Effects of body characteristics on lap belt fit and comfort

Abstract

Proper seat belt fit is crucial for occupant safety and comfort, as incorrect positioning may reduce injury protection and increase discomfort, potentially leading to misuse. This study investigates how individual body characteristics influence lap belt fit and perceived comfort, with particular focus on anthropometry and seating posture, determined by the seatback recline angle. A controlled laboratory study was conducted using a test rig representing a vehicle seating environment. Thirty adult participants, balanced by sex and distributed across Body Mass Index (BMI) categories, were evaluated in two seatback configurations (upright and semi-reclined). Three lap belt anchorage conditions were assessed: a manufacturer-defined position (P1), a forward-shifted position (P2), and a user-selected position. Objective measurements (anthropometry, belt geometry relative to the anterior superior iliac spine, and buckle position) and subjective assessments (perceived comfort, perceived safety, and usability) was used. Statistical analyses were used to identify relationships between body characteristics and belt fit. The results show that seatback recline angle has the strongest influence on lap belt fit, followed by BMI and anthropometric measures. Higher BMI is consistently associated with a more superior lap belt position relative to the pelvis, reflected as an upward displacement of both belt segments. However, the belt position relative to the pelvis in the anterior–posterior direction is strongly affected by seating posture, with reduced anthropometric predictability in the semi-reclined condition. P2 generally reduced perceived abdominal pressure and improved usability, particularly in terms of buckle accessibility and ease of buckling, but introduced tradeoffs such as increased perceived thigh loading and shoulder belt interference. Despite these improvements in local comfort, P2 was perceived as less safe than P1, however, perceived safety does not necessarily reflect actual restraint performance. User-selected positions minimized discomfort in most cases but were typically located outside regulatory acceptance regions, indicating a mismatch between user preference and standards. The findings highlight the combined influence of anthropometry, seatback recline angle, and restraint geometry on lap belt fit, showing that no single factor explains occupant variability. This study contributes empirical evidence supporting the need to consider occupant diversity and seatback recline angle in future restraint system design, particularly in the development of more adaptive anchorage positions.