Baessler K, Metz M, and Junginger B. Valsalva versus straining: There is a distinct difference in resulting bladder neck and puborectalis muscle position. (coreexercisesolutions.com)
These muscles include the sphincter muscles, the urogenital triangle, and the levator ani (which is made up of the pubococcygeus, illiococcygeus, and puborectalis muscles). (hab-it.com)
When you squat the puborectalis muscle relaxes making elimination much easier. (nutritionfacts.org)