Hi Arash, the following may give you some idea altough its quite difficult to determine the strain using ultrasound during dynamic activities such as running

J Exp Biol. 2014 Sep 1;217(Pt 17):3159-68. doi: 10.1242/jeb.100826. Epub 2014 Jun 19.

Tendon elastic strain energy in the human ankle plantar-flexors and its role with increased running speed.

Lai A1, Schache AG1, Lin YC1, Pandy MG2.

Cheers

Steve

Hi Arash, this paper can help you. "Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon?". Journal of Biomechanics 44 (2011) 822–826.doi:10.1016/j.jbiomech.2010.12.015.

@ Stephene , @ Carlos,

Thank you very much for your suggestions.

By reviewing the suggested papers, I found what I wanted about AT's strain range in running.

Regards,

Arash

Normally it takes less than 4% of the maximum load or ultimate strength. During normal walking the load needed to keep the weight of a subject is normally about the toe region. At this region small amounts of load results in a high elongation and strain.   

It is difficult to suggest a definite figure, because it depends on the water content of the tendon which itself is related to age of the subject. However, it is normally about 30-35% of maximum (ultimate) strain in a mature person and slightly more than this figures in younger subjects.

Dear Dr. Oryan,

Thank you for your answers.

Will you please suggest any particular study regarding the relationship between age and strain range?

Regards

In considering the peak load on any tendon it helps to recognize the many variable factors.

Lever length 

Stride length 

Leg component mass

*Status of the involved muscle body ( full relaxed length - or compromised and the "tonus" of the muscle belly )

Running may overload muscle - this overuse may result in a tightened muscle body which will not modulate the peak load - which will then be experienced at the tendon and bone connection.  Repetitive loading of the tendon degrades the collagen - resulting in a compromised tendon  ( tendinosis ).  Thus further tendon damage is more likely. 

Please see:  http://www.ncbi.nlm.nih.gov/pubmed/20086639

                     http://www.ncbi.nlm.nih.gov/pubmed/20086644

Also:   http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312643/

Obviously age may affect tendon injury and healing, -  but nutrition may be of unrealized value.  A healthy gut biome may provide better healing - tendon soup is a recommended adjunt to healing in many cultures.

All this may miss your original question of strain ( load ) as a discrete value - but it is intended to raise questions which may be of some help.

@ Gregory,

Thank you very much for your suggestions.

I read them, however, I couldn't quite understand that repetitive exposure to load and unload (fatiguing exercise) affects muscle contractile characteristics first and then it causes tendon over-elongation or the other way around?

This seems pretty important when it comes to creating a vibration model.

Repetitive exposure to load might affect the tendon in the same way as it occurs to engineering materials such as steel. However other factors like loss of collagen fibers might accelerate the fatigue process. I've read some papers on muscle force changes with fatigue. But I don't know if there is any research regarding the AT's stain-stress diagram during a fatiguing exercise?

Dear Arash:

Sorry to just now get back to you

The basic idea - chronic muscle tension by over activation of the muscle spindle cells changes the loading factor on the tendon ( both ultimate load and load as a function of time ).  This challanges the repair and remodeling of the collagen comprising the tendon - and the weaker tendon can then mechanically fail because the interim collagen fibers are straight ( rather than curled ).  When enough of the tendon collagen is substandard tensile strength is reduced - a lesser force can cause rupture.

I would therefore consider tendon problems ( tendinosis ) and tendon rupture as a result of chronic muscle hypertonus.   Massage to return the muscle to a relaxed state enables the collagen to remodel ( in a curled form for the most damage resistant composition ).

                                                AS AN EXAMPLE:

The basketball player takes off for the layup on the same foot every time, - this overuses that calf muscle - causing overactive muscle spindle cells to shorten the calf muscle body - over time reducing the ability of the calf muscle body to absorb peak muscle loading, - and causing the tendon to experience more load than previous, + and the resulting tendinosis ( collagen degradation ) and compromised healing results in a sub competent AT with much greater likelihood of damage.

Hope this makes this proposition clearer.  You have to consider what seems to be the basic cause leading to tendon damage - not simply an isolated tendon load value.

Normally it occurs in the first 10 percent of the linear region of the load-deformation and/or stress-strain curves. Even during jumping it never passes the yield point, where the tissue changes its behavior from elastic to plastic criterion. 

Dear Gregory,

Thank you so much for your comprehensive explanation.

Kind Regards,

Arash