Monday, March 8, 2010

Bone Elasticity and Height Increase

Previously, I've presented articles about microfractures(or microcracks which are microfractures for the cortical bone).  But, that is only part of the theory of height increase via tensile strain.  The theory is microcracks in the osteons caused be tensile microstrain(deformation of the bone in a longitudinal fashion) to increase height.

Structural characterization of human rib cage behavior under dynamic loading.

 "The influence of the structural properties, i.e. geometry and initial rib slopes, and of the costo-vertebral joints on the deformation capacity of the rib cage during a dynamic loading was studied. Each rib cage was loaded four times by increasing, up to 40% of the initial thickness, the mid-sternum deflection. The spine was rigidly fixed without constraining the costo-vertebral joints and the rib motions were computed from 3D video analysis. In addition, numerical simulations were performed with subject-specific models obtained from the rib cage geometry and a method for model personalization. The objective was to numerically evaluate the sensitivity to solely changes in geometry. The rib rotations were determined from the motion of 3D-markers close to the costo-vertebral joints and the 3D rib deformations were assessed from the motion of markers along the ribs. The rib rotations varied with the costal level (mean value 5.8 degrees [max. 7.9 degrees, min. 3.5 degrees], 2.9 degrees [4.8 degrees, 1.0 degree], 2.5 degrees [4.8 degrees, 1.1 degrees] and 2.2 degrees [3.5 degrees, 0.8 degree] for rib 2, 4, 6 and 8 respectively) and among the subjects (mean variation from 3.3 degrees to 7.1 degrees). The rib deformations were mainly in the sagittal plane for the upper ribs and in the rib plane for the lower. Although, no statistically significant correlations were found with different morphometrics parameters, a link (R2>0.4) was found between the initial rib slope and the amount of rotation and deformations, according to the assumption described by Kent et al (2005). The costovertebral joint was described by a functional rotation axis (i.e. helical axis) that does not correspond to the physiological axis of rotation. The orientation and the position of this helical axis changed with the level of deflection and varied with the costal level."

Morphometrics refers to the mathematical and scientific tools to study shape.  The use of morphometric parameters in this study likely refers to a means of standardizing the shapes of various ribs so they could be compared.  So the rib twisted mostly in the sagittal(the up-down) plane.  The loading in this study can be interpreted to be a compressive load and against a bone from the side(like if you put a dumbell on the side of your tibia).  Deflection refers to how an object is bent.  If you bend a bone from the right and later or simultaneously the left then you should get equal deformations in both directions.  A bending motion produces tensile strain in addition to a laterally compressive one.  Bending the bone by 40% is tough to achieve though.  But if we assume that bending a bone by 40% increases the bending by 8 degrees then bending by 5% increases the deformation by 1 degree.  But we'd still need a strain gauge or some other tool to get specific correlations between weight used and degree of bending.

The role of dynamic flexion in spine injury is altered by increasing dynamic load magnitude.

"Evidence indicates that loads and postures that an individual is exposed to alter their risk of reporting low back pain or incurring a spine injury. In vitro research indicates cyclic flexion under static compressive loads can lead to disc herniation, while repetitive compression in neutral or flexed postures leads to vertebral failure. However, no research has examined the likelihood of altering injury site (disc vs. bone) when dynamic load exposures are varied concurrently with cyclic flexion. METHODS: Fifty porcine cervical spinal units were assigned to one of five groups based on peak normalized loads of 10%, 30%, 50%, 70% and 90% of the unit's predicted tolerance. Specimens underwent passive range of motion tests to determine individualized range of motion. Once individualized loads and angles were determined, specimens were cyclically compressed and flexed based on profiles obtained from a floor to waist height lift until failure occurred or 12h elapsed. After testing specimens were dissected to identify injury site, and cumulative exposures sustained to failure were calculated. FINDINGS: Disc injury was not observed when peak loads exceeded 30% of the tolerance, while they comprised a higher percentage of the total injuries incurred when decreasing from the 30% to 10% groups. Those specimens exhibiting disc injury tolerated significantly greater: cycles to failure (9000 vs. 930, P<0.0001), cumulative compression (10872.7 vs. 1089.5MNs, P<0.001), shear (1822.1 vs. 150.6MNs, P<0.001) and angular excursion (130809.7 degrees vs. 12714.7 degrees , P<0.001). INTERPRETATION: If the spine is exposed to greater levels of load, in the presence of repetitive flexion, it is more likely to experience vertebral fracture. However, if the spine is exposed to many cycles of low peak loads, injury is more likely to occur to the intervertebral disc than to the vertebral bone or endplate."

 So you can see how important the usage of high loads is and dynamic exercise is as well.  Under static compressive loads there was an injury to vertebral discs(definitely won't increase height as injury to intervertebral discs isn't anabolic) and with dynamic loads there was a fracture caused to the vertebrae itself(the bone).

Excursion is a synonym for deviation so an angular excursion would be an angular deviation.  A common floor to waist lift is the deadlift.  The amount of cumulative compression and shear tolerance decreased greatly as weight decreased in the floor to waist exercise.  However, compression and shear doesn't usually increase length in the cortical bone only in the periosteum(but as the bone at the top of the head is a flat bone and an increase in periosteal shear will increase periosteal width and therefore increase height).  Angular excursion tolerance decreased greatly as well(angular excursion tolerance could be defined as the tolerance of bone to being bent in an unnatural way).

The heavier the weight you use the more shear strain and bone deformation the bone can handle before injury.

So, not only does using light weights targets the intervertebral discs(for other bones equivalent would be articular cartilage or tendons/ligaments), but heavier weights seem to have a greater effect on the bone than do lighter ones(you need bones to grow taller).  So to increase height, heavy weight is needed(as this reduces the amount of bone elasticity needed to cause microfractures and increases the strength of stimulus for a method like LSJL).  The larger the weights you use the smaller amount of time you would need to stress the bone to cause microfractures.  For example, you would have to temporarily stretch your radius and ulnar bone lengths a lot longer to cause microfractures holding a five pound dumbell then you would holding a sixty pound dumbell.

The same principle should apply for lateral synovial joint loading, the larger the weight you use the more likely you are to experience the desired forces(hydrostatic pressure in the epiphysis of the long bones) versus undesired forces(articular cartilage damage).

No comments:

Post a Comment