Journal of Pregnancy and Child Health
Open Access

Pregnancy; Center of Pressure (COP); Kinematic; Kinetic; Balance; Stability; Motion Patterns; Gait Modes

Introduction

Pregnancy is one of the most common and critical events in the life of women. The entire pregnancy period can categorized into three trimesters (1-12 weeks, 13-26 weeks, and 27 to the end) [1]. During pregnancy, women experience substantial changes in their bodies, such as physiology, morphology, and hormonal systems. Body laxity increases because of the relaxin hormone and reaches its maximum level in the second trimester [2]. The physiological structure of the chest as well as trunk geometry and muscle function change during pregnancy to adapt to fetal growth. These changes affect the range of motion of various body segments and sports performance [3], possibly resulting in extended discomfort and pain. Studies have shown that 90% of pregnant women suffer from back pain, 50% complain of waist pain, 20% complain of pelvic and genital pain, and 20% complain of lumbar radiculopathy [4]. The rectus abdominis muscle plays an essential role in trunk movement, pelvic stability, and restraint of the contents of the abdominal cavity. The separation of the rectus abdominis occurs during pregnancy and the first week after delivery. The long-term separation of the rectus abdominis adversely affects the body shape and position of the internal organs.5 Physical and physiological problems can result in emotional and sleep problems in pregnant women.

Because pregnancy is a contraindication for many treatments and drugs, the management of these problems is still challenging, and current treatment methods are not satisfactory [4, 6-8]. Physical exercise, body orthoses, and medicine are commonly recommended for the prevention and treatment of pregnancy problems and body function recovery during the postpartum period. However, debates exist among studies on the effectiveness and safety of these treatments. In a review study, interventions to prevent and treat pelvic and back pain, such as increasing the amount of exercise and using specially designed pillows, were found to significantly reduce waist pain [4]. However, some studies found that the outcomes of these treatments were unsatisfactory [9]. The postpartum rehabilitation protocols have not been fully explored. Evaluation of the effects of interventions is critical for determining a fitness protocol. Biomechanical research on pregnancies have been conducted through various approaches. However, there are few studies that provide comprehensive knowledge, cutting-edge developments, and research and practical gaps in this area. Therefore, this study aims to comprehensively review biomechanical studies on non-pregnancy, pregnancy, and postpartum periods, preparing for future research on the pregnant cohort.

Method

Data Sources and Searches

Articles were searched in the electronic databases of PubMed, Scopus, and Cochrane from inception until June 2, 2021. Pregnant/ pregnancy and biomechanics were used as searching keywords, and we obtained 117 literature from Pubmed, 2891 from Scopus, and 11 from Cochrane. Duplicate results were removed, and 2918 studies were finally collected.

Selection Criteria for the Analysis

The included pieces of literature were required to meet the following criteria: 1) the research object of the literature is healthy pregnant women; 2) the research direction is in the range of biomechanics, which can be related to the trunk, lumbar spine, hip joint, knee joint, ankle joint, wrist joint, foot, muscle strength, muscle endurance, joint strength, plantar pressure, and motion analysis; 3) with full text; and 4) written in English or Chinese. Two authors independently performed the selection using predetermined criteria, and a third and fourth researchers resolved disagreements through discussion or arbitration until consensus was reached.

By browsing article titles and abstracts, 821 works of literature with full text were selected for further confirmation. After reading the full text, 100 studies were confirmed. Figure 1 shows the selection procedure. The classifications of the selected 100 studies are listed in Table 1.

Figure 1: Flowchart of the literature search process using PRISMA 2020 flow diagram for new systematic reviews which included searches of databases and registers only.

Table 1: The classification of selected studies.

In these studies, the postpartum cohort was used as the control group. Considering that the body anatomy and functions might not have completely recovered to the pre-pregnancy level several months after delivery, it might not be reasonable to set the postpartum period as the control group. An additional selection criterion was added for this review: 5) cohort studies comparing pregnant women with the same group of women in the pre-pregnancy period or with nonpregnant women. Fourteen articles were included in the final analysis (Table 2).

