Disclaimer: I did some research for this question; I did not know all of this off the top of my head :P. Sources were found through the footnotes in Rabbi Ribiat's The 39 Melachos.
It is forbidden to body-build on Shabbos. :P Joking aside, there is an issur (prohibition) against boneh (building) on Shabbos, which can also be applied to body parts, such as pus-filled pimples (Kesuvos 6b, see Rashi ad loc) [ though the Rambam calls this מכה בפטיש, and not בונה (Hil. Shabbos 10:17 [16?]) ].
This is derived (Shabbos 95a) from חוה, who was delivered to אדם הראשון with braids in her hair. [ See also Rambam Shabbos 22:26 says this only "looks like boneh, which would mean that this is only rabbinically prohibited (R' Ribiat) ]
דרש רבי שמעון בן מנסיא ויבן ה' אלהים את הצלע מלמד שקילעה הקב"ה לחוה והביאה אצל אדם....
Rabbi Shimon ben Menasya expounded: "ויבן ה' אלהים את הצלע" "And Hashem built the rib [into חוה];" this teaches that Hashem wove חוה['s hair into braids], and brought her to אדם.
( Translation mine, comments in brackets based on Rabbi Ribiat. )
So, let's assume that boneh (and/or makeh b'patish), is forbidden to do on the human body on Shabbos.
What about retainers? Orthodontic retainers, correct me if I'm wrong, are used to maintain the position of teeth by holding them in a position for an amount of time. People who wear their retainers nightly, have their teeth move slowly out of position over the course of the day, and put the retainer in at night to return them to where they are supposed to be. (This last sentence is mostly conjecture.) Could that be a problem of boneh or makeh b'patish?
Similarly, what about tightening braces with rubber bands? Might placement of rubber bands on the braces (pulling the teeth in a given direction) also be boneh or makeh b'patish?