AMERICAN SOCIETY OF BIOMECHANICS Presented at the Twenty-First Annual Meeting of the American Society of Biomechanics Clemson University, South Carolina September 24-27, 1997

INTRODUCTION

This study involved a measurement of biomechanical deviations in gait during pregnancy. This information will increase the understanding of how gait is altered during pregnancy. In addition, this information will be useful to increase the understanding of factors which contribute to various musculoskeletal disorders during pregnancy (such as low back pain, hip pain, and other painful conditions).

REVIEW AND THEORY

Significant hormonal and anatomical changes occur during pregnancy which dramatically alter body mass, body mass distribution, joint laxity, and tendon and muscle strength. The effects of these changes on standing posture have been fairly well documented, however, little has been reported about the effects of these changes on walking gait. The walking gait of pregnant women has been characterized as a “waddling gait” (Heckman and Sassard, 1994). Characteristics of a waddling gait include: wide base of support, external foot progression angle (“toeing out”), and large pelvic obliquity (tilt of the pelvis side to side in the coronal plane) range of motion. However, a review of literature revealed no studies documenting overground walking kinematics and kinetics of pregnant women. The purpose of this study was to assess the gait of women during pregnancy and compare it to their non-pregnant gait.

PROCEDURES

Ten women were studied in the 2nd half of the last trimester of pregnancy and again one year postpartum. Lower extremity joint kinematic and kinetic data were collected using a Vicon 370 3-D Motion Analysis system (60 Hz) and an AMTI force platform (600 Hz). The subjects performed repeated walks at a self selected pace across the room (about 10m) until 6-8 strides of kinematic and kinetic data were obtained. Pelvic angles; hip, knee, and ankle joint angles, moments and powers; and dynamic base of support width were compared for the pregnant and non-pregnant conditions. Selected maximum, minimum and range of motion values for the pelvic and joint angles; selected maxima and minima for the joint moments and powers; and dynamic base of support width (measured as the average width of the ankle joint centers during double support) were analyzed using a one-way ANOVA with repeated measures at a 95% significance level.

RESULTS

Statistically significant findings (p<0.05) for selected kinematic and kinetic variables analyzed for the pregnant and non-pregnant conditions are presented in Table 1 below. Significant increases in anterior pelvic tilt, hip adduction during stance, double support time, and hip abduction/adduction power (H3-F); and decreases in maximum hip extension, and single support were found during walking during pregnancy (p<0.05).

Variable	Pregnant	1 Year Post	p- value
Maximum Pelvic Tilt	18.7°	14.3°	0.0109
Max. Hip Extension	2.5° (flex.)	5.3° (ext.)	0.0429
Average. Hip Adduction (during stance)	5.6°	3.6°	0.0158
Double Support Time	27.9 % cycle	25.5 % cycle	0.0342
Single Support Time	36.1 % cycle	37.4 % cycle	0.0305
Hip Ab/Add. Power (H3-F)	0.76 Nm/kg-s	0.59 Nm/kg-s	0.0490

Table 1: Significant (p<0.05) deviations in gait due to pregnancy. Means for each condition and p-values from statistical analysis are presented.

Selected findings which were not statistically significant are presented in Table 2. Mean dynamic base of support was slightly larger during pregnancy than 1 year post, however this difference was not significant (p> 0.05). Pelvic obliquity range of motion and foot progression angle during stance were also not significantly different (p>0.05) for the two experimental conditions.

Variable	Pregnant	1 Year Post	p-value
Dynamic Base of Support	0.207 m	0.192m	0.4585
Pelvic Obliquity ROM	9.5°	10.3°	0.3731
Foot Progression Angle	8.2° (external)	8.3° (external)	0.8721

Table 2: Selected variables which were not significantly different (p>0.05) for the pregnant and non-pregnant conditions. Means for each condition and p-values from statistical analysis are presented.

DISCUSSION

Little evidence was found to support the popular contention that the gait of pregnant women can be described as a “waddling gait”. The results of this study show that the dynamic base of support is not significantly wider, pelvic obliquity range of motion is not greater, and the foot progression angle is not significantly more externally rotated which are characteristics of a ‘waddling’ gait. The changes in gait during pregnancy quantified by this study are mild and can be explained by the increase in body mass and width, and the change in mass distribution about the trunk.

Increases in hip abduction/adduction power during late stance (H3-F) with pregnancy are apparently a result of an increase in body mass. Hip abduction/adduction power generation at this time period has been identified as a measure of the muscular power of the stance side hip abductor muscles used to raise the unsupported side of the pelvis (Eng and Winter, 1995). With the increased body mass during pregnancy, an increased load must be lifted by the hip abductor muscle group. Significant increases in double support time and decreases in single support time during pregnancy appear to be compensations to minimize the time spent in single limb support when this increased muscular effort is required to support an increased body mass with only one foot.

The increase in width of the pelvis during pregnancy apparently was compensated for by an increase in maximum hip adduction during stance. Evidently the hip adduction angle was increased during the single support phase of gait in order to keep the foot centered under the body to avoid a wide base of support. Walking with a wide base of support necessitates large side to side excursions of the center of mass and is energy inefficient.

The significant increase in anterior tilt of the pelvis (top of the pelvis tipped forward) during pregnancy that was observed can be explained by the increase in the amount of body mass located in a low anterior position on the trunk during pregnancy. An increased anterior load on the lower trunk causes a forward rotating moment tending to rotate the pelvis forward. Unfortunately, trunk position was not measured in order to identify how trunk posture was altered to compensate for the increased anterior pelvic tilt. Typically, increased lumbar lordosis is necessary to bring the upper body backwards into a stable position when anterior pelvic tilt is increased.

The results of this study indicate that women alter their gait in order to compensate for the increase in body mass and change in body mass distribution during pregnancy. It should be noted that these deviations, although statistically significant, were mild and a surprising normal kinematic and kinetic profile was generally appreciated.

REFERENCES

Eng J.J., and Winter D.A., J. Biom., 28, 753-758, 1995.

Heckman J.D., Sassard R., J. Bone Joint Surgery, 76-A, 1994.