Table 2: Chronological order of publication.

Results

General Information Involved in Systematic Review Studies

Table 2 shows the general information of the 14 reviewed studies, including the authors, years of publication, country where the studies were conducted, and titles and publishing journals.

The 14 articles were published between 2002 and 2019. Most of the articles were published in 2015. In 2010, two articles were written by the same author, based on one study from different perspectives. Five studies were conducted in the USA, two in Australia, two in Portugal, two in Japan, one in China, one in Poland, and one in Germany.

Pregnant and non-pregnant women were included in the cohort studies. Some studies explored different pregnant trimesters in one pregnant group, and used a group of non-pregnant women as controls. Other studies investigated the same group of subjects from the non-pregnant to the pregnancy period. Some studies included the postpartum period, whereas others did not.

Age

The age range of the subjects (including control groups) was 1916- 4010 years, as shown in Table 3. The investigated periods and target populations were also provided.

Table 3: Publications: Characteristics of the papers on the locomotion of women throughout pregnancy.

Investigated periods

As shown in Table 3, these studies investigated different trimesters of pregnancy. Two studies [15, 23] surveyed women from the prepregnancy to pregnancy periods, and one study [15] involved the postpartum period. During pregnancy, the two studies selected different stages. The former15 studied the end stage of pregnancy, while the latter [23] studied the first trimester. Other studies included non-pregnant women as the control group. Three studies [10, 12, 17] performed repeated measurements in the control group; the other studies were only conducted once. In total, six articles [10-12, 15, 17, 18] covered the postpartum period.

Sample size

The largest sample size was 81, of which 41 were pregnant and 40 were non-pregnant [13, 14]. The second largest was 72, of which 36 were pregnant and 36 were nonpregnant [19]. The smallest sample size was 15, of which 8 were pregnant and 7 were non-pregnant [22]. The second-lowest was 16, of which 8 were pregnant and 8 were nonpregnant [20]. The rest of the research sample sizes were between 21 and 36 (4 of 20+, 5 of 30+)

Objectives

The objectives of this study are presented in Table 4. The main concept of the research was to determine the biomechanical changes in pre-, in-, and post-pregnancy periods. All studies focused on kinematic or kinetic changes in pregnancy compared with non-pregnancy.

Table 4: Objectives of the studies.

Among them, six studies focused on balance and stability [11-14, 19, 20]: one study explored the changes in balance during pregnancy [11]; one investigated the stance width of pregnant women during pregnancy and postpartum periods [12]; one estimated the dynamic postural stability in mid and late pregnant group and non-pregnant control group [13]; one study compared dynamic postural stability among pregnant fallers, pregnant non-fallers, and non-pregnant women [14] and also explored the static balance and fall rate changes during pregnancy [11] as well as the monthly rate of falls [12] and whether regular exercise during pregnancy was related to the incidence of falls [14] one investigated the characteristics of the Center Of Pressure (COP) progression during pregnancy and regionalized COP progression characteristics of pregnant women at different gestational stages [19] one study analyzed balance strategies during pregnancy from a kinematic perspective [20]. Five studies were conducted on the biomechanics of the trunk [10, 17, 18, 22, 23]. Estimations of these studies include: pregnant related kinematic changes of the trunk segment and changes in mediolateral width in the supporting basement by testing five tasks (seated and standing forward flexion, left-right flexion, seated-sitting axial rotation) [10]; changes in the thoracic spine during pregnancy and postpartum, and the movement between the thoracic and lumbar spines in comparison with nonpregnant women [17]; inspection of the causes of low back pain during pregnancy, and measuring the spinal posture and pelvic position during pregnancy [18]; and Japanese pregnant Body Segment Interstitial Parameters (BSPs) and variations in BSPs over time. The lower trunk segment moment, COM location, and COM velocity of pregnant women when performing motor tasks were studied based on the estimated BSPs. The motor tasks included standing up from a chair, picking up a square tray, turning to the right, walking a few steps, turning toward the destination, performing targeted movements when standing up from a chair or walking [22] and the biomechanical changes of the pelvis in the first trimester [23].

