simtk.org

Hello,

I am trying to record the tibial acceleration during running. To do so I created a new market and attached it to the Tibia. Then, I used the output reporter tool to record the position, velocity and acceleration for this specific marker. However the values returned in the output file doesn't seem realistic.
Has anyone had the same problem and found a solution? Should I use another proccess to obtain tibial Acceleration ?

Thank you

Hello Francisco,

See the response to this post: viewtopicPhpbb.php?f=91&t=17211&p=0&sta ... 737f58abd5

Hope that helps!

Thanks,

Nicos

Just be aware that an accelerometer does not measure acceleration.

The marker acceleration is the acceleration expressed in the global reference frame. An accelerometer measures global acceleration but expresses it in a local reference frame (the sensor axes). And there is an effect of gravity.

The equation for the 3D accelerometer signal (s) would be:

s = inv(R) * (a - g)

where g is the acceleration of gravity and a is the acceleration of the sensor. R is the rotation matrix that transforms local to global coordinates (hence we use its inverse).

The equation shows that you get no signal from a body in free fall, when a = g.

Another way to simulate the accelerometer would be to model the sensor itself. An accelerometer has a small mass sitting on a load cell. The load cell output (force) is divided by the mass (known) to get a value in acceleration units. In an OpenSim model, you can use a weld joint to attach a small mass to the leg. The reaction force in the joint, divided by the mass value, would be the accelerometer signal.

Ton van den Bogert