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™.