Something that Tyler has often asked me to look into and try out myself through email is to ask about the idea of what if we apply just the right magnitude of tensile loading to cause the needed deformation we need to lengthen the leg.

This type of reasoning I have always had a lot of resistance against because of so many failed attempts before. Sky tried it in his group with LimbCenter.org and I am sure thousands (if not millions) of other crazy height increase seekers throughout human history have thought about and tried to put heavy weights on their legs to hopefully “stretch out” the bones.

I refer to the posts from before

• “Long Bone Tensile Strength, Loading Capacity, Compression Strength”
• Is Bone Distraction Or Bone Breaking Always Needed To Increase Height?
• “Sky’s Mistake, Why He Never Increased In Height”

From the first post, I had quoted the phrase…

The human femur is nearly twice as strong in compression as it is in tension; the ultimate strength is of the order of 12,000 lb./sq.in. in tension and 20,000 lb./ sq.in. in compression, and more or less the same relationship holds for oxen (Evans, 1957,chap.14).

From wikipedia….

It has relatively high compressive strength, of about 170 MPa (1800 kgf/cm²)^[4] but poor tensile strength of 104–121 MPa and very low shear stress strength (51.6 MPa),^[5] meaning it resists pushing forces well, but not pulling or torsional forces. While bone is essentially brittle, it does have a significant degree of elasticity, contributed chiefly by collagen

It is interesting to note that in multiple sources it is stated that due to the hollow nature of human bone, the overall structure of human bone is as strong as reinforced concrete for just tensile loading.

The thing to remember is that cancellous bone is 5X more ductile (malleable) than cortical bone and when surgeons decide to do the cosmetic surgery of distraction osteogenesis, they have to put a reasonably sized crack on the bone, to crack the cortical bone. The trabecular bone tissue is still left there which is not supposed to be cracked apart, which is what is being pulled apart when the screws on the external fixator are turned. I am referencing the study “Limb lengthening by callus distraction (callotasis).”

Remember that even with only cancellous bone, it still takes a bit of effort to use the screw to pull just the cancellous bone tissue apart, when the callus is being formed from the body’s natural healing properties. Apparently the medical term for where the surgeon takes a hammer and chisel and hammers on the human tibia (or other long appendage bone) until the cortical bone cracks is called “proximal submetaphyseal corticotomy“. The name itself means that the cortical bone is separated or cut open and that the cut is made somewhere below the metaphysis area, which is the middle area. The abstract above states “dynamic axial loading is instituted to promote corticalization.” The statement seems to suggest that after the distraction and lengthening is done, you put a compressive load on it to stimulate cortical bone growth.

Incidently there is an article from LiveScience.com that shows that overweight men have thicker femur bones.

The thing again is that the long bone in humans is very strong, steel-like. The 100-120 MPa when converted to US units gives a strength around 15,000-17,000 psi (Pound per Square Inch) which is  about the same and in agreement with the other sources I have found.

Calculations: If we tried to estimate just how much pounds as force would be needed to reach the maximum stress/strain which would lead to breakage on the average human leg, which is about 1.4 inches thick or 4 cms thick and have a intermedullary cavity about 2 cms thick, then the overall calculated area is

pi*2^2 – pi*1^2= 3*pi = about 9.42 cm^2 and 9.42500 (cm ^ 2) = 0.0009425 m ^ 2

if we put this into the ultimate tensile strength at 120 MPa (that is a mega-pascals) we get 113,100 newtons which converts into 25,426 pounds force.

Most compact cars themselves would not even weigh over 4,000 lbs-forces. To get around 25,000 of force pulling down or up on your cortical bone is a very high amount of force. Of course we are talking about the thickness of the longest bone in our human bodies, the femur. Maybe the tibia would be smaller. However this is a very basic physics problem. Let’s go back to the original question

Can we refine the answer to the original question a little more? would the bone in our lower limb break or stretch first?

It would seem that when bones extracted from already dead animals become dryer from the lack of blood, plasma, and water running around and through them. Dryness is usually correlated with loss of elasticity. We remember that for parts of the human to have elasticity, there must be an extracellular matrix filled with material that is elastic.

For cartilage and skin tissue, that would be collagen. For our purposes, since we are talking about Hyaline cartilage, we are looking at Type II Collagen, which is secreted by chondrocytes. We read from the first source that it is collagen that seems to give the long bones their tensile strength. Collagen fibrils arranged in lamellae results in the tensile strength of bone. Collagen has a low Youngs Modulus (stress/strain) aka (F/A)/(delL/L) , good tensile strength, and poor compressive strength. Elasticity seems to mean that the strain that is produced from the stress applied over a variation of stresses, is linear. 

So to fully answer, the question, it would seem that the long bones in our body, given the fact that the collagen is what creates it’s tensile strength, suggest that if we did load the bones up to a certain point, they will elongate first, but since the bones are “elastic”, once you let go of the load, the bones will go back to their natural length again. It seems that if you wish to really stretch out the bone to lengthen the bone, you would have to put a load on it that is beyond it’s normal stress/strain curve.

Of course the bone will eventually fracture, but there might be a certain amount of tensile loading we can put on the bone, past the point of it’s elasticity, and before the load becomes so much that the bone will fracture.

So let’s outline what the result/answer will be…

• The tensile strength from bones is from the collagen fibers in the bone, which actually make the bones “elastic”

• The long bones will elongate before they will fracture when you put steady rate of increased loading on the bones in a tensile way.

• However, once the load is let go, the bone will snap back to its original length like a rubber band.

• The bones will fracture once the load is increased over a certain point. 

• We might be able to calculate what will be the exact, most optimum load we put on the human leg, after elastic elongation, and before fracture, which can lead to real bone lengthening. 

• The load we calculate can be very high, if we are only thinking of using a gradual, steady linear increase in load on the bone. It might be more effective to do sudden jerks on the leg.