Influence of Pregnancy and Footwear on Lower-Back Muscle Activity During Quiet Standing

Introduction. An estimated 50 to 80 percent of women experience back pain during pregnancy. This pain ranges in severity but can sometimes interfere with daily activities. Lower back pain can start early on in pregnancy, and is typically reported between the fifth and seventh month when most changes take place. Though not the only factor, localized weight gains are quantifiable and potential causes of lower back pain during pregnancy. This study aimed at quantifying the effects of pregnancy stage and footwear condition on lower-back muscle fatigue, used as a predictor of low-back pain. Our main hypothesis was that both pregnancy and footwear condition affect lower back muscle fatigue. Data collection protocol. Ten non-pregnant healthy female subjects between the ages of 18 and 30 years old with a BMI no greater than 26 were recruited for the standing trials. To simulate the effects of localized weight gain during pregnancy, weights were added to subjects’ upper trunk, lower trunk, bottom, thighs, and lower legs, following a pregnancy-specific anthropometric model (Haddox, Gait & Posture, 2020). Each subject participated in four sessions to model 0, 3, 6, and 9 months pregnant. Subjects were equipped with two electromyographic (EMG) electrodes to record muscle activity, one on each of the erector spinae muscles. Subject then stood motionless for five minutes for each of the following four footwear conditions: barefoot, sports shoes, flats, and heels. Excluding barefoot, the order of the conditions was randomized during each session. Data processing. The EMG data collected was exported into Matlab. The median frequency of each signal was computed for the entire trial except the first ten seconds that were excluded to account for initial subject positioning. The final output was the averaged median frequency between the left and right muscles. The lower the median frequency the higher the muscle fatigue. Results. Our main result indicated a trend for the barefoot condition to exhibit higher median frequencies than all the other footwear conditions throughout pregnancy. In addition, the median frequency tended to decrease for the barefoot condition throughout pregnancy, while remaining constant for the other footwear conditions, which exhibited similar values. Discussion. These results only partially validated our main hypothesis since, due to the limited number of subjects tested so far, we could not perform a rigorous statistical analysis. However, our findings highlighted the role of localized weight gains on lower back muscle fatigue, and thus on potential low back pain. As expected, footwear conditions also affected muscle fatigue. However, an interesting result is that wearing shoes tended to minimize the effect of pregnancy, since muscle fatigue remained similar. Current work focuses on adding more subjects and replicating this study for five-minute gait trials at a self-selected speed.