Hair Crease after a Ponytail
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A: Well, the crease you mention is caused by the same principles that allow you to curl the hair with rollers or a curling iron.
Fortunately, removing such a bend can usually be accomplished by applying a little tension combined with additional heat (such as a warm flat iron), or simply by misting the affected hair with water and running a comb through it to loosen the bend.
If you opt for the spray/mist method, I recommend using a leave-in conditioner, so that you can be sure to keep the hair healthy and smooth the cuticle of any roughing that may have occurred as a result of using a ponytail elastic.
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See also:
Flat iron styling
Ponytails and hair damage
How to put your hair in a ponytail
Is it okay to put your hair up in a ponytail right after coloring it?