Hi fathyi
You can look and consider this picture plz :
- angles at hips,knees,ankles should be 90 degree
-Head positioned erected over shloulders,screen at the level of vision OR LITTLE below it
-Elbows angled at 70-135
-Wrists shouldn't be felxed or extended but should be positioned neutrals WITHOUT ANY ANGLES
-No slouched shoulders
-striaght back,with small pillows at lumber area of spine
-Feet should be positioned flat on the floor.
- Put a small distance between posterior aspect of knees and chair so no compression on it's structures while it's at 90 degree...
hope this could help u!!!
Regards