If users bottom slides forward when pedal, it means, the seat is too deep. Arrange a cushion or 2 on the seat back. It is better to be "flat shaped" cushion, like the seat cushion of COGY.
If the seat is too shallow, loosen the velcro on seat back, and make the seat deeper.
video = https://www.youtube.com/watch?v=sa36V5fJUK8
Seat back hight
If the user needs more hight on seat back, we can make it about 20cm higher using additional accessories. This will sometimes helps user to keep the good posture instead of leaning back too much.
This video is the method to make it up to 4cm higher.
For additional accessories,
To make the seat higher, use extra cushion.
To make the seat lower, loosen the velcro at the bottom.
Steering unit angle
Adjust the steering unit angle to find the most comfortable position for user.
video = https://www.youtube.com/watch?v=u8UG5VmRtHE