Electrical stimulation can also be applied to the anal sphincters and pelvic floor muscles, inducing muscle contraction without traditional exercises (similar to transcutaneous electrical nerve stimulation, TENS). The evidence supporting its use is limited, and any benefit is tentative. In light of the above, intra-anal electrical stimulation (using an anal probe as electrode) appears to be more efficacious than intra-vaginal (using a vaginal probe as electrode). Rarely, skin reactions may occur where the electrodes are placed, but these issues typically resolve when the stimulation is stopped. Surgically implanted sacral nerve stimulation may be more effective than exercises, and electrical stimulation and biofeedback may be more effective than exercises or electrical stimulation by themselves. TENS is also sometimes used to treat FI by transcutaneous tibial nerve stimulation.