Five studies conducted gait analysis [15-17, 21, 23] investigation of gait parameters in one group of three different states (before pregnancy, during pregnancy, and after delivery [15]); quantification and comparison of the lower limb kinematics and spatial/temporal parameters of gait between pregnancy (in the second and third trimesters) and non-pregnancy [16]; study on temporal-spatial characteristics of step width, stride length, and speed as pregnancy progresses, in early postnatal period and compared with non-pregnant [17]; quantification and comparison of lower limb kinetics of gait between women in mid and late pregnancy and non-pregnancy groups [21] and analysis of kinematic changes in the locomotor system associated with fetal development in the first trimester of pregnancy [23].

Parameters

Fourteen articles focused on different biomechanical parameters. Their results are described in Table 5. Six articles studied balance and stability using the parameter COP [11-14, 19, 20]: the path length and Average Radial Displacement (ARD) of the COP when eyes are opened and closed [11]; traditional parameters about the COP, including standard deviation of the displacement about the mean and mean sway velocity in the Anterior Posterior (AP), Medial Lateral (ML), and Combined Radial (RAD) directions; 95% power frequency in the AP and RAD directions, and Angular Deviation (Ang Dev) of the principal sway direction from the AP axis [12]. Reaction time and the movement of the COP (reaction time, initial sway, total sway, and sway velocity) [13, 14]. COP parameters, including maximum Velocity (Vmax), Average Velocity (Vave), Duration (DCOP), ML and AP displacements in each region [19]; the anterior COP displacement at the maximum Functional Reach Test (FRT) distance, and FRT is a static balance test [20].

Table 5: Parameters.

Five studies investigated the biomechanics of the trunk [10, 17, 18, 22, 23]: angular motion of the thoracic and pelvic segments, and the relative rotation between the two segments during forward flexion activities (seated and standing), side-to-side flexion (seated and standing), and axial rotation (seated) [10]; the trunk kinematic cluster, which included the range of motion of the thoracic and pelvic segments, and the thoracolumbar spine in the sagittal, coronal, and transverse planes [17]; anthropometric data of the whole body, including the trunk segment, such as age (years), height (cm), initial weight (kg), weight gain (kg), abdominal circumference (cm), and completed back pain assessment and disability questionnaires [18]; Body Segment Inertial Parameters (BSPs); motion analysis [22]; and position of the pelvis and mean width of the Base of Support (BOS) in the double support phase [23].

Five studies conducted gait analysis [15-17, 21, 23]: gait characteristics, including Velocity (v), Gait Frequency (f), Length of Steps (l), Time of Single (SS) and Double Supports (DS), width of the BOS in double support phase, and ranges of motion of the lower limb joints, including the ankle, knee, and hip [15]; kinematic and kinetic parameters, including angular displacement and range of motion of the ankle, knee, and hip joints, walking speed, double limb support time, time of support and flight phases of both lower limbs, stride width, stride length, right and left step length cycle time16; kinetic parameters, including the three components of the Ground Reaction Force (GRF) normalized to units of body weight, net joint moments, and powers of the ankle, knee, and hip joints [21]; anthropometric measurements, gait registration, and assessment of the feet load pattern23; velocity, stride length, and step width [17].

