In dictionary terms it is:

“the amount of extension of an object under stress.”

In tennis terms, it means the same thing when talking about tennis racquet strings.

How much does a string stretch under the reference tension load or otherwise stretched (impact)?  The proliferation of wrist, arm and shoulder injury has brought attention to the property of “stiffness.”  The problem is that your stiffness may be different than my stiffness, so there needs to be an “index” associated with each string, in my opinion.  I have that data on over 500 tennis strings, but that is just me.

The images show the results of high elongation (left) and low elongation (right) string upon breaking.

Several years ago a player asked me “where is the string that is missing?”  Well, it is not missing.  The ends you see should be connected!

If the string has little elongation when it breaks there is nothing “pulling” it apart like the high elongation string. So each time you hit the ball, the string either elongates a bunch or it doesn’t.

In the case of the high elongation string, on the left, it absorbs a good portion of the “shock” associated with a hard hit, whereas the low elongation string, on the right, lets your body do the absorbing to a great extent.

So, it is reasonable to use very low reference tensions for low elongation string (35 to 45 pounds; 16 to 20.5 Kg) and higher tensions (45 to 60 pounds; 20.5 to 27.2 Kg) for high elongation strings.

You may ask, “how do I know how stiff a string is?”  If you see the word “polyester or co-polyester” it is likely that string wil be stiff compared to natural gut, most nylon based multi-filament construction, and PEEK (Zyex) material.  In my opinion, there is no “bad” string just “bad” applications.  If in doubt…ask!