The Biomechanics of Pregnancy

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Introduction

Pregnancy, as a natural and physiological process, produces in a woman a series of changes involving the motor system. Weight gain, especially changes within its distribution requires functional adaptation of the musculoskeletal system. These changes make both the posture and gait pattern of pregnant women different from non-pregnant subjects.

The addition of anterior mass on the trunk in a pregnant woman changes the body's center of mass if there is not a concomitant change in posture. Consequently, kinematic adjustments are made to improve stability, allowing safer gait for the pregnant woman, but may be only mechanical in nature. The adjustments do reduce the kinetic effect of the increased mass as total mass normalized moments decrease.

Biomechanical Considerations

During pregnancy, a number of biomechanical and hormonal changes occur that can alter spinal curvature, balance, and gait patterns by affecting key areas of the human body. This can greatly impact quality of life (QOL) by increasing back pain and the risk of falls.

The overall postural effect of pregnancy by the final month is as follows:

• Anterior tilt of pelvis with hyperextended knees
• Accentuated lumbar lordosis with a short radius curve
• Posterior gravity line
• Hyper kyphosis of the upper thoracic
• Protracted shoulders
• Anterior angulation of the cervical region
• Extension of the occiput on atlas.
• Associated with these postural changes is a waddling gait pattern.

The video below demonstrates how posture changes as the fetus grows.

Changes in the Spine

• Pregnant women with anterior translation of their center of mass have been shown to lack positional adjustment of lumbar lordosis, and the force of gravity, when more distant from the hip, generates a larger hip moment and an unstable upper body.

• Many pregnant women demonstrate a sway-back posture, whereby the upper trunk is displaced posterior to the lower body causing the center of gravity to shift further backward, and increases the tone of head and neck muscles, causing the head to shift forward to compensate for the change in center of gravity and prevent falling.

Changes in the Knee

• As pregnant women experience an anterior shift in center-of-gravity, their knees hyperextend to maintain balanced, upright posture. Knee hyperextension tenses the anterior cruciate ligament (ACL) as it impinges against the femoral notch, which may cause the ACL to adapt and lengthen throughout pregnancy.
• Knee laxity during pregnancy reaches a constant level before the fifth month of pregnancy, which suggests that laxity increases early in pregnancy; it decreased significantly by about 14% 4 months postpartum.
• The knee is delicately balanced to maintain stability while also allowing for a wide range-of-motion during activities. Perturbations to this balance, such as those caused by the non-uniform changes in joint laxity which persist following pregnancy, could potentially increase the risk of developing OA and other musculoskeletal disorders in women in their post-reproductive years.

Changes in the Ankle and Foot

• Changes in the biomechanics of feet during pregnancy and puerperium, indicates an increase in circumference measurement and a reduction in plantar arch while the length and width of the foot increases with increased body weight and ligamentous laxity.
• During pregnancy, forces exerted at the soles of the feet shift from the posterior to the anterior with consequent increases at the forefoot and, more prominently, at the midfoot.
• Flat feet add stress on the foot and causes inflammation of plantar fascia and increases strain on the feet, calves and sometimes the back.
• The plantar loading would redistribute as the body weight increase and centre of mass relocate, which is reported to be correlated with foot complaints.

Biomechanical Alterations in Gait

Walking is the most essential daily activity for the human body; however, there are many biomechanical changes in gait during pregnancy. Pregnant women altered gait pattern to adapt the weight gain and the shift of centre of gravity. The gait of pregnant women undergoes following modifications:

Gait Speed

Natural locomotion of pregnant women is characterized by slower speed and lower frequency and length of steps, as compared to the pre-pregnancy and postpartum state. Pregnant women tend to avoid the large relative phase between pelvic and thoracic rotations that is typical for high walking velocities, possibly because the moments of inertia of their pelvis and thorax have increased which renders the control of relative phase more critical.

Gait Cycle

A significant decrease in the length of the gait cycle is observed in pregnant women. There is an increase in double support time compared to post-partum and nulliparous women. These are fine adjustments that minimize the time on one leg to reduce muscle solicitation. Thus, pregnant women exaggerate transition phases in order to increase the security of gait.

