Biomechanical changes during pregnancy: definition, factors involved and clinical scope
Definition
Biomechanical changes during pregnancy are physiological adaptations of the musculoskeletal system and motor control to accommodate uterine growth and weight changes. They involve redistribution of loads, changes in joint alignment and mobility, and adjustments in postural coordination. Variations in spinal curvature are often observed (e.g. a tendency to greater lumbar lordosis) and in the pelvic position (possible pelvic anteversion), as well as an alteration of the projection of the centre of gravity, with marked individual variability.
Factors involved
The main determinants are interdependent and are expressed progressively:
- Hormonalincreased relaxin and progesterone is associated with increased ligamentous laxity and capsular, favouring extended ranges of motion.
- MechanicsThe following factors: increase in body mass and changes in its distribution, uterine growth and increase in mammary volume modify levers and joint moments in the spine, pelvis and hips.
- Neuromuscular and posturalAdjustments in the activation pattern of the trunk and pelvic musculature, changes in the tension of the thoracolumbar fascia, and variations in the base of support and the gait pattern.
- Thoracic-respiratoryThe following is the result: elevation of the diaphragm and ribcage with changes in costovertebral mechanics and thoracic mobility.
- Plantar supporttendency towards increased pronation and forefoot widening which may influence the upward kinetic chain.
Clinical Outreach
These adaptations are associated with common conditions such as lumbopelvic pain, discomfort in the pelvic girdle (including the pubic symphysis), overloading of paravertebral and scapular musculature, and a variable feeling of instability. Changes may also be observed in the balance and gait, with possible repercussions on functional tasks such as prolonged standing, turning and stair climbing. The clinical expression is heterogeneous: not all pregnant women present symptoms and their intensity fluctuates according to trimester, musculoskeletal history, activity level and comorbidities, so interpretation must be based on individual clinical criteria.
Biomechanical changes during pregnancy and hormones: how relaxin and progesterone can influence joint stability
Hormonal effects on periarticular tissues
During pregnancy, the relaxin and, to a lesser extent, the progesterone, hormones that are associated with changes in collagen and in the water content of ligaments and capsules. These changes may increase the ligamentous laxity and range of motion, especially in the pelvic girdle and, to varying degrees, in limbs. Laxity is not pathological in itself, but reduces the contribution of the passive stabilisers and requires greater involvement of muscle control to maintain the joint stability.
Biomechanical adaptations and their interaction with stability
Uterine growth shifts the centre of mass anteriorly, increases lumbar lordosis and promotes pelvic anteversion. These adaptations redistribute loads on the pubic symphysis and sacroiliac joints, and may be accompanied by changes in gait and plantar support. When coexisting with increased laxity, joint displacement may increase under habitual loads, requiring adjustments to neuromuscular control and proprioception to sustain efficient movement patterns.
Individual variability and clinical considerations
The magnitude of these changes varies according to factors such as previous hypermobility, number of pregnancies, activity level and weight gain. Pelvic discomfort or a feeling of joint “looseness” may occur without acute injury. Not all pregnant women experience instability; the presence of severe pain, blockage or marked functional failure is not typical and may suggest additional conditions requiring specific assessment.
Posture and the spine during pregnancy: shifting of the centre of gravity and adjustment of lumbar lordosis
Displacement of the centre of gravity
The increase in uterine and breast tissue volume displaces the centre of gravity slightly forward (and, to a lesser extent, upwards). To support the projection of the weight within the support base, As a result, many pregnant women adopt compensatory adjustments, such as widening the separation of the feet or modifying the gait pattern. This change increases the moment of force on the hip and lumbar spine, demands greater stabilising activity and can make postural oscillations more sensitive, with relevant inter-individual variability.
Lumbar lordosis adjustment
As a sagittal equilibrium response, the sagittal equilibrium response usually increases the lumbar lordosis through increased pelvic anteversion and extension of the lumbosacral region, with the aim of keeping the trunk aligned over the pelvis. This adjustment redistributes compressive and shear loads on the L4-L5 and L5-S1 segments and modifies the participation of the lumbar and gluteal extensor musculature in the face of a progressively stretched abdomen. Not all gestations show the same degree of change: some show discrete increases and others maintain a curvature close to the baseline.
