The purpose of this study was to investigate the influence of arm position on thoracic response and injury severity in side impacts. A total of sixteen non-destructive side impact tests and four destructive side impact tests were preformed using four human male cadavers. Single-axis strain gages were placed on the lateral and posterior regions of ribs three through eight on the impacted side, and the lateral region of ribs three through eight on the non-impacted side. Thoracic rods attached to ribs five, seven, and nine were used to measure lateral rib deflection. For the non-destructive tests, four test conditions with different arm positions were evaluated for each cadaver by performing displacement-controlled, low-energy, lateral impacts, 16 kg at 3 m/s, with a pneumatic impactor. The results of these tests showed that the highest average peak forces, peak rib deflections, and peak rib strains were observed when only the ribs were impacted and lowest when the shoulder was impacted. In addition, higher average peak forces, peak rib deflections, and rib strains were observed when the arm was placed parallel with the thorax versus 45 degrees. For the destructive tests, two test conditions were evaluated by performing high-energy lateral impacts, 23.4 kg at 12 m/s, with a pneumatic impactor. Only one destructive test was performed per cadaver with the arm placed at either 45 degrees or parallel with the thorax. Using rib fractures as the parameter for AIS, both tests conducted with the arm at 45 degrees resulted in an AIS=4 due to the large number of ribs with multiple fractures which resulted in a flail chest. Conversely, both tests conducted with the arm parallel with the thorax resulted in an AIS=3. The analysis of thoracic strain gage time histories showed that, in general, the rib fracture timing varied with respect to thoracic region. Using scaled rib 5 deflection, lateral fractures on the impacted side occurred between 4.2 mm and 34.9 mm, posterior fractures on the impacted side occurred between 19.0 mm and 37.8 mm, and lateral fractures on the non-impacted side occurred between 60.2 mm and 74.3 mm of deflection. It was found that AIS=1 injuries occurred at scaled rib deflections of 4.2 mm to 8.6 mm (2% to 3% compression), AIS=2 at 9.6 to 17.4 mm (4% to 7% compression), and AIS=3 at 13.1 mm to 20.1 mm (5% to 9% compression) measured at rib five. In conclusion, the results of the current study show that in low-energy side impacts both the arm and shoulder reduce impactor force, rib deflection, and rib strain. In high-energy side impacts, the position of the arm has a considerable effect on both the total number and distribution of rib fractures.