Recently there was a paper about the role of muscle cell replenishment (from stem cells) in order for mice to keep their strength. Very roughly put, the stem cells signalling pathways get screwed up as you age (accumulation of damage), this causes the replenishment of muscle cells to be corrupted and therefore the mice become fatigue.

This could be fixed (It is reversible)

http://www.nature.com/nm/journal/v20/n3/full/nm.3465.html

Was that roughly what you were asking for?

The loss is generally at the bonds between particles. Components are the same but the forces or interactions between them changed. Thus for example in fatigue of rigid materials the generation of microcracks, which grow and attach to the accumulation cycle is shown.

What happens with biological materials is quite different. First elongations and/or microcracks are generated in tissues which are after remodeled causing this regeneration and muscle growth.

Yes: eugitaF

Recently there was a paper about the role of muscle cell replenishment (from stem cells) in order for mice to keep their strength. Very roughly put, the stem cells signalling pathways get screwed up as you age (accumulation of damage), this causes the replenishment of muscle cells to be corrupted and therefore the mice become fatigue.

This could be fixed (It is reversible)

http://www.nature.com/nm/journal/v20/n3/full/nm.3465.html

Was that roughly what you were asking for?

With ageing, material properties of muscles, tendons and bone change.

Also maximum force/csa (stress) of muscle declines.

So the constituents' properties do change with age!

In addition to the excellent comments above, I wanted to add that there might be addtional components originating from hormonal changes, such as declining levels of testosterone in men and women and declining levels of growth hormone:

http://www.ncbi.nlm.nih.gov/pubmed/12062358

These changes might mediate the impairments of recovery and replenishment associated with aging.

My question is slightly different. If you look at the constituent atoms (be it living material or non living) aging does not change these. So on physics (and chemistry) view point, it is the electrons of individual atoms that provide the interaction and thus the strength in materials through so called interatomic interactions. These can not change- at most the interacting atoms have been dragged too far in distances or angles to revert back in previous equilibrium positions- may have undergone structural phase transitions. But on remould or on another external perturbation these could be restored to earlier structure and thus should behave as 'young' or at earlier time. But on the other hand we know materials loose strength on persistent use and recycle. I do not understand why.

I agree that atoms and their electrons do not change but in the human and animal body atoms are bound together to form molecules and the structure of molecules changes with ageing. Proteins become methylated, their folding changes. This is one of the biggest problem accompanying many diseases with ageing.

I'll try to answer for inorganic materials. Basically, if you deform something and deform it back, even though it is macroscopically the same as when you started, the material contains a lot more stress after the process than before (even though averaged over the whole material the final stress is zero). When you continue cyclic deformation, eventually there are small parts of the material where the micro-stresses are high even though the macro-stress across the whole sample is low. In these cases the material then breaks at the locally highly stressed areas, and the crack propagates through the material. These small cracks that slowly grow are also called fatigue cracks.

Agreed Manuel, but these cracks are because deviations occur from defined equilibrium positions due to stresses which are outside elastic limits. My question is if you bring them back to elastic limits, the material should be perfectly same as it was in the past or at young age. And this restoration could take place when the building blocks which are atoms or molecules are allowed to reassemble like crystallization. Or remoulding or recycling as usually done in plastics. Despite that however we rate recycled plastic inferior - my question is why?

@Kristian Bendtsen your link is returning error. Could you please check and resend your link to nature? Or it may be that it is not accessible from home due to paid article.

@Vijay Jindal

The link works for me.

I think I got your question now Vijay. But I don't see why Manuel Schanbel's answer is not what you were looking for.

Could a rephrasing of your question be "Why does remoulding of a metal not create a perfect crystal structure?", and I guess that has something to do with the speed of cool.

@Vladimir I want to address firstly why any number of cycles in recycling does not completely restore mechanical properties. Product could be anything from fiber, plastic to muscle, metal to bone. The properties of the product only depend upon the constituents which do not change. If we take care to slowly regrow from the same constituents why impairment in quality?

Sorry but this is going to be a long answer,

As far as engineering materials go, I actually don't understand your question. Taking steel as an example, most steel available right now have an important fraction of it coming from recycling. From a practical standpoint, the fatigue properties of a part made from this new steel batch would be the same as those from an earlier batch of the same alloy. Some differences may exist because no two batches have the exact same amount of each alloy elements and impurities, and no two batches have the exact same heating and cooling rates and times, but that is pretty much it. If you look at polymers, If you remelt and cool a thermoplastic with similar parameters (temperature, moisture, no additional solvents...) as those used in the original process, you are likely to pretty much 'reset' its fatigue life.

Bear in mind that there will always be a huge scatter in fatigue properties because the fatigue phenomenons have important interactions with local properties, which are not homogenous.

