Friday, March 5, 2010

Microstrain in Bone

Bone is an elastic tissue.  You can temporarily(or even possibily permanently) stretch your bones.  That is in fact the definition of microstrain which is a measurement of how the bone changes in length.  Our theory is that microfractures induced in a stretched state will result in height increase(Note: This is different then the easy height theory which states to create mirofractures first and then stretch them).  In order to put this theory into practice we have to know what kinds of exercise induce what kind of microstrain and how the bone is changed in length(is it compressed or lengthened or twisted or bent?).  A bone in it's lengthened state is clearly an optimal one for increasing height via microfractures but it's also possible the other three states can help as well or at least be non-detrimental.

We are limited somewhat in the exercises we can perform at this point.  For example, you cannot hold very much weight when you are inverted via inversion boots(what I prefer to do is a semi-inversion and hold dumbells during a decline bench).  So, it's important to know how much microstrain the exercises we can do cause.

Flexible multibody simulation approach in the analysis of tibial strain during walking.

"The motion capture data were used in inverse dynamics simulation to teach the muscles in the model to replicate the motion in forward dynamics simulation. The maximum and minimum tibial principal strains predicted by the model were 490 and -588 microstrain, respectively, which are in line with literature values from in vivo measurements."

A range of 500 microstrain is hardly a great deal however walking is a very easy to perform exercise so that means it's possible to achieve much higher microstrain with more difficult exercises(increase in load or speed).  Note that negative 588 microstrain is reached so walking does compress the bone but it also stretches the bone out by 490 microstrain(you need 1500 microstrain for bone adaptation).
Here's an article about bending the bone affects the microstrain on the bone.  It's possible to produce a bending force on the bone by say laterally loading the midshaft. 

Endochondral ossification in vitro is influenced by mechanical bending. 

"Bone development is influenced by the local mechanical environment. Experimental evidence suggests that altered loading can change cell proliferation and differentiation in chondro- and osteogenesis during endochondral ossification. This study investigated the effects of three-point bending of murine fetal metatarsal bone anlagen in vitro on cartilage differentiation, matrix mineralization and bone collar formation. This is of special interest because endochondral ossification is also an important process in bone healing and regeneration. Metatarsal preparations of 15 mouse fetuses stage 17.5 dpc were dissected en bloc and cultured for 7 days. After 3 days in culture to allow adherence they were stimulated 4 days for 20 min twice daily by a controlled bending of approximately 1000-1500 microstrain at 1 Hz. The paraffin-embedded bone sections were analyzed using histological and histomorphometrical techniques. The stimulated group showed an elongated periosteal bone collar while the total bone length was not different from controls. The region of interest (ROI), comprising the two hypertrophic zones and the intermediate calcifying diaphyseal zone, was greater in the stimulated group. The mineralized fraction of the ROI was smaller in the stimulated group, while the absolute amount of mineralized area was not different. These results demonstrate that a new device developed to apply three-point bending to a mouse metatarsal bone culture model caused an elongation of the periosteal bone collar, but did not lead to a modification in cartilage differentiation and matrix mineralization. The results corroborate the influence of biophysical stimulation during endochondral bone development in vitro. Further experiments with an altered loading regime may lead to more pronounced effects on the process of endochondral ossification and may provide further insights into the underlying mechanisms of mechanoregulation which also play a role in bone regeneration." 

1000to1500microstrain is not very much and there was no change in length. If you bend a bone you are going to get some tensile strain(bending a bone provides strain in all directions).  The increase in periosteal length though can have synergestic effects in height increase endeavors. 

Measurement of microstructural strain in cortical bone. 

"Typical maximal in vivo strains in humans have been measured, and found to range from around 1,200 με (principal compressive strain) to about 1,900 με (maximum shear strain) (Burr et al. 1996). In these experiments, strains were measured on the surface of the tibia using a strain gage that covered an area of several square mm, which would typically encompass thousands of bone cells." 

A shear strain is a strain that acts against the bone(for example if you rubbed a dumbell up and down against it). 

"Cracks were observed to be produced (by a few cycles of loading) at global strains as low as 2,500 με, similar to the present monotonic global strain level (2,000 με).  10,000 cycles at a global strain of 1500 με, highly significant (P < 0.005) differences were observed in microdamage levels in canine radii and ulnae as compared with uncycled controls. On the other hand, for an applied strain of 625 με, no statistically significant difference in the level of microdamage was observed." 

So this gives us a good idea that we need at least 2,500 microstrain to cause microcracks(remember that microcracks occur in the cortical bone so if you are trying to gradually strength individual osteons with tensile strain you need to cause at least 2,500 microstrain).  Microcracks may actually reduce the strain on bones so that it explains any diminishing returns we may see from exercise. 

In vivo measurement of human tibial strains during vigorous activity. 