Measurement Tools

Table 6 summarizes the measurement tools used in the 14 studies. The most frequently adopted tool is force plates [11-14, 16, 17, 21-V23] followed by motion analysis systems, including an expert vision motion analysis system [10, 17] Vicon [15, 20, 22] Visual 3D Software (C-Motion Inc., Germantown, USA), [16, 23] and Qualisys Track Manager [21]. In one of the study, researchers used a radiation-free spine and surface topography system (Formetric, Diers International GmbH, Germany) to measure spinal posture and pelvic position [18]. In another study, spatiotemporal parameters were measured based on electrodes attached to the novel Pedar insole plantar pressure measurement system (Novel GmbH, Munich, Germany) [19]. Two studies used questionnaires: self-evaluation questionnaire-perceived sense of balance, track fall incidences (five categories) [12] the German version of the Roland-Morris disability questionnaire, and the Oswestry Low Back Disability Questionnaire (ODQ). The level of back pain was measured using a Visual Analog Scale (VAS) [18]. Some studies used clinical tests, such as the Motor Control Test (MCT) [13, 14] functional reach test [20] and clinical balance scale [23]. Other tools have also been used to assist investigations, such as a height-adjustable chair [10]. Butterworth digital low-pass filter [16] Frankfurt plane, Stadiometer, Harpenden, and Skinfold caliper (Table 6) [23].

Table 6: Measurement tools.

Outcomes

Because the investigated periods in these studies were inconsistent, the study results related to the review purpose from those 14 studies could not be classified according to time. Instead, they were analyzed according to the research goals. The results of these 14 studies are summarized in Table 7.

Table 7: Results.

Balance and stability changes during pre-, in-, and post-pregnancy

Pregnancy affects balance and stability. Postural balance changes were indicated by stance width, COP, perceived sense of balance, and sway [11-14]. Postural stability was maintained in the first trimester, reduced as the pregnancy progressed, and did not recover at 6 to 8 weeks postpartum [11]. In the first trimester, the path length of the COP of pregnancies in eyes closed/opened and the ARD of the COP of pregnancies in eyes opened were not significantly different from those in the non-pregnant group; however, there was a significant difference in the ARD of the COP when eyes were closed, which indicated that in the first trimester, the pregnant balance was not significantly different from that in the non-pregnant [11]. The same parameter values in the second trimester, third trimester, and 6 to 8 weeks postpartum were significantly higher than those in non-pregnant women, indicating a balance decrease in those three periods [11] This indicates that pregnant women have an increasing need for visual cues to maintain balance [11].

A study12 was conducted on a cohort of 16 weeks gestation to 6-month postpartum, compared with an age-matched non-pregnant control group and found that the Sense of Balance (SB), which was the self-assessment balance ability. It was found that the higher the score was, the more unstable the subject became; specifically, it increased from 16th week of pregnancy to 6 weeks after delivery, and then decreased. Six months after delivery, the value of the SB decreased to the lowest; however, it was still higher than that in the non-pregnant group.12 During pregnancy, postural sway in the AP and radial directions increases, while after delivery it decreases.12 ML sway remained unchanged during pregnancy, but increased after delivery [12]. The preferred Stance Width (SW) was found to increase during pregnancy (from weeks 16 to 40) and drop to control levels after delivery, indicating decreased balance during pregnancy and increased balance in the postpartum period [12].

The response time to perturbation did not change during pregnancy [13] and also did not significantly differ between pregnant fallers, pregnant non-fallers, and non-pregnant women [14] however, the movement of the COP, in terms of initial sway, sway velocity, and total sway, changed remarkably [13-14]. From the non-pregnancy period to the second trimester, the initial sway remained stable without perturbations, but was reduced in the third trimester [13] and pregnant fallers had significantly less initial sway than pregnant non-fallers and non-pregnant women [14]. The amplitude of the initial sway increased with the level of perturbations, and backward perturbations resulted in larger amplitudes than forwarding perturbations [13-14]. Without perturbation, sway velocity was the lowest in the third trimester. It increased with the perturbation level, and forward perturbations resulted in a more remarkable sway velocity than backward perturbations. The third trimester showed less total sway than the second trimester in the control group. Forward perturbations elicited more total sway than backward perturbations. Large perturbations produced more total sway than medium and small perturbations [13, 14]. Pregnant fallers had significantly less sway velocity and total sway than other two groups +. There was no difference in these aspects between the pregnant non-fallers and non-pregnant groups [14].

Balance strategies changed during pregnancy. Pregnant women in their second and third trimesters were found to rely more on the ankle joint strategy than the non-pregnant, and with the progression of pregnancy, the ankle joint contributed more to balance maintenance; however, analysis of the first trimester and postpartum periods were not included [20].