As for the fatigue of living being and their organs, I have a hard time getting the grasp of the 'remould' equivalent... All I know is that the aging process often involves changes in the molecular structures or in the organisms ability to deal with external stimulus. There is a reduction in the regenerative abilities and part of the 'fatigue' is in part due to accumulations of matters that prevent all functions to perform as they originally did and so on.

If we have not answered you adequately, can you link us to an article showing what you actually mean?

Best regards.

As Emery stated, entropic processes are definitely important in aging and fatigue. A fully reversible process is purely conceptual and nature precludes its event.

It is however unclear to me if the original question is articulated more around the living or the non-living realms. For me, the answer appears quite different for these two cases. For the non-living materials used in engineering structures, even though entropy plays an important role at the root level, the way we manufacture them and manipulate them makes them part of a system that is more open than closed and losses can be more or less easily compensated for by external means that equate to changing the boundary of the system to make entropic losses less significant. The entropic process takes place but is mitigated.

In living organisms, although the processes are not closed systems per se, there is a lot less control over the way things evolve and the boundaries of the system are defined at the beginning of the game. The processes are homeostatic and we have only little control over their evolution. Therefore, as aging goes on and the functions start to fail, we can only observe the decay, be we do not have easy access to the process so we can't mitigate them.

Entropy will increase as time grows because of environment and so the disorder. But there is no prohibition for reversing this process completely. As ,long as the constituent atoms are allowed to find their equilibrium positions at the given initial conditions, they will have to finally attain the same structure with similar strength as existed originally. So infinite number of recycling processes can not alter the properties of constituents. May be we are forced to accept impurities in the process of recycling and if that could be avoided we could completely reverse the degenerated stuff. And this could apply to living and non living?

''Nothing is lost, nothing is created, everything is transformed.'' A. Lavoisier

I'd say that its just a matter of process control. Therefore, in theory, you are possibly right for some non living inorganic materials (minerals for example) over which we have a great control during processing. Then comes organic yet non-living materials (polymers for example) which already behave differently and for some of which, like thermoset polymers, reversibility with heat is not a possibility because of combustion and where the process for reversibility are more chemical than physical. For these, we exert a little less control during processing. However, form a practical standpoint, it is impossible to have complete control over any process and therefore impossible that ''the constituent atoms are allowed to find their equilibrium positions at the given initial conditions''.

Finally, in living materials, we have very little control over the process because by nature, the living organism control their internal functions. Therefore, we can give them signals to act differently, but the organisms keeps control over the process. It is therefore even more unlikely, in my eyes at least, that full reversibility of living materials aging process is achievable. As I see it, If we get full control of living materials, it is not living anymore as life implies a form of autonomous response to external stimuli. Now this would be more of a philosophy rather than physics question.

If i would say something it is that mechanical properties do not solely depend on the constituents, but also on how those constituents connected and arranged together. Constant use introduces microcracks that spoils or damages the connectivity of the constituents, thus "weakening" the overall behavior. So, the idea that "but the constituents do not change" is in fact irrelevant to aging.

The "arrangement and connection" is dictated by constituent atoms. If there are foreign constituents in unknown quantity, all variations in arrangement and connection strength become possible, A pure iron in a village or elsewhere will have same structure and strength at similar temperature and pressure. Structure is determined by interactions between constituent atoms in an equilibrium configuration determined by minimum energy configuration. So also the strength through the potential energy or cohesive energy at and around minimum energy and its second order derivatives with respect to structural variables like distances and angles.

In metals, fatigue has a lot to do with dislocations, which are at the crystal level and not at the atomic level. As energy is provided by loading or other sources, these dislocations move around and group themselves to form microscopic cracks that further progress into macroscopic cracks and eventually cause fracture. The thing is that along with these cracks, there are strain concentrations. Therefore, at the crack tip, strains may be quite high above the elastic limit. Of course, although it is not exactly the same for non-crystalline media, it is analogous, with molecular or microscopic changes leading to fatigue rather than atomic changes.

Perhaps one way to come around the problem is to see it from a weakest link perspective. The forces/energy required to change the local microscopic properties are lower than those required to alter the structure at the atomic level. The fatigue or failure process is therefore not happening at the atomic level. It is a bit like the amount of energy required to make electrons climb to the next orbital is (luckily) much lower than the energy required to split the nucleus.

I think it is reversible if a person can bear it .

Here is a paper: that you can find in my ResearchGate.

Fangxu Li, Zhilai Yang, Yang Lu, Yan Wei, Jinhui Wang, Dazhong Yin, Rongqiao He. Malondialdehyde suppresses cerebral function by breaking homeostasis between excitation and inhibition in turtle Trachemys scripta. PLoS ONE 2010 5(12): e15325.