
This study evaluates the impact of the Japet exoskeleton on the force exerted in L4L5 discs according to various parameters: the location of the load, the mass of the load, as well as the position of the body during load carrying.
AnyBody Technology offers its community the WilkeSpinalDiscPressure library. It is composed of three files that served as a basis for us: “Standing Lift Close”, “Standing Lift Flexed” and “Standing Lift Stretched Arms”. The Japet exoskeleton was implemented in each of these models. The efforts provided by the exoskeleton are 4 kg per actuator (i.e. a total of 16 kg). To perform a rotational movement, the pelvis/thorax segment follows a B-spline curve allowing a rotation of the trunk ranging from 0° to 25°. Finally, since the disk is represented by a dot in the AnyBody model, we will not talk here about intradiscal pressure but about the force exerted on the disk.
Impact of the exoskeleton on the L4L5 disk depending on the location of the load
To assess the impact of the exoskeleton on the disk depending on the location of the load, the model carries a load of 10kg and is held in an orthostatic position. Three tests are carried out: load carried against the body, load carried with half-extended arms and load carried with outstretched arms (Figure 1.)
Figure 1. Impact of the exoskeleton on the force exerted on the L4L5 disk as a function of the position of the 10kg load
As demonstrated by Wilke et al. a few years ago, the forces exerted on the L4L5 disk increase when the distance of the load from the body increases. It should be noted that the exoskeleton makes it possible to reduce the force exerted on the L4L5 disk by 135N (i.e. 13.8 kg) on average, regardless of the location of the load.
Impact of the exoskeleton on the L4L5 disk according to the mass of the load
To assess the impact of the exoskeleton on the L4L5 disk according to the mass of the load, the model is in an orthostatic position and carries various loads with half-stretched arms. The loads are 0kg, 5kg, 10kg, 15kg and 20kg.
Figure 2. Impact of the exoskeleton on the force exerted on the L4L5 disk as a function of the mass of the load carried
Figure 2. shows that wearing the Japet exoskeleton while maintaining a load, allows a decrease of 135N (or 13.8 kg) on average on the L4L5 disk, regardless of the mass of the load.
Impact of the exoskeleton on the L4L5 disk depending on the position of the operator when carrying a load
To assess the impact of the exoskeleton on the L4L5 disc according to the position of the operator during load carrying, the model carries a load of 10kg with half-extended arms for two types of movements: axial rotation and forward bending.
Figure 3. Impact of the exoskeleton on the force exerted on the L4L5 disk during a 10kg load bearing during axial rotation
For rotation angles of 0° to 20°, the exoskeleton makes it possible to reduce the L4L5 intra-disc force by 113N on average (decompression between 11% and 12%). At 25° of rotation, the force is reduced by 92N, i.e. 9% decompression.
Figure 4. Impact of the exoskeleton on the force exerted on the L4L5 disc during a 10kg load bearing during a load on the ground, a 45° forward bend and a 90° forward flex
The Japet exoskeleton allows a decompression of 4% of the L4L5 disc when taking on the load (10kg) on the ground, as well as when carrying the load in front bending of 90°. When flexed forwards by 45°, the decompression of the disc is 8%.
Conclusion
The results of the simulation show that the exoskeleton makes it possible to reduce the efforts on the L4L5 disk regardless of the location of the load, its mass, or even the posture when carrying the load. However, the device acts more effectively on angles of rotation between 0° and 20°, as well as on flexures less than 45°.
These results are questionable since it is not real movement resulting from motion capture, but movement created artificially. However, the results obtained are encouraging. It would be appropriate to carry out a similar study on the basis of real recordings in a work situation, and on a large scale.
References
Bassani, T. (2017). Validation of the AnyBody full body musculoskeletal model in computing lumbar spine loads at L4L5 level.INRS. (2018, 10 26). Retrieved from INRS: http://www.inrs.fr/risques/lombalgies/statistique.htmlRajaee, M.A. (2005). Comparative evaluation of six quantitative lifting tools to estimate spine loads during static activities. Applied ErgonomicsRasmussen, J. (2009). Validation of a biomechanical model of the lumbar spine. 22nd Congress of the International Society of Biomechanics.Wilke, H.-J. (2001). Intradiscal pressure together with anthropometric data - a data set for the validation of models. Clinical BiomechanicsZee, M.D. (2007). A generic detailed rigid-body lumbar spine model. Journal of Biomechanics.