The characteristics of the COP also changed depending on the change in pregnancy to maintain balance [19] pregnant women had a visibly lateral shift of the COP displacement, slower COP Vave in the hindfoot and midfoot, faster COP Vave in the forefoot, Vmax over all regions decreased as the pregnancy progressed, COP moved forward with slower velocity over the rearfoot and midfoot, and faster over the forefoot as the pregnancy progressed.

Changes of the trunk motion pre-, in-, and post-pregnancy

The motion of the trunk was influenced by pregnancy. No difference was found in the displacement of the thoracic or pelvic segment or the range of motion of the thoracolumbar spine during early gestation [10]. The pelvic width and range of motion of the pelvis in the sagittal and coronal planes did not change significantly from prepregnancy to the 12th–16th pregnancy week; only the lateral plane pelvic rotation was significantly reduced [23]. As pregnancy progressed, the influence on the motion of thoracic or pelvic segment or the range of motion for the thoracolumbar spine and support width were varied during seated and standing trunk forward flexion, trunk side-to-side flexion, and trunk axial rotation [10]. Thoracic segment displacement decreased during trunk forward flexion and trunk axial rotation; pelvic segment displacement decreased significantly at the late gestation in seat forward flexion, but not during trunk axial rotation, and had no difference in standing forward flexion. Thoracolumbar spine range of motion decreased as pregnancy progressed during trunk axial rotation. However, it did not significantly differ during seated forward flexion and was less during standing forward flexion. The base of support was not significantly affected as pregnancy progressed during trunk axial rotation, but was significantly greater than non-pregnant when standing during side-to-side flexion and forward flexion. There was no significant difference for motion of thoracic or pelvic segment or the range of motion for the thoracolumbar spine doing seated and standing side-to-side flexion. Another study [17] showed that, as pregnancy progresses, the range of motion of the pelvic segment and thoracolumbar spine declined significantly in the rotation around the vertical axis. Further, the range of motion of the pelvic part around the antero posterior axis (lateral tilt) also showed a significant decrease.

In addition to trunk motion, the shape of the trunk was found to be influenced. One reviewed article 18 indicated that thoracic lordosis increased during pregnancy, whereas no increase was found in lumbar lordosis. The lateral deviation decreased during the second trimester, third trimester, and postpartum period [18]. No significant change was found in the position of the pelvis during and after pregnancy; however, the pelvic tilt increased as pregnancy progressed, and once after delivery (postpartum-12 weeks after delivery) [18].

After delivery, the thoracic or pelvic segment or the range of motion for the thoracolumbar spine and support width showed no significant difference between seated and standing trunk forward flexion, trunk side-to-side flexion, and trunk axial rotation as pregnant [10]. After delivery, the range of motion of the pelvis was smaller, while the thoracic region had a larger range of motion than in late pregnancy [17]. For postpartum to 12 weeks after delivery, lateral deviation and pelvic tilt did not recover [18].

Pregnancy influenced the body mass, segment COM location, trunk segment moment of inertia, and lower trunk length of the women [22]. There was no remarkable difference on absolute and relative lower trunk masses with pregnancy between the 16th and 18th weeks compared to non-pregnant women. As pregnancy progressed, 24th–25th and 32nd-33rd pregnancy weeks, the absolute and relative lower trunk masses of the maternal group were significantly greater than those of the non-pregnant control group. The body mass, height of the uterine fundus, and abdominal girth increased significantly from 16th–18th to 32nd-33rd pregnancy weeks. The positions of segments COM were more anterior in the pregnant group than in the nonpregnant group, while they showed no significant difference in superior–inferior and the ML directions. Heavier segment mass and smaller radius of gyration in 16th-18th and 32nd-33rd pregnancy weeks owing to the longer segment length in pregnancies, than non-pregnant group were found. The lower trunk segment moment of inertia in the anterio-posterior direction in the 32nd-33rd pregnancy weeks was larger than that of the non-pregnant women. The length of the lower trunk segment slightly reduced as the pregnancy progressed, which may be caused by the increased spinal curvature.

