## 1 Introduction  
Lumbopelvic pain (LPP), which includes low back pain (LBP) and pelvic girdle pain, are common discomorts experienced by women during pregnancy [1]. Symptoms can have a negative impact on the quality of life of the mother in both prenatal and postnatal stages [2, 3]. Body weight gain, especially in the abdominal area, and a shift in the position of the body's center of gravity due to fetal growth are distinct changes experienced during pregnancy and a common risk factor of LPP. In addition, an expanded gravid uterus can stretch and weaken the abdominal muscles [4]. These changes in the physical and musculoskeletal system affect the posture and movement of pregnant women and impose strain on muscles and joints, which contributes to pain at various locations such as the lower back [5, 6]. For example, the sit-to-stand (STS) motion is a cause of LBP during pregnancy [7]. Hence, physical methods are required to manage musculoskeletal disturbances during pregnancy, together with the coaching of proper movement to avoid physical loading [8].  
Clinical motion analysis typically employs visual observation as the dominant means of assessment; however, more quantitative and objective motion assessments are required to obtain meaningful results [9]. For example, the inertial measurement unit (IMU) allows objective motion measurement without limitations related to the measurement environment or disturbances in motion. Therefore, the IMU has been widely used to analyze motions including gait in a straight path and STS [10; 11]. Our previous research investigated the differences in torso motion between pregnant women with and without LPP during STS using an IMU [12]. In the study, an IMU was attached to the lower torso, and angular velocity data in the sagittal plane were used to assess the flexion and extension movements of the trunk. Some key indices were then proposed, that is, maximum peak, minimum peak, and peak-to-peak (PP). The maximum peak represents forward motion of the torso, and the minimum peak represents backward motion, with larger values indicating faster motion. PP is the difference between the maximum and minimum peaks and therefore represents a shift in motion from forward to backward. The results of the study revealed that the maximum peak was greater in the LPP group than in the non-LPP group. However, the role of other motion characteristics as risk factors of pain, such as additional stress on lumbar muscles due to muscle load, remains unclear.  
Physical movement therapy based on assessments of motion analysis and muscle load, such as the exerted muscle force, increases the effectiveness of physical therapy for relieving pain. Evaluation of the erector spinae (ES) muscle force is particularly useful for managing LBP because muscle load and fatigue are two of the main risk factors associated with LBP during pregnancy [1]. Although the musculoskeletal model can be used to estimate muscle force, it is difficult to estimate the co-contraction activation of agonist and antagonist muscles, such as the ES and rectus abdominis (RA) [13; 14]. To address these issues, we previously proposed a method for estimating muscle torque from the musculoskeletal model using a genetic algorithm (GA) for pregnant women [15].  
STS motion assessment should consider the ES muscle load in order to manage LBP in pregnant women. However, the link between muscle load during STS motion and LBP has not yet been revealed for pregnant women. Therefore, the aim of this study was to propose a method for assessing motion and muscle load during STS motion using an IMU. A secondary aim was to investigate the relationship among STS motion, muscle load, and LBP during pregnancy using the proposed assessment method.