Hi Motjaba, 

I am unsure as to the most reliable, however, the following papers may give you a benchmark to work off. 

http://journals.lww.com/nsca-jscr/Abstract/2003/02000/Maximal_Strength_Testing_in_Healthy_Children.25.aspx

http://www.tandfonline.com/doi/abs/10.1080/02701367.2002.10609041

For your age group, exercises including hand-held dynamometry may represent the easiest exercise - as it is a rather simple movement compared to other exercises to measure muscle strength such as chest  press, squat etc etc. Hope this helps 

http://www.tandfonline.com/doi/abs/10.1179/108331905X55730

http://jbjs.org/content/90/5/1053.abstract

Best.

Dear Mojtaba 

AoA 

i think you can use the tests as you can demonstrate easily in your environment and facilities and also give the motivational stimulus to your said samples groups. although if you have digital equipment such as hand grip dynamo meter then you can get strength test of both right and left hand of kids and get the data easily. on the contrary if you do not have any facility then get manual test push up test and count these push of each age group samples by paper pen bass and than use that information for what ever your research concern.

dont hesitate if you have any query regarding your task 

regard 

dr. Muhammad rafiq 

Body weight tests are useful such as pushups and pullups but do not really measure maximum strength. If you have access to a forceplate or load cells the Isometric Mid Thigh Pull test is very safe and reliable to implement even with 11-13 year olds. 

Thanks for all the answers

Intuitively, reliability is going to require that technique be a minimal factor. That is the real value of grip strength, simple and safe. A test involving an exercise which allows for free movement at the end point (free weight exercise) would be more difficult to use as opposed to a selectorized machine movement. 

free weight Muscular Endurance Tests like the Maximal Push/ pull Ups and Max Sit ups as well as Burpees per unit time (60 secs). These are fairly easy to administer and I find previous comments indicate that they do not measure maximal strength. I would not recommend the measurement of maximal strength with weights for this age group as it may interfere with musculo-skeletal development. Dynamometry tests, YES, But are we really sure that they would give us absolute maximal values? they give us a value in the shortest time possible.

In younger population a dynamometer is a good tool, with some database, to perform a test to ascertain the strength:

http://www.ncbi.nlm.nih.gov/pubmed/14970972

http://adc.bmj.com/content/59/5/453.short