Strain guages attached to the polygraph chair (usually under the front legs) are used in an attempt to detect and deter the use of physical countermeasures that would result in a shift of weight distribution, such as pressing one's toes to the ground. They are now obsolescent and are being replaced with piezo-electric sensor pads that are usually placed on the seat of the polygraph chair, but which may additionally be placed on the arm rests and under the examinee's feet.

Although it has not been demonstrated that such guages and sensor pads are able to reliably detect an anal sphincter contraction employed as a countermeasure in the context of an actual examination, as a precaution, one might employ mental countermeasures (which in any event would be my first choice if I were to use countermeasures) or tongue biting, combined with breathing manipulations.

See the message thread "Counter-countermeasure Techniques" for discussion of strain guages, sensor pads, and other counter-countermeasures:

http://antipolygraph.org/cgi-bin/forums/YaBB.pl?board=Proc;action=display;num=10...

See Chapter 4 of The Lie Behind the Lie Detector.