Why it is important?
At this point, we allowed the two IMUs positioned on proximal and distal segments to measure different quantities for each underlying body part. But, in our example, how does the actual calculation of the rotations of the elbow joint take place? Although on a mathematical level it is possible to use the measurements of the IMUs and combine them as they are, this determines a measurement of the articulation that is completely distant from reality. This is due to the following factors:
​1. Each IMU works independently of each other, therefore provides data that depends on how it is oriented in space. If two IMUs are INITIALLY oriented differently from each other, their readings will not be directly compatible and therefore combinable
​2. The degrees of freedom that can be describable mathematically depend on which biomechanical assumption has been made for a certain joint. For example, if the orientation information provided by the IMUs were used as is, this would be equivalent to assuming that the adjacent joint is a 3-degree-of-freedom ball joint.
​3. Each joint is normally measured and calculated separately from the others. However, if there is a dependence between one joint and another, at a biomechanical level, or if one wants to take advantage of it, it would not be enough to treat the IMUs separately, but it would be necessary to "fuse" the information of both
The ingredients
​Due to the aforementioned factors, when describing the movement of a joint, it is necessary to create a "biomechanical model" that takes into consideration:
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The initial position of the IMU units on the body
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The type of joint you want to describe
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The type of bond between the various joints within the body
Calibration
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​Defines the link between the IMU and the underlying body part
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Defines anatomical reference systems
Joint models
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they define the joint degrees of freedom
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they define the joint constraints
Kinematic chain
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​Defines how each joint is related to the others
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Defines the anthropometry of the avatar