The Biomechanics of Pregnancy
Introduction[edit | edit source]
Pregnancy, as a natural process produces a number of physiological changes in a woman's body. Weight gain in pregnancy challenges the musculoskeletal system to adapt accordingly. These progressions make both the stance and stride different for pregnant ladies from non-pregnant. These changes make both the posture and gait pattern of pregnant women different from non-pregnant subjects.
The body's center of mass changes as the fetus grows. The kinematic adjustments are made in order to improve stability, allowing safer gait for the pregnant woman in opposition to the anteriorly added mass on the trunk. However these modifications might be just mechanical in nature, diminishing the motor impact of the increased mass as total mass normalized moments decrease.
Biomechanical Considerations[edit | edit source]
Various biomechanical and hormonal changes occur during pregnancy that can alter musculoskeletal alignments by affecting the key areas of the body such as spinal curvature, balance, and gait patterns and can greatly impact the quality of life (QOL) by increasing back pain and the risk of falls.
- Anterior tilt of the pelvis.
- Hyperextended knees.
- Increased lumbar lordosis.
- Posterior shift of gravity line.
- Hyper kyphosis of the upper thoracic spine.
- 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[edit | edit source]
- Lack of positional adjustment in the lumbar curve occurs as the center of mass translates anteriorly during pregnancy.
- The upper body becomes and unstable as the force of gravity becomes distant from the hip, generating a larger hip moment.
- In pregnancy women exhibit a sway-back posture. The upper trunk moves posterior to the lower body causing the center of gravity to shift posteriorly, and causing an increase in the tone of head and neck muscles, causing the head to shift anteriorly in order to compensate the shift of centre of gravity and prevent falling.
Changes in the Knee[edit | edit source]
- Pregnant women tend to compensate anterior shift in center-of-gravity by hyperextending their knees, so that they can maintain their balance and upright posture. Anterior cruciate ligament (ACL) adapts to the demands put on the knee because of hypertension and lengthens throughout the pregnancy as a result of impingement against the femoral notch .
- Laxity in the knee increases early in the pregnancy and decreases significantly in the postpartum period 
- The changes in the joint laxity can persist following pregnancy making the women prone to developing OA and other musculoskeletal conditions 
Changes in the Ankle and Foot[edit | edit source]
- An increase in the circumference of the foot and reduction in the plantar arch occurs as a result of biomechanical changes during pregnancy and puerperium. Whereas weight gain and ligamentous laxity can cause an increase in the foot's length and width.
- During pregnancy, the exertion of forces shift from the posterior to the anterior feet with resulting increments at the forefoot and, predominantly, at the midfoot.
- Inflammation of plantar fascia can occur as a result of added stress due to flat feet.
- The centre of mass relocates as the bodyweight increases, redistributing the plantar load, which is reported to be correlated with foot complaints.
Balance Strategies in Pregnancy[edit | edit source]
An increment of 15 to 25 percent in the woman's body weight occurs during pregnancy which implies relatively greater stress on the joints and associated structures (e.g. tendons and ligaments) whereas the growing fetus and increase breast volume causes the centre of gravity to shift to the front. Considering all the biomechanical alterations during pregnancy, balance tends to decrease causing a negative impact on postural stability and high risk of fall, especially in the third trimester.
To avoid falls and control medial-lateral balance pregnant women tend to rely on feedforward mechanism instead of feedback mechanism. Increase in postural sway in anteroposterior direction causes degradation in the perceived balance. However, reduction in perceived and actual stability is compensated by increase stance width which helps preserve the lateral balance in pregnant women.
Furthermore, Women's reliance on the visual cues increases during pregnancy. Whereas, there is degradation in forward reach distance in pregnant women compared to non-pregnant and the ankle joint strategy takes over the hip joint strategy.
