Seat belt pretensioners (SBPT) cause deflection of the rib cage anteroposteriorly even in a side crash. Since the maximum value of anterior-posterior (A/P) deflection is generally smaller than that of lateral deflection in a side crash, probability of thoracic injury has been assessed by lateral deflection. However, the contribution of A/P deflection may be relatively large in low-speed crashes where lateral deflection tends to be smaller. The objective of this study is to investigate the relative contribution of A/P deflection due to SBPT activation to the predicted number of fractured ribs (NFR) with human body model (HBM) simulations. The HBM simulations showed that predicted NFR increased from 1 to 4 by the activation of SBPT at a delta-V of 16 km/h, which would raise the fatality rate of the elderly population. The injury metrics named Bent Rib Index (BRI) and Thoracic Area Index (TAI) using the combination of A/P and lateral deflection showed a better correlation with predicted NFR than that using lateral deflection alone (LCmax). The coefficient of determination were 0.87, 0.88 and 0.76 for BRI, TAI and LCmax, respectively.