Biomechanical modulating factors
The ligament laxity related to hormonal changes, mobility of the sacroiliac joints and symphysis pubis, body mass index, previous tone and neuromuscular control, type of footwear and fatigue influence the magnitude and pattern of these adaptations. Postural adjustments are dynamic across trimesters and depend on the interplay between foetal growth, muscle compensatory capacity and individual tolerance to loading, without automatically implying the onset of symptoms.
Gait and balance in pregnancy: gait variations, base of support and postural control
Footprint variations and base of support
During gestation, changes in foot mechanics and stance are often observed. Increased body mass and increased tissue laxity may favour some changes in the foot mechanics and stance. pronation and a partial flattening of the medial arch in some individuals, resulting in a wider footprint and adaptations of the stance pattern. In parallel, the medial arch is often widened, resulting in a wider footprint and adaptations of the stance pattern. support base with a wider foot space and slight external rotation, a common strategy to gain stability during gait without the need for increased compensatory efforts.
Gait parameters that tend to change
- Pitch lengthcan be reduced slightly by prioritising shorter and more controlled steps.
- Double support timetends to increase, seeking greater security in the transition from one foot to the other.
- Running speedoften decreases moderately, especially on uneven or fatigued surfaces.
- Footprint variabilityThe shoe may be larger, reflecting continuous adjustments to changes in the centre of mass and to the comfort of the shoe.
Postural control and balance
The centre of masses moves progressively forward and slightly upward, requiring adjustments to the postural control to maintain stability. Compensations such as increased lumbar lordosis and some pelvic anteversion are common, as well as increased reliance on hip strategies in addition to the ankle strategy when managing disturbances. Sensory integration (vision, vestibular and somatosensory systems) remains key; fatigue, changes in plantar sensation - due to oedema or variations in foot volume - and type of footwear can modulate moment-to-moment stability. These adaptations do not follow a single pattern and may vary from trimester to trimester and from person to person, with possible periods of increased vulnerability to stumbling, particularly in unpredictable environments.
Pelvic floor and abdominal wall: relationship of pregnancy biomechanics to support and continence
Biomechanical changes of pregnancy
Uterine growth and hormonal action increase tissue laxity and modify lumbopelvic alignment, with anteversion of the pelvis and adjustment of breathing. These changes alter the synergy between diaphragm, abdominal wall y pelvic floor, The load distribution and the intra-abdominal pressure. More costal breathing and diaphragmatic ascent may reduce the deep abdominal contribution to support tasks, which conditions the support response and continence to efforts such as coughing or lifting.
Abdominal wall-pelvic floor interaction and continence
Distension of the linea alba and possible diastasis recti modify the transmission of forces in the abdominopelvic “cylinder”. When the activation of the transversus and obliques is less efficient, the pelvic floor assumes greater demand to maintain the visceral support and the continence. The complex of the anal elevator and the endopelvic fascia bear increasing loads due to increased uterine mass and abdominal contents; if anticipatory coordination to pressure increases is delayed or insufficient, stress-induced urinary leakage or pelvic heaviness may occur, although this does not occur in all pregnant women.
Modulatory factors and clinical signals
The response is variable and depends on the interaction between mechanical demand, muscle capacity and connective tissue properties. They can modulate risk:
- Prior history of incontinence or weakness of the pelvic floor.
- High body mass index, chronic cough or constipation (increase pressure).
- Activities with impacts or repeated lifting of loads.
- Marked ligament laxity and ligament fatigue abdominal wall.
Clinical manifestations include leakage on exertion, urinary urgency, difficulty holding gas, and a feeling of pelvic heaviness or descent. The presence and intensity of these signs are not an absolute predictor of outcome, but are indicative of the load on the abdominopelvic structures during pregnancy.
