Determining the sense of the exercise being performed during training of the pelvic floor muscles (PFM) is difficult because it is challenging to understand visually and it is not accompanied by any joint movement. The training limb position is considered one of the most essential factors in PFM training. There are scattered few reports recommending the supine position for the diagnosis and treatment of a weakened PFM.
In addition, vaginal pressure measurements are thought to be easier to perform and standardize in the supine position than in the upright position. However, this has not been sufficiently verified. Therefore, this study aimed to clarify the relationship between posture and PFM. We examined the effect of transition from the supine position to the upright position on the PFM and determined the angle at which the PFM are likely to contract.

Twenty healthy women without any orthopedic diseases in the lumbar region, pelvic girdle area, or lower limbs were included in the study. The mean and standard deviation of age, height, weight, and body mass index of the participants were 20.6±0.5 years, 160.3±5.3 cm, 50.6±5.6 kg, and 19.7±1.9 kg/m2, respectively. All the participants had no history of orthopedic, gynecological, or urological diseases in the pelvic girdle and lumbar region.
Pelvic floor elevation (length of the pelvic floor shifted cephalad from rest), descent (length of the pelvic floor shifted caudally from rest), and distance from the abdominal wall to the bladder floor (to capture changes in bladder shape) were measured using an ultrasound system (SONIMAGE MX1 Konica Minolta) with a 3.5 MHz convex probe. 
　The participants were requested to drink 500 mL of water 1 h before the measurement to allow the bladder to be filled with urine. To measure the base of the bladder, the probe was placed on the upper pubic rim and tilted 60º to the head. The depth was adjusted to enable the visualization of the entire bladder.
The measurement was recorded as an image when the participant was still and the pelvic floor was maximally elevated at the end of expiration and as a video during exercise. The displacement of the pelvic floor to the cephalic side was defined as the rate of pelvic floor elevation, and the displacement to the caudal side was defined as the rate of pelvic floor descent. Both displacements were measured with reference to the bladder floor at rest.
(1) The length from the abdominal wall to the bladder base was measured when the tilt angle was changed from supine to upright position at 10-degree intervals from 0 to 70 degrees on the Tilt Table.
(2) The displacement of the bladder base was measured when the pelvic floor muscle group was voluntarily contracted at an inclination angle under the same conditions as in (1).
IBM SPSS Ver. 24 (IBM Corporation) was used for all analyses, and the significance level was set at 5%.

Changes in the position of the pelvic floor according to the angle of inclination (Fig. 1)
The distance from the abdominal wall to the bladder base increased caudally with increasing tilt angle. However, there was no significant difference in the corresponding increase in urine volume in the bladder over time.
Pelvic floor elevation rate and standing tilt angle during voluntary contraction of PFM (Fig. 2)
Analysis of variance by repeated measurements was conducted for the angle and pelvic floor elevation rate. There was a significant difference in the pelvic floor elevation rate between the angles (F[7,133]=85.49, p<0.01), and Tukey multiple comparisons at a level of significance of 5% showed a significant difference between 0º and 20º.

The length of the bladder from the abdominal wall to the bladder base increased when the tilt angle from the dorsal supine position to the standing position was changed from 0º to 70º, indicating that the bladder was deformed caudally by gravity. The distal 1/3 of the urethra is fixed, but the pelvic floor supports the bladder base and the proximal 2/3 of the urethra. Therefore, it is thought that the urethra descended caudally in response to the supportive force of the pelvic floor as the tilt angle increased.
The pelvic floor elevation rate was significantly higher when the standing tilt angle was 20º than when it was 0º. This is because the pressure exerted by the pelvic organs on the pelvic floor was greater in the standing position than in the supine position, and the length-tension curve suggests that the pelvic floor became extended to exert muscle tension. It is thought that the optimal amount of the load was at an inclination angle of approximately 20º.

Based on the point mentioned above, it is thought that if the contraction sensation of PFM cannot be perceived, it is better to start PFM training after confirmation of the contraction sensation in a limb position with an inclination of approximately 20º, rather than in the dorsal supine position, which is the degravitational position described thus far.