The Joint
Our body works following rules during movement. Each of the joints in our body can undergo displacements or rotations. Among the rules that exist, it is always up to the part of the body closest to our center of gravity to move the part of the body furthest away.
In order to move our elbow, our arm includes muscles that are attached to the forearm and therefore act as a lever to move the elbow. The arm in question is called the "proximal segment" in this case while the forearm is called the "distal segment". The movement of the joint always occurs thanks to the action of the proximal segment against the distal segment. In this case the joint we are talking about is also called an "adjacent" joint.
Degrees of freedom
The rotation in the joint space can be described through the so-called "degrees of freedom".
The joints of the body are not like those of a robot. The dynamics with which they move and rotate cannot always be described as if they were dealing with a mechanical joint. On the other hand, mechanical joints can also be of different types.
Thorax
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Flexion-Extension
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Lateral rotation
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Axial rotation
Humero-Thoracic
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Flexion-Extenstion
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Abduction-Adduction
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Internal-External Rotation
Elbow
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Flexion-Extension
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Pronation-Supination
3D measurement of a joint
In the previous example, for the description of the movement of the elbow, the movement analysis reasons in the opposite way: since the movement analysis observes the movement in order to be able to describe it, it is necessary to measure how much and how the distal segment moves compared to the proximal segment (although, as we said before, it is the proximal one that actually acts on the distal!). For this reason, by positioning 2 IMUs, one on the distal segment and one on the proximal segment of the elbow, it is possible to calculate how the elbow is moving by measuring how much the IMU located on the distal segment is rotating with respect to the proximal segment. To do this, among the quantities listed previously, we would find the orientation of each IMU in space in order to understand how one, relative to the other, is changing their orientation.
But it's not enough. There are still some fundamental elements to take into consideration.
