If you shorten track suit pants often, and do not have an over locker you can use twin needle.  I would recommend the widest one possible, because I think it gives a better finish.

Personally I prefer to over lock the edge of the new hem, and sew two rows of stitching, the first through the overlocking and the second in the exact same position all the way around next to the first row of stitching.

If you are operating a clothing alteration business, you can buy yourself a little Coverstitch machine.  They are around AUD$600 and they are wonderful.  They give the professional cover stitch finish like you see on new clothes. It gives a twin stitch on the outer and over locked on the inside.  There is also a very easy way to unpick these seams.  I have provided illustrations and information in my book Genie Clothing Alteration Secrets Revealed.

Happy sewing

Judith aka genie