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You have not mentioned your age. Forward bending is restricted, if there is any problem in the vertebral
column, or if you are not used to doing such thing. If forward bending causes pain, you should consult an orthopedic surgeon to exclude vertebral pathology such as slipped disk.
It is better to avoid forward bending exercises, as they may create problem later in life. If you cannot touch your toes, do not attempt to do so.