So this is assembled forearm and upper arm. So upper arm is using aluminum extrusion to connect joint 3 with joint 4. By using aluminum extrusion we can lengthen and shorten the arm and change the reduction ratio of joint 3 by changing a few parts. The length of extrusion in this picture is 200mm. Gear reduction of joint3 is 8:1 planetary gear ratio and then 32:28 belt reduction. The total reduction of joint3 is 9.14285:1.