Gait changes during pre-, in-, and post-pregnancy

Pregnant influenced some aspects of the women’s gait. No significant changes were found in the average ranges of motion of the ankle, knee, and hip joints in the sagittal plane before, during, and after pregnancy [15]. However, another study indicated that, compared with the non-pregnant group, the extension and adduction of the right hip during the stance phase between the second and third trimesters decreased [16]. The maximum extension and abduction of the right thigh, maximum flexion of the left knee, and maximum plantar flexion of the right ankle of pregnant women between the second and third trimesters showed remarkable differences [16]. Most pregnant subjects experience an increase in left knee flexion and a decrease in right ankle plantar flexion [16].

The results of the reviewed studies were not the same in terms of spatial and temporal parameters.

In the first trimester, the basic kinematic gait parameters did not change significantly [23]. There was a significant decrease in the length of the right and left steps; therefore, a significant reduction in the stride length was found in the non-pregnant group, as well as in the second trimester and third trimester of pregnancy [16] One reviewed study17 indicated that this decreased trend was linear.

No significant changes in walking speed, stride width, right and left step time, cycle time, time of support, and flight phases were found during the non-pregnant stage and second and third trimesters [16]. However, one study indicated that the strip width has an increasing linear trend as pregnancy progressed [17] and another reviewed study [15] showed that v significantly decreases from pre-pregnancy to pregnancy and increases significantly after pregnancy. The f had the same change pattern, and the value of v and f returned to the prepregnancy state at 6 months postpartum. No significant difference was found in the l between the pre- and post-pregnancy periods, although it was significantly lower during pregnancy [15]. The duration of DS increased as pregnancy progressed, and was significantly longer than that before and after pregnancy [15]. The BOS in the pregnancy period was higher than that before and after pregnancy, and returned to the same level as that before pregnancy [15]. After birth, the stride was readapted [17]

The change patterns of the Ground Reaction Force (GRF) in the second and third trimesters were almost the same, but the values were different [21]. In the left stance, the last peak of the vertical GRF showed a significant decrease, which was approximately 5% of BW [21]. Compared with that in the pre-pregnancy period, the ML GRFs showed remarkable differences in the second and third trimesters [21].

Pregnancy impacts the joint moments, muscle participation patterns, and joint power [21]. Most muscle participation patterns changed at the end of the stance phase, and asymmetry may occur between two lower limbs. The second peak of the right hip joint moment in the sagittal plane significantly decreased in those pregnant in second and third trimesters compared with those in the nonpregnant group, indicating a reduction participation of hip flexors. This peak value showed significant difference between the left and right hips in the third trimester, implicating an asymmetric muscle participation pattern. Increased involvement of the external rotator muscle in the second trimester of pregnancy was observed at the first peak of the left hip moment. The participation of the knee extensor muscles decreased, and the knee flexor muscle increased during the second trimester of pregnancy compared to that during the pre-pregnancy period. Ankle dorsiflexor participation decreased during the second and third trimesters compared to that during the pre-pregnancy period. A significant decrease in the involvement of the lateral malleolus muscle, associated with the first peak of left ankle moment in the frontal plane, was observed during the second trimester of pregnancy. In the final stages of the stance phase, joint production and mechanical energy absorption in the lower limbs decreased during pregnancy, compared to that of the non-pregnant group. Meanwhile, no significant difference of the joint power of the hip, knee, and ankle in the three anatomic planes of pregnant women in the second trimester compared with those in nonpregnant and in the third trimester.

Quality Assessment

A quality assessment was conducted using the NHLBI Quality Assessment Tool for Observational Cohort and Cross-sectional Studies.26,27 For 0-4 “yes” from 14 questions in the assessment tool, a study will be graded as “poor Quality,” while 5-10 “yes” means “fair Quality” and 11-14 “yes” means “Good Quality.”