Biomechanical Alterations in Gait[edit | edit source]
Locomotion is the most fundamental everyday action for the human body; however, in pregnancy, several biomechanical changes can be noticed in the gait. Alteration in the gait pattern is the result of adaptations made as opposed to the weight gain and the shift of centre of gravity. The gait of pregnant women undergoes following modifications:
Gait Speed[edit | edit source]
The gait of pregnant women is described by slower speed as compared to the pre-pregnancy and postpartum state. The large relative phase between pelvic and thoracic rotations is avoided by the pregnant ladies that is typical for high strolling speeds, possibly because of the increase in the moments of inertia of their pelvis and thorax which makes the command of relative phase more critical.
Gait Cycle[edit | edit source]
There is a significant decrease in the length of the gait cycle in pregnant women and an increase in double support time compared to post-partum and nulliparous women, exaggeration of the transition phases occur in order to increase the security of gait. 
|Spatial and Temporal Parameters of Gait|
|Stride Length||decreases as pregnancy progresses|
|Stride Width||remains unchanged|
|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|
|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|
|Pelvis||Increase in pelvic separation width and reduction in the amplitude of the unilateral elevation of the pelvis.|
|Hip||Decrease in abduction.|
|Joint Kinetic Parameters|
|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|
|Hip||Increase in hip abductors moment|
|Knee||Increase in knee adductors moment|
Sit-to-Stand during Pregnancy[edit | edit source]
With the progression in pregnancy rising to stand from a chair becomes more difficult, which may lead the body to adapt according to the biomechanical modifications and musculoskeletal demands.   Following temporal-spatial, kinematic and kinetic strategies can be used by the pregnant women 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)
Utilizing the biomechanical changes owing to increased uterus volume, the pregnant women will deliberately restrict the propulsive impulse in order to keep up with upright balance and upstanding position at the end of rising to stand. To guarantee safe movement, pregnant ladies ought not to start steps until arriving at a steady standing situation subsequent to rising.
The video below shows the safe way of transference from sitting to standing during pregnancy
Stand-to-Sit during Pregnancy[edit | edit source]
- The following changes occur in the kinematic performance of stand-to-sit during pregnancy:
- Hip and spine motion change in pregnancy while sitting from a standing position as hip range of motion and standing angle changed in favor of spine motion.
- Postural changes identified with gestational lordosis influence the transference from a standing position to sitting.
- Coordination changes away from the hip during the initial and terminal phases of sitting:
- Joint coordination shifts from hip dominant to spine- and shoulder-dominate coordination not long before the beginning of sitting movement.
- Hip-knee joint coordination not long before seat contact shifts from hip to a knee-predominant movement during pregnancy
Discomforts of Pregnancy[edit | edit source]
A wide variety of musculoskeletal problems can be reported during pregnancy as it induces a variety of biomechanical, hormonal, and vascular changes. The growing fetus alters the body’s centre of gravity applying mechanical stress on the body. Laxity in the joints develops secondary to hormonal changes. Compression of soft tissues in pregnancy can occur because of fluid retention. Consequently, making a pregnant woman prone to musculoskeletal injuries.
- Low back pain
- Pelvic girdle pain
- Hip pain
- Diastasis Recti Abdominis
- Leg problems
- Carpal tunnel syndrome
- Urinary incontinence
Implications for the Physiotherapist[edit | edit source]
During pregnancy, the biomechanical alteration of the posture should be examined by a physiotherapist early in the ante-natal program. Monitoring postural changes initially and revaluating as the pregnancy progresses is important in order to take suitable preventative and therapeutic measures, and to encourage the maintenance of 'good posture'. A simple inclinometer or clinometer can be used to monitor the postural changes, and the data gathered by the comprehensive recording could help the physiotherapist to explore the influence of postural changes on the development of pain during pregnancy.
Clinical Bottom Line[edit | edit source]
The research proposes that there is a huge difference in each individual's postural adaptations during pregnancy, with, perhaps, each woman solving the problems in her own way but the influence of relative weight gain on postural and biomechanical adaptations during pregnancy demonstrates the importance of antenatal exercise programs .
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