Over 50% of pregnant women develop low back pain (LBP) over the course of their pregnancy. Even so, the exact physical cause of LBP in pregnant women remains unknown. The weight of a developing fetus results in an increasing moment about the spine that must be counterbalanced by increased strength and endurance of the postural muscles. In the general population, poor trunk muscle strength and low muscular endurance levels have been identified as risk factors among those likely to develop LBP. The shift of the electromyogram's (EMG) power spectral density toward lower frequencies has been related to muscle fatigue. The purpose of this study was to develop a testing equipment system and protocol to measure the fatigability of the low back and quadriceps musculatures that contribute to the maintenance of posture and are therefore influential in offsetting the increasing moment created by a developing pregnancy.

The Isometric Trunk Extension Chair was designed and built to allow symmetrical isometric contraction of the extensors while retaining the subject in a repeatable semi-kneeling position that will be safe and comfortable for use by a pregnant woman. The Quadriceps Extension chair was designed and built to allow isometric contraction of the quadriceps muscle. Fatigability of the back extensor and quadriceps muscles was assessed using surface EMG. The primary findings of this study were the interclass correlation coefficient repeatability measures for all major variables of the trunk extension tests on nonpregnant women. The developed testing system provides a reliable means of measuring fatigability in the quadriceps and trunk extensors at the LI and L5 spinal level in women. The 60% MVC trial testing system may be used in future studies to study fatigability of these muscles and their connection to LBP during pregnancy.