The J2-M3400 is a reinforced ceramic construct, which has the equivalent of the medullary cavity in human bones. This is where the elements of the neural network are placed for protection. This makes the ceramic skeleton of the EM-J2 Series twice as strong as a comparable human one.
Overall the skeleton is built to resemble the human one. The knee however is built differently with a ball joints style for extra strength. The traditional joints that are floating such as shoulder and hips are made so that the connecting limb actually locks into the socket and the socket itself is lined with a frictionless surface. To remove a limb from the socket, the socket must be unlocked which causes a section of it to release.
A lattice of Osmiridium filaments in a polymer strand acts like ligaments and hold the bones together. They are secured with Ionic Adhesive and are flexible enough for the joints to work within the normal range of motion.
The rib cage in the more advanced models can actual expand in a life like fashion.
The hips of J2-M3400 vary depending on the gender of the android. This is purely an aesthetic feature.