Hi Mike – thanks for taking the time to contribute to the discussion. Let me address your questions in order:
1) You are quite right: the picture of the man whose arm is bent at less than 158° is not an indication of what is going on in the bones or in his joints. To know what is going on there, we would have to ask him what he is feeling. Paul Grilley explains this in detail in his DVD. In the interest of brevity (my article was already quite long) I did not include this guy’s full background. Paul did ask him and knows that he is not feeling any tensile resistance in his muscles, there is no pathology. This guy had reached the limit imposed on him by his bone structure, as indicated by the feeling of compression in his joint. He is not going to be able to go further, short of some sort of surgical intervention. I can sympathize with this guy because my own right arm similarly cannot open to 180°. I too reach compression before my right arm can openly completely. I can easily feel that I am reaching a bone on bone compression in my elbow, it is not a pathology or a “tightness” in my joint capsule. (Interestingly, my left arm can open 180°.)
2) You say that comparing a power lifter to a yogi is not a valid comparison because of what happens after
the weight dropped. However, before the lifter drops the weight, there is stress in her joints just as there is stress in a yogi doing, say, side plank. This is
a valid comparison because both are stressing their joint in a place that is beyond 180°. Both are spending some time there, even though the yogi will tend to stay there longer than the weight lifter. The fact that a yogi will smoothly come out of her pose does not negate the stress that she felt while in the pose.
Your point is more appropriate when talking about the stresses that occur while in the pose and the use of muscles when coming out. You say that in a hyperextended and locked out joint that “the stabilizing muscles on one side of the joint are slack while those on the other side are at full length.” If that were so, it would be true even in a non-hyperextended but locked out joint. When I lock my left elbow at 180° in side plank, my biceps are stretched to the maximum that they can possibly stretch. (They can’t go further because I am locked out – there is no more ROM for them.) But I don’t agree that my triceps are slack. Try it yourself: go to where you lock out and see if your agonist muscles are lax. They are contracting to help me stay in the pose and the antagonist muscles, the biceps, are stretched. But, my point is – this is the same whether you are hyperextended or “normally” extended: the biceps will be stretched to their max and the triceps will be engaged. This is not an “uneven” relationship – it is the natural relationship of that body.
Yes, we could microbend the elbow and make it more muscularly active: but why reserve that command only for the hyperextending student? Even “normal” 180° students, (like my left arm) will lock in side plank. If your intention is to activate the muscles the whole time you are in the pose, then why not tell everyone to microbend their elbows? Why – because it is hard. Try it yourself: how long can you hold side plank with a microbend in your elbow compared to being locked out? (Maybe only ½ as long? That is the point of Turbo Dog – it is great for building strength, but you can’t stay as long.)
3) I would disagree with your statement that bone density is only increased by the stress placed on the bones by the muscles. NASA has been wrestling with the problem of bone density loss for astronauts who remain in orbit for months. They have experimented with all sort of muscular exercises to create a suitable stress on the bones to keep them strong, but so far have found no way to maintain bone density in a microgravity environment. [They are coming closer, as this study
shows, but to recreate what gravity does for us every day they have to get really creative and the amount of muscular exercise is extreme.] We need gravity to stress the bones; and that comes from the body’s weight creating stress on the bones. A NASA study
of isometric exercise has shown that such exercise can maintain muscle strength but at the cost of muscle cells! (The astronauts doing only isometric exercises in orbit lose myofibrils.)
The examples you use of women jumping to help strengthen their bones and failing are dynamic
stresses of the bones: jumping creates a transient stress on the bone and the joints. That is a yang form of exercise and i agree that this is not the way to strengthen yin tissues. Yin tissues require yin stress: long held stresses. A 300-pound basketball player dunking a basketball places tremendous transient stress on the arches of his feet when he lands, but he doesn’t suffer fallen arches. He may break his feet, but he won’t change their shape. A 90-pound waitress, standing on her feet all day long, may suffer fallen arches over time: that is a yin stress.
You said, “Direct bone on bone force is actually rarely good.” While we use the term “bone on bone,” to be precise, in our articulating joints we never have direct bone connecting with bone: we have bone connecting with cartilage connecting with synovial fluid connecting with cartilage connecting with bone. To maintain health we need this whole system to undergo stress: stress is what stimulates the chondrocytes in the cartilage to create more cartilage and synovial fluid. Stress in the bone stimulates the osteoblasts to build more bone. If we ever get to bone contacting bone in a synovial joint, we have reached the pathology of osteoarthritis and that is not good, I would agree. But, to say “no one should ever try to stress their bones or their joints” is unhealthy and unhelpful, because no stress is just as damaging to our body as too much stress. Too little stress leads to osteoporosis. Life requires a balance between the too little and the too much.
My main point is – if you prevent people who can hyperextend from going to where they can, you risk too little stress to their joint and it may atrophy over time. Yes, they could go too far and the joint could be damaged but the opposite extreme is not better.
Thanks again for your thoughts.