Table 8 shows the quality of the 14 studies. All studies were fair (5- 10), and the mean score was 7.071/14. The table also lists assessments of the 14 reviewed studies using the quality assessment tool

Table 8: Assessments of the 14 reviewed Studies using the quality assessment tool.

All the authors have fully described their study goals. Nine authors did not clearly specify and define their study participants, such as describing their demographics and location of the study participants [10-12, 17-22]. Only four studies [13-15, 23] can fully represent the target population because they had at least 50% eligible persons attended the research, while the sample size of the rest of the research may have caused the research to be biased. Only two [15, 23] studies have formulated inclusion and exclusion criteria, and used the same essential criteria for all subjects involved. No study explained the reason for the sample size, and did not discuss statistical power. This was not a “fatal flaw,” although none of these studies mentioned any information about efficacy or sample size, as observational cohort studies can be exploratory studies that usually do not provide information about strength or sample size [26]. This indicates that the authors did not consider whether the sample size was sufficient to answer the prespecified question. Only five studies [10, 12, 15, 17, 23] can confirm that pregnancy state existed before the possible body’s biomechanical changes, and they provided evidence for the causal relationship between pregnancy and possible biomechanical changes in the body, because the control group performed several tests as experimental groups [10, 12, 15, 17] or studied the pre-pregnancy statement of the same pregnant group [23] while control groups from other studies were only tested once.

All studies had sufficient time to observe the impact of pregnancy on human biomechanics, because pregnancy time was regarded as the exposure level, time was the only measurement, and Q9 and Q10 were not applicable. None of the studies mentioned the use of blinding.

Discussion and Conclusion

During pregnancy, women experience substantial changes in physiology, morphology, and hormonal systems. These changes have profound effects on the biomechanics of the human body, particularly the musculoskeletal system, resulting in discomfort, pain, and decreased body stability. Sufficient biomechanical knowledge is critical for understanding the etiology and precautions of musculoskeletal disorders. With awareness of health problems in the pregnant cohort, identification, intervention, and precaution of problems have garnered attention. Studies have been conducted to determine the biomechanics of pregnancy. In addition, there have been review studies on summarization.

Because the pregnancy periods in these studies are inconsistent, further studies are required to determine which period is most biomechanically representative, and sample size should be determined according to certain statistical methods, which are not indicated in these studies. None of these 14 studies had such a large sample size to represent the universal pregnant population. The diversity of age, work, height, weight, and living habits makes it difficult to confirm a representative sample size. Although these studies provided valuable information, extended parameters that are necessary for a full understanding of pregnancy biomechanics can be further explored. For example, plantar pressure in the second and third trimesters and postpartum period [23] remains unclear.

Existing measurement methods are not comprehensive, such as measuring waist circumference, which can only measure changes in the body circumference. However, they cannot measure body composition, muscle, fat, or inorganic modifications, and the ratio of these components affects the body’s activities [25]. Selecting a better, and more specific evaluation method can improve the quality of research experiments. For example, adding body composition analysis, which can provide the weight distribution and composition of body segments. In addition, it provides clues to determine whether the increase in weight is owing to the fetus, fat, or muscle; the increased part is the limb or the abdomen [24]. Meanwhile, this measurement method cannot repeatedly measure exposure (pregnancy). Therefore, more comprehensive experiments involving pre-pregnant, in-pregnant, and post-pregnant individuals are required.

Contribution to Authorship

Y.W., L.W., Y.P., Q.T. and M.Z. conceived the study. Y.W., L.W., Y.P., Q.T. and M.Z. provided plan. Y.W., L.W., Y.P., Q.T. and M.Z. carried out and analyzed. L.W. drafted the manuscript; all authors contributed to revisions and approved the final version.

Details of Patient's Consent

Not applicable

Details of Ethics Approval

Not applicable

Funding

This research was funded by the National Natural Science Foundation of China (No. 11972315 and No. 11732015).

Acknowledgement

Not Applicable

Conflicts of Interest

The authors report no conflict of interest.

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