The socio-economic interest in occupational health has never been bigger. Very few individuals go through life without experiencing some kind of occupational injury ranging from mild and temporary back pain to permanent disability. Many of these injuries are in the musculoskeletal system and can be traced back to the working environment, but while insurance premiums are exploding along with workers' compensations, we remain in doubt of the precise causes of many of the conditions, and interventions are very often prescribed without much firm knowledge of their effects.
Musculoskeletal modeling is the ideal tool for these situations because it allows for a quantitative comparison between different working situations and can assess the consequences of prospective interventions.
Workplaces with elements of materials handling or lifting are among those with the highest propensity for injury in back, shoulders and arms. The overall explanation is that any repetitive work task that degenerates tissue faster than it can be repaired will eventually traumatize the human body. Our muscles typically have much smaller moment arms than the exterior loads, and therefore small differences in the working postures or loading parameters can cause large differences in the tissue loads
The AnyBody Modeling System™ allows for detailed analysis of a particular work task and for assessment of the consequences of a change in the workplace design. How will the sideways movement of a handled material influence the joint forces, and is this likely to influence the shoulder pain the worker is experiencing
The mechanics of the human body is far to intricate to allow for a precise answer to this type of question without the type of rigorous mechanical model provided by the AnyBody Modeling System™.
Pushing is one of the most common manual work tasks and can be investigated very easily with the AnyBody Modeling System™. In this particular example a hospital employee is pushing a bed with a total mass of 200 kg forward at an acceleration of 0.3 m/s^2. The duration of the push is 1.5 second. It is possible to choose three different heights of the horizontal handle bar: 0.9m, 1.1m and 1.3m. The question is: which handle bar height results in the minimal load on the body?
A musculoskeletal model enables simple investigations of muscle efforts and joint forces depending on postures and other workplace parameters. The two figures below chart the overall muscle effort as well as the elbow joint forces through the push. It is evident that the high handle bar is preferable on both accounts, especially when the bed is close to the body.
The design of portable equipment such as backpacks has a large influence on comfort and fatigue. Variation of the center of gravity changes the posture and muscle activation in a complicated fashion, and the same can be said for the way the straps put their forces on the bones of the body. This example shows a posture change from forward flexed to upright followed by a change of center of gravity of the backpack further away from the body. Notice how the model automatically compensates its posture to maintain balance.
Quantification of joint forces
When you play the animation of this example you will see a working situation similar to the operation of a steering wheel in a fork lift. This is a typical repetitive working situation, and it demonstrates very well how the posture and muscle activations are responsible for the generation of joint forces. The bulging of the muscles in the animation (exaggerated) shows that a larger effort is necessary at the point in the cycle where the hand is retracted and pushing laterally on the wheel. This is because the moment arms of the muscles are unfavorable in this situation.
Even though the torque in the wheel is constant, the graph of simulated gleno-humeral joint reactions shows that a large peak in the reaction force appears in this interval. This is entirely due to the unfavorable mucle joint moments and the large muscle forces. Hence, it can be influenced considerably by changing the parameters of the wheel such as the position of the center, the inclination of the axle, and the diameter of the handle position. The precise adjustments can be identified with the AnyBody Modeling System™.
Meat plant worker
The picture shows a worker in a meat plant cutting with a knife. The knife force is 50 N and varies in direction as indicated by the red arrow. The worker is resting his left hand on the table and the model automatically computes the forces transferred to the table. Please notice that it is not necessary to measure this support force except for model validation purposes.
Clicking the picture will play a video that illustrates the muscle activations in response to varying knife forces.