The hair's elasticity is the measure of how much the hair will stretch
(and return to a normal state). Healthy hair, when wet, will stretch up to
50% of its original length and return to its normal shape without breaking,
while dry hair will only stretch about 20%.
Elasticity is rated as being low, normal, or high. Hair's elasticity comes
from the side bonds in the hair shaft. Hair with normal and high elasticity
is easily styled with wet-roller sets, thermal styling tools, etc., while hair
with low elasticity may prove hard to curl, or lose its curl quickly.
To test the elasticity of your hair, you need to select strands of
hair from four different areas on the head. Be sure the hair is wet. Hold
the strand securely and stretch the hair. If the hair stretches and returns
to its original length when released, then it has good elasticity. If the hair
breaks or doesn't return to its former shape, you have low elasticity.
Hair with low elasticity is highly susceptible to breakage, will be hard
to curl, and will not perm well.