vsEMG values of the maximal isometric contraction of PFM till fatigue (a); holding time of maximal isometric contraction of PFM till fatigue (b); thickness of TRA muscle during maximal isometric contraction of PFMs till fatigue (c). PFMs, pelvic floor muscles; TRA, transverse abdominal; vsEMG, vaginal surface electromyography; COG, the control group; SIG, the sitting study group; SUG, the supine study group.