"Our understanding of mechanical controls on bone remodeling comes from studies of animals with surgically implanted strain gages, but in vivo strain measurements have been made in a single human only once. That study showed that strains in the human tibia during walking and running are well below the fracture threshold. However, strains have never been monitored in vivo during vigorous activity in people, even though prolonged strenuous activity may be responsible for the occurrence of stress fractures. We hypothesized that strains > 3000 microstrain could be produced on the human tibial midshaft during vigorous activity. Strains were measured on the tibiae of two subjects via implanted strain gauges under conditions similar to those experienced by Israeli infantry recruits. Principal compressive and shear strains were greatest for uphill and downhill zigzag running, reaching nearly 2000 microstrain in some cases, about three times higher than recorded during walking. Strain rates were highest during sprinting and downhill running, reaching 0.050/sec. These results show that strain is maintained below 2000 microstrain even under conditions of strenuous activity. Strain rates are higher than previously recorded in human studies, but well within the range reported for running animals." 

Now what we are looking for to increase height is tensile strain but we see that running can produce 2000 microstrain which is only 500 less microstrain needed to microcrack.  So an exercise like running that causes tensile strain and is 25% more strenuous would be sufficient to grow taller with osteonal microcracks. 

Bone deformation recorded in vivo from strain gauges attached to the human tibial shaft. 

"A strain gauge rosette was attached to the midshaft of a man's tibia. This demonstrated that during every stride the bone surface was subjected to a number of discrete deformation cycles. During each cycle the bone was deformed from a particular direction, released at least partially and then deformed from another direction. This feature has been observed from a number of sites in experimental animals. The largest deformation occurred while the subject was running; the principal tension then reached 850 microstrain applied in line with the bone's long axis at 13 times 10-3 microstrain per second. When walking the largest deformation occurred prior to 'toe off'; compression was then the larger principal strain about minus 400 microstrain applied at 37 degrees to the bone's long axis at minus 4 times 10-3 microstrain per second. These strain values are the same order of size as those recorded from the long bones of sheep and pigs during their locomotion." 

Why does running cause so much more microstrain than walking? Impact.  So how do you generate impact in a way such as to cause microcracks and tensile strain, you tap the bone laterally. 

I'm not sure if something like this would work in terms of measuring microstrain: INFSC000B:Infinity Strain gauge Meter.  It does say it has weighing applications so maybe it is capable of measuring human microstrain.  It's expensive so does anyone have any ideas that are cheaper?

Mechanical Strain Regulates Osteoblast Proliferation through Integrin-Mediated ERK Activation

"Mechanical strain plays a critical role in the proliferation, differentiation and maturation of bone cells. As mechanical receptor cells, osteoblasts perceive and respond to stress force, such as those associated with compression, strain and shear stress. However, the underlying molecular mechanisms of this process remain unclear. Using a four-point bending device, mouse MC3T3-E1 cells was exposed to mechanical tensile strain. Cell proliferation was determined to be most efficient when stimulated once a day by mechanical strain at a frequency of 0.5 Hz[0.5Hz is the same frequency used in LSJL studies] and intensities of 2500 µε with once a day[this is much higher microstrain however], and a periodicity of 1 h/day for 3 days. The applied mechanical strain resulted in the altered expression of 1992 genes, 41 of which are involved in the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of ERK by mechanical strain promoted cell proliferation and inactivation of ERK by PD98059 suppressed proliferation, confirming that ERK plays an important role in the response to mechanical strain. Furthermore, the membrane-associated receptors integrin β1 and integrin β5 were determined to regulate ERK activity and the proliferation of mechanical strain-treated MC3T3-E1 cells in opposite ways. The knockdown of integrin β1 led to the inhibition of ERK activity and cell proliferation, whereas the knockdown of integrin β5 led to the enhancement of both processes. This study proposes a novel mechanism by which mechanical strain regulates bone growth and remodeling."

"Mechanical loads include mechanical strain and compressive and shear stresses."<-LSJL involves shear stress.

"The mechanical microenvironment within a tissue can influence the fate of a cell. Such local mechanical stimuli result in mechanotransduction, which is the conversion of a physical signal into intracellular biochemical cascade signals"<-The goal of LSJL is to change the microenvironment within the epiphyseal bone marrow to influence mesenchymal stem cells to differentiate into chondrocytes.

"Osteoblasts are important mechanical receptors that can transform mechanical stimuli into biochemical signals and secrete bone matrix to promote bone matrix mineralization"<-osteoblasts could transmit biochemical signals to MSCs too encouraging chondrogenic differentiation.

"[applying] drag forces to integrin β1 on the apical surface of adherent human MSC and confirmed that the expression of vascular endothelial growth factor (VEGF) and collagen I were induced by integrin β1-mediated mechanical forces, which are involved in osteogenesis"<-So integrins are mechanotransducers.  How do we apply forces to integrin Beta1 to encourage expression of Collagen II and Sox9?

