Stretching machines can be of help, but they will allow you to stretch in only a few positions, in my opinion they aren't worth the money. The positions and type of stretch you can do on your own are almost infinite and it won't cost you anything. You can find a lot of information about stretching on the internet or in book, i'm also pretty sure your martial arts instructor can guide you to do the right stretches to enhance your martial arts skills. If your looking to kick higher, I would look at dynamic stretches, but knowing what you want to achieve could help us guide you in the right direction.
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Thats what she said...