EE was haul truck mentor. Operator backed in on left side to get loaded. First dipper was loaded onto HT with no issue. The shovel swung onto HT on the second pass the dutchman made contact w/ bed tail. Mentor felt some stiffness. On 1/3 the mentor stated that their back was hurting still from the contact. Mentor was issued PT & attended first session on the 13th of January.