Dear Siamak,

Thanks for your great comments.

I will try a Fourier series approximation for each joint angle.

Best regards,

We use a kinematic model of lower limbs to measure the speed of gait. Essentially you calculate joint positions from angles during gait and position of hip from kinematic model. The best results are obtained if you know when the left and right feet are on the floor. When foot is on the floor you can attach the kinematic structure at that foot and then you calculate from angles how the free foot moves. And then you switch between the left and right foot. We also do not have GRF, but we have an 2 IMUs on each foot so that we can estimate when the foot is on the floor.

Depends on your configuration there are also other solutions for estimate the speed of gait.