Re: lengthening the tendon?
It really would depend on which tendon.
Sometimes with Hallux (big toe) fusions, there is pain in the ball of the foot and this can be helped by lengthening the Gastrocnemius (calf) muscle. It is a part of the Achilles, but lengthening it does not result in weakness.
There are some other tendons that could be lengthened if it is the joint between the 2 bones of your big toe - namely the Extensor Hallucis - and this would prevent the toe from elevating if the muscle contracted.
I guess that we need some more information.