A muscle attachment site is used to connect muscles, stretch receptors, and springs to other rigid body parts in the biomechanical model of your organism's body. You can add body parts and position them anywhere in space relative to parent part. You specify which muscle attachments you want to use to connect your muscle or spring and the force is applied at each of these attachment points with the force vector pointing from one attachment to the next one in line. You can string a muscle along multiple attachments, so you are not limited to just two point. This gives you the ability to have muscles bend over obstacles in the way. A good example of this is shown in the locust leg in figure 1. The flexor muscle in the tibia does not connect directly from the femur to the tibia. Instead, the muscle is pulled over a lump and then connects to the tibia. This changes the angle of the application of force on the tibia and allows the much smaller flexor muscle to keep the leg flexed against the larger force begin generated in the extensor muscle. (The flexor muscle can only generate around 0.7 N of force, whereas the extensor muscle can generate around 15 N of force!!!). You can produce this same type of behavior in AnimatLab by using three attachment points and stringing the muscle between these points. Once this is done then tension force is produced that goes from the tibia attachment towards the lump attachment instead of directly towards the femur attachment.
When you add a muscle or spring it initially has no attachment points specified. When you select the Attachments property you can open the Edit Attachment Points dialog. You can specify the order of the attachments that you want to use for the spring or muscle. The muscle is attached to those points in the order in which you specify them, so if you order them badly you can get weird loops. To add a new attachment select it in the drop down and hit the add button. This will add it to the list on the right. To remove an attachment from the list select it and hit the remove button. To change the order of the items select an item and hit the up/down arrow buttons to move it.
You can add attachments just like you add any other body part type. They are visualized using a small amber sphere. Attachments have no mass, so they do not contribute to the dynamics of the system. They are only points in space relative to the parent object that lets the attached muscle or springs no where to apply forces. The attachments maintain the same relative location and orientation to the parent part regardless of the movements of the biomechanical organism as a whole or the parent part during the simulation. This ensures that your muscle will always apply its forces at the correct location on the parts, but as the parts move and rotate the vectors connecting those parts change just as with real muscles.
The attachment only has a subset of the standard properties that all rigid bodies have. Rotation and density properties are missing because attachments are points and can not be rotated, and they do not contribute to the dynamics of the simulation and thus have no mass or density. To see a description of the properties common to all bodies follow this link
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