A model of pelvic floor muscle activity

Investigators: Ms Margaret Sherburn, Prof Mary Galea, A/Prof Kerry Landman, Mr Steve Martin

Real time transabdominal ultrasound assessment of pelvic floor function as an outcome measure has not previously been investigated. Previous research has indicated that this method of measuring the pelvic floor muscle activity has good reliability, both intra-tester & inter-tester (Sherburn et al, 2005 in press), however the interpretation of transabdominal US measurements is in its infancy. As yet there are no normative data available with this new use of US technology or standardised measurement parameters.

The pelvic floor muscles form a diaphragm within the bony pelvis. Ultrasound displacement measures are sagittal plane linear measurements of the amount of lift of the central portion of this diaphragm (Fig. 1). It is unknown whether the rate of displacement throughout a contraction is linear, or whether the rate of displacement changes as the muscular diaphragm approaches its maximum excursion. It is also unknown whether this displacement alters after a pelvic floor muscle training program, when the muscles become stiffer with increased strength and the length/tension curve for the muscle alters.

Another factor that may impact on pelvic floor muscle activation is the bony architecture of the pelvis. It is known that altering the biomechanical characteristics of skeletal levers within a dynamic system alters the muscle activity within that system (e.g. gait kinematics), and a recent study indicates that having a pelvis of a particular shape may predispose individuals to neuromuscular injuries, leading to the development of pelvic floor disorders.

Figure 1 . Visualization of the pelvic floor using transabdominal diagnostic ultrasound in the sagittal plane in a female subject in supine lying. Panel A demonstrates the pelvic floor at rest. Panel B demonstrates the movement of the pelvic fascia associated with contraction of the pelvic floor. The dotted line between two white crosses (X) is the degree of excursion of the pelvic floor during a maximum contraction .

The study comprises two phases:

1. Mathematical modelling of diaphragmatic muscle action: This will be a theoretical study using known morphological parameters of the pelvic floor to create a mathematical model that will be able to explain how the linear measures obtained by ultrasound relate to pelvic floor muscle activation changes. This modelling will investigate what displacement patterns are most likely and what they mean for changes in muscle strength/force production.

Finally, it is hoped that by using a mathematical model we will be able to predict changes in linear displacement (representing muscle force production) by understanding the patterns of change of displacement of the pelvic floor.

2. Testing the mathematical model of pelvic floor muscle action, as measured by transabdominal ultrasound, from external bony landmarks of the pelvis : This study aims to measure pelvic diameters to test the model of pelvic floor muscle behaviour on a homogeneous population of young fit adult women who can perform pelvic floor muscle contractions. The hypothesis to be tested is that women with wide pelvic diameters, as measured from external bony landmarks, will produce different linear displacements of the pelvic floor when performing maximal voluntary contractions of the pelvic floor muscles, compared to those with narrow pelvic diameters.

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