It is measuring how much and how the body moves. To fully understand how the body is performing we need to know angles and forces.
Take a block of wood it can’t bend, stretch or squash (well not very much anyway) if we draw three lines through three points on this block of wood we can describe three ways to rotate it. Because the points don’t get further apart or closer together, the wood does not stretch remember, we can always know where the block is and how it’s rotated based on these three points or markers.
Now we can orient this block of wood wherever we like by rotating it around these three lines. If we put two blocks of wood together, we can easily measure the angle between them by looking at the lines and the angles between them.
These blocks of wood could just as easily be parts of the leg or arm and we can mark the points by placing small markers on the body and tracking them using cameras. We can work out the joint angles by looking at the lines just as before with our blocks of wood. This is biomechanical modelling.
Now, the important thing about muscles is that they produce forces (muscles can only pull though!). They produce forces to create torques, or turning forces, about a joint, just like the force needed to tighten or loosen a nut with a spanner. The amount of turning force depends on how much force is applied and how far away it is to the centre of the joint.
In movement science we can use sensors in the floor to measure the amount of force. Because we are tracking the position of the joint with our markers placed on the body, we have all the information we need to work out what the muscles are having to do to maintain our posture or move our limbs (because all the forces have to add up: this is one of the famous laws of Issac Newton). This is called kinetics and we can plot the changes in forces during movement just as we did with the joint angles. Again, it’s important to remember that the forces act in 3 dimensions and, unlike many techniques, our measurements capture all of them.
Here is what the forces look like in a graph of someone walking.
Changes in the patterns of forces and of joint angles can tell us huge amounts about why movement is healthy or unhealthy, whether it is likely to cause harm in the long run or if its improving with treatment.