Table below

Spatial and Temporal Parameters of Gait
Stride Length	decreases as pregnancy progresses
Stride Width	remains unchanged
Step Length	Decreases
Step Width	Increases as the pregnancy progresses
Single Support Time	Decreases
Double Support Time	Increases between trimesters
Base of Support	Increases
Ground Reaction Force	Decrease in late pregnancy.

Joint Kinematic Parameters
Sagittal Plane
Pelvis	Increase in anterior tilt about 5 degrees
Hip	Increase flexion during stance phase
Knee	Increase flexion during terminal stance
Ankle	Decrease dorsiflexion and plantarflexion
Frontal Plane
Pelvis	Increase in pelvic separation width and reduction in the amplitude of the unilateral elevation of the pelvis.
Hip	Increase in abduction.

Joint Kinetic Parameters
Sagittal Plane
Hip	Significant increase in the hip extensors moment
Knee	Significant decrease in the knee extensor moment
Ankle	Significant decrease in the ankle plantar flexor moment
Frontal Plane
Hip	Increase in hip abductors moment
Knee	Increase in knee adductors moment

Sit-to-Stand during Pregnancy

Rising to stand from a chair becomes more difficult to perform as pregnancy progresses, which may lead to altered biomechanics affecting the musculoskeletal demands on the body segments. Pregnant women may use following temporal-spatial, kinematic and kinetic strategies during the transfer:

• Reduction in pre-extension time.
• Increased base of support.
• Decreased hip joint extension velocity.
• Decreased and delayed vertical Ground Reaction Force (GRF)

The biomechanical alterations may have been used to control the momentum which would otherwise be amplified by the increased mass of the trunk. The need to control upright balance at the end of rising to stand is paramount and subjects will voluntarily limit the propulsive impulse to maintain upright stance under varying conditions. During rising, pregnant women dampened the propulsion is attributable to increased uterus volume, and they enhanced the forward propulsion at gait initiation. To ensure safe motion, pregnant women should not initiate gait until reaching a stable standing position after rising.

Stand-to-Sit during Pregnancy

The physical and physiological changes during pregnancy may necessitate a change in stand-to-sit kinematic performance as follows:

• Hip and spine motion change for pregnant women performing a sitting task as Hip range of motion and standing angle changed in favor of spine motion.
• Postural changes related to gestational lordosis affect stand-to-sit changes.
• Coordination changes away from the hip during the first and last parts of sitting:

1. Joint coordination shifts from hip dominant to spine- and shoulder-dominate coordination just before the start of sitting motion.
2. Hip-knee joint coordination just before seat contact shifts from hip to a knee-dominant motion during pregnancy

Discomforts of Pregnancy

Pregnant women are often burdened with musculoskeletal symptoms due to the physical, hormonal, and anatomical changes that occur throughout pregnancy. These symptoms are associated with musculoskeletal dysfunctions, modified gait, joint laxity, muscle imbalance, and increased body mass. (1) Musculoskeletal pain experienced by some pregnant women may be related to the increased joint moments associated with the increased mass(2)

• low back pain
• pelvic girdle pain
• leg problems
• urinary incontinence

(Spinal curvature and characteristics of postural change in pregnant women)

Implications for the Physiotherapist

As part of their ante-natal care of pregnant women, physiotherapists are concerned to encourage the maintenance of 'good posture'. Examination of the standard deviations for initial posture and the variety in magnitude and nature of postural changes during pregnancy highlight the importance of the physiotherapist assessing each patient's posture early in the ante-natal program. It is obviously important not only to determine initial postural alignment, but also to evaluate it progressively, so that suitable preventive and therapeutic measures may be taken.

Monitoring of postural changes, using the simple inclinometer or clinometer, together with comprehensive recordings of pain during the ante-natal period could produce important data for further guidance of those concerned with the care of the pregnant woman. The physiotherapist could explore whether the woman's initial posture had a significant influence on the development of particular postural changes during pregnancy, or on the development of pain. The results of the study reported here suggest that further research relating to posture and pain during pregnancy would be justified

(The Relationship of Low Back Pain to Postural Changes During Pregnancy)

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