"2000 and 2500 µε [are within the physiological range]; and 5000 µε is above physiological range [according to mechanostat theory loads above about 2000 microstrain do not promote bone growth]. Strains of 2000 and 2500 µε markedly promoted cell proliferation, whereas a strain of 5000 µε inhibited cell proliferation[but could 5000 microstrain encourage stem cell differentiation into chondrocytes]. The mechanical strain of 5000 µε enhanced PI positive stained percent and lactate dehydrogenase (LDH) activity in the culture medium of the cells, indicating that the strain of 5000 µε resulted in cell necrosis and overloading, which is unsuitable for cell growth"<-It's possible though that excessive microstrain could kill bone cells allowing for cartilage cells.  Microstrain did not activate the TGF-Beta signaling pathway and TGF-Beta is involved in chondrogenic differentiation.

"30 min steady fluid shear stress of 20 dynes/cm2 increased the expression of the following early mechanoresponsive genes: integrin β1 in C57BL/6J mouse osteoblasts, Wnt, estrogen receptor, insulin-like growth factor-I, and bone morphogenetic protein"<-BMP and IGF-1 encourage chondrogenic differentiation.

Mechanical loading and how it affects bone cells: the role of the osteocyte cytoskeleton in maintaining our skeleton.

"During physical activity, mechanical forces are exerted on the bones through ground reaction forces and by the contractile activity of muscles"

"1,000 microstrain equals a 0.01 % change in length of the bone compared to its original length. Vigorous exercise induces bone strains up to 1,000 microstrain in humans."

"controlled bouts of whole bone loading resulting in 1,000 to 3,000 microstrain are anabolic in experimental animal models of bone-loading"

"strains resulting from habitual activity suffice to prevent bone loss compared to complete unloading, even though these habitual strains hardly ever exceed 400 microstrain."

"cultured bone cells release nitric oxide (NO) in response to mechanical stimulation in the form of a fluid shear stress, and that the amount of NO released linearly correlates to the rate of the applied fluid shear stress"

"mechanical loading of mouse tibia enhanced fluid transport through the lacuno-canalicular system, demonstrating the correlation of canalicular fluid flow with mechanical load. It is not the amount of strain applied to a whole bone that influences bone formation, but the rate at which the strain is applied"

"The osteocyte cell membrane is surrounded by interstitial fluid and proteoglycans(ECM). Microtubules  originate from the centrosome and form a scaffold along which numerous molecules shuttle. Microtubules also form the core of the primary cilium. Osteocyte cell processes contain mostly actin, cross-linked by fimbrin at places where processes bifurcate. The processes contain αvβ3 integrins, possibly located on top of collagen “hillocks”, and protein tethers. These tethers may deform as a result of interstitial fluid flow and subsequently “tug” at the actin cytoskeleton, or pull open stretch-activated ion channels, allowing calcium and other ions to enter the cell and activate a multitude of chemical signalling cascades. The fluid flow may also tug directly at integrins, such as α5β1, present along the cell processes and body and associated with hemi-channels. Activated hemi-channels release signalling molecules such as ATP, triggering a signalling cascade. The mechanical signal may be transduced from sites where integrins are triggered, via the actin cytoskeleton, to distant sites, including the nucleus, which is connected to actin via LINC complexes. Forces applied to actin fibres can likely also open stretch-activated ion channels. Integrins are often clustered together at focal adhesions, which in osteocytes are predominantly located at places where the cell processes intersect with the cell body. Focal adhesions contain tyrosine kinases, such as focal adhesion kinase, which make focal adhesions a prime location for transducing mechanical signals into a chemical response."

Exercise-induced bone formation is poorly linked to local strain magnitude in the sheep tibia.
"we characterize the in vivo strain environment of the sheep tibial midshaft during treadmill exercise and examine whether this activity promotes bone formation disproportionately in the direction of loading in diaphyseal regions that experience the highest strains. It is shown that during treadmill exercise, sheep tibiae were bent in an anteroposterior direction, generating maximal tensile and compressive strains on the anterior and posterior shaft surfaces, respectively. Exercise led to significantly increased periosteal bone formation; however, rather than being biased toward areas of maximal strains across the anteroposterior axis, exercise-related osteogenesis occurred primarily around the medial half of the shaft circumference, in both high and low strain regions. Overall, the results of this study demonstrate that loading-induced bone growth is not closely linked to local strain magnitude in every instance. Therefore, caution is necessary when bone shaft shape is used to infer functional loading history in the absence of in vivo data on how bones are loaded and how they actually respond to loading."

" Functional loading is expected to have a predictable effect on diaphyseal shape by promoting bone formation disproportionately in areas of the shaft surface that are routinely subject to the highest levels of strain. Thus, bone shaft cross-sectional shapes that display structural reinforcement in a particular plane (e.g., elliptically shaped cross sections) are hypothesized to have experienced more frequent bending loads and higher strains across the axis about which the bulk of bone tissue is distributed"

", during quadrupedal walking, the ulna of a macaque monkey primarily experiences coronal bending, with the highest strains produced on the medial and lateral shaft surfaces; however, macaque ulnar diaphyses are shaped to better resist sagittal bending"

"Instead of being biased toward areas of maximal strains across the anteroposterior shaft axis, significant exercise-induced bone formation occurred primarily around the medial half of the diaphyseal circumference, in both high strain regions and directly adjacent to the neutral axis where strains were lowest. "<-this provides evidence that bone growth is based on fluid flow.

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