I did not take enough biology or physiology in school so I will do some research and then come back to explain in detail how the long bones functions. I wanted to focus on the tensile strength, leading capacity, density, and composition of long bones. For use to be able to manipulate or change something, we must learn and study it as much and as deeply as possible.
One of the major things I wanted to learn was this….
“How much force or load can the long bone, specifically the human femur withstand, in terms of a compressive load, a tensile load, and a lateral load?”
From the article “DIFFERENCES IN THE TENSILE STRENGTH OF BONE OF DIFFERENT HISTOLOGICAL TYPES”, We learn that “…bone containing Haversian systems has less tensile strength than that with no Haversian systems in it .”
THE STRENGTH OF BONE
“When an object is acted on by an external force there will be an intermolecular resistance inside the object which will tend to prevent the object deforming. The magnitude of this resistance is called the STRESS. The stress will, however, only tend to prevent the object deforming. The amount that the object is actually deformed is called the STRAIN. There are three ways in which forces can act upon objects to produce strain.They can produce compression,tension or shear. In compression the molecules of the object come closer together:in tension they separate from each other,and in shear they slide over one another. From the biological point of view it is nearly always the tension strength of bone that is important. Most accidental fractures of bone are tension fractures, even the spiral fractures associated with torsion(Evans, Pedersen & Lissner, 1951). Compressive and shearing fractures are usually the result of impact, which is relatively uncommon in nature, ….”
The reason for tension fractures being more common than compression fractures is the that bone, like most brittle substances, is stronger in compression than in tension (Evans,1957,chap.4).
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).
Me: I really hope you got that part. That shows just how much force/sq. in our upper leg bone can withstand before it can deform and stretch.
“…it is well known that dry bone is stronger than wet bone.”
“…Some authors have pointed out that where as dry bone behaves as an almost perfectly elastic substance, wet bone creeps. In a perfectly elastic substance strain is always proportional to stress. A substance that is imperfectly elastic will behave more or less like an elastic substance up to a certain stress, called its elastic limit, but after that point the amount of strain caused by a given increase of stress is larger…”
“…the energy absorbing capacity of bone is more or less proportional to the total strain.”
” There are in fact two reasons for supposing that it is the Haversian systems themselves that make the bone weaker: first,they reduce the actual amount of bone present, and secondly, they reduce the total amount of calcium in the bone. “
From source 2, I find the answer to the question, “What substance in bone contributes to tensile strength?” The answer is Collagen fibrils arranged in lamellae results in the tensile strength of bone. 🙂
From the Wikipedia article on human bone HERE, I paste the section about the Mechanical properties of bones…
The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the osseous tissue that gives bones their rigidity). It has relatively high compressive strength, of about 170 MPa (1800 kgf/cm²) but poor tensile strength of 104–121 MPa and very low shear stress strength (51.6 MPa), 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. All bones consist of living and dead cells embedded in the mineralized organic matrix that makes up the osseous tissue.
Me: So if we did stretch the human bones using a tensile force, it would require around 100-120 MPa of Newton/m^2. It is supposed to be mostly brittle but there is s slight bit of elasticity.
There are two types of bone, explained below…
Compact (cortical) bone
The hard outer layer of bones is composed of compact bonetissue, so-called due to its minimal gaps and spaces. Its porosity is 5–30%. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone.
Trabecular (cancellous) bone
Filling the interior of the bone is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone), which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone. Its porosity is 30–90%. If, for any reason, there is an alteration in the strain the cancellous is subjected to, there is a rearrangement of the trabeculae. The microscopic difference between compact and cancellous bone is that compact bone consists of haversian sites and osteons, while cancellous bones do not. Also, bone surrounds blood in the compact bone, while blood surrounds bone in the cancellous bone.
From source 4 we learn more about the mechanical properties of the long bones.
Besides the metabolically active cellular portion of bone tissue, bone is also made up of a matrix (a bonding of multiple fibers and chemicals) of different materials, including primarily collagen fibers and crystalline salts. The crystalline salts deposited in the matrix of bone are composed principally of calcium and phosphate, which are combined to form hydroxyapatite crystals. As you can see, the chemical formula for hydroxyapatite crystals includes molecules of calcium (Ca), phosphate (PO4), and hydroxide (OH):
However, the compressional strength of bone is greater than that of even the best reinforced concrete, and the tensile strength approaches that of reinforced concrete. But, even with their great compressional and tensile strengths, neither bone nor concrete has a very high level of torsional strength (the strength to endure twisting). In fact, bone fractures often occur as a result of torsional forces that are exerted on an arm or a leg.
Except for the articular cartilage that covers the very ends of each epiphysis, the bone is completely enclosed by a tough covering called the periosteum. Within the periosteum lies a bony layer called compact bone, which is solid, strong, and resistant to bending. The epiphyses are composed largely of spongy (cancellous) bone, which provides the greatest amount of elastic strength since the epiphyses are subjected to the greatest forces of compression.
Me: So we know that in terms of compression forces and tensile forces, the epiphysis are super durable.
In humans, different bones stop lengthening at different ages, but ossification is fully complete by about age 25. During this lengthening period, the stresses of physical activity result in the strengthening of bone tissue.
From source 4 we learn some more critical elements
Collagen has a low Youngs Modulus (stress/strain) aka (F/A)/(delL/L) , good tensile strength, and poor compressive strength.
Cancellous bone is 25% as dense, 10% as stiff and 500% as ductile as cortical bone
Cortical bone is excellent in resisting torque
Cancellous bone is good in resisting comression and shear
- Longitudinal loading
- Tensile strength ~130 MPa
- Compressive strength ~190 MPa
- Transverse loading
- Tensile strength ~50 MPa
- Compressive strength ~130 MPa
Cortical bone has adapted to a situation where compression loading is greater than tensile loading
Tensile and compressive yield strengths are close to the respective ultimate strength
Bone loaded above its yield stress deforms by a relatively large amount compared to its elastic behaviour
Prior to fracture, cortical bone has undergone relatively large deformations
Ultimate tensile strength is slightly more sensitive to strain rate than Young’s modulus
Strain plotted with time for adult human cortical bone under tension
If cortical bone is loaded at a certain level for enough time, it will break, although the stress level is well below yield and ultimate strengths
If creep occurs without fracture, a permanent deformation results : viscoplastic behavior
The tensile behavior of trabecular bone is much different from its compressive behavior after yielding : failure occurs by fracture of the individual trabeculae => the specimen can take less and less load until final fracture occurs
Me: Overall, if we really wanted to, we can theoretically create a machine that can apply in the range of 60-120 MPa of tensile force/area at a very steady rate and get the human long bones to elongate, and hoping that the bone is not brittle from age or fracture, the bone will slowly elongate However, the numbers are clearly showing that applying a force from the side or by twisting the bone will lead to the bone more easily fracturing and deforming.