So what is it about Sanchin that makes it so coveted by so many styles of Karate as well as Gong Fu? There are so many factors from Iron Shirt to Chi Gung to a complete fighting system. But here is one part of Sanchin and Iron Shirt not usually discussed.
The secret is in the full body fortification, from the muscles (which is obvious), to the Tendons (Autonomic system) and even Ligament development (which ties in heavily with bone density development). It is the unique exercise of dynamic tension combined with Tendon/Ligament tension not only strengthens the practitioner right down to the bone, but also serves to protect nerve and blood structures.
Tendon development is not happen as quick as muscle development (within 3 months), yet it is quicker than the ligament or bone development, (these take about 6 months for tendons and longer for ligaments to adapt to constant or repeated stress induced on your system). These time estimations also depend on your training, age, nutrition and other factors .
To add a bit of detail for those interested, most of the tissue in your body is composed of two basic proteins, Collagen and Elastin. The ratio of these two proteins determine whether a structure is muscle (more elastin) or a tendon (more collagen). The more Collagen, the greater the density and rigidity in a structure, as example; ELASTIN > Muscle > Tendon > Ligament > Bone > COLLAGEN.
For the Kyusho practitioner, this bears greater study and understanding. Arteries, have a lot of elastin, particularly near the heart, your Other structures have varying ratios of elastin/collagen and they will all adjust to accomodate the stress loads you work with in in your training program. The greater the stress loads, the greater the fortification of the whole body... including the arteries, nerves, muscle, tendon, ligament, bone and even your skin.
This can be over trained and cause greater stresses on the internal function of the body (but great for a combative warrior).