Low back injuries are common and highly prevalent with occupational lifting. Changes in load knowledge during lifting can result in biomechanical errors and spinal perturbations which can alter muscle excitation, kinematics, and load perception. The objective was to identify changes in trunk muscle excitation and kinematics following unexpected increases and decreases in load during lifting. A repeated lifting task with ten boxes lifted from the floor to a waist-level table was performed. Three loading conditions were used: a known load (KL) of 7.5kg, a high load (HL) of 15kg, and a low load (LL) of 2.5kg, with participants unaware of the changes in load. Bipolar sEMG was used to analyze muscle excitation in the RA, EO, TES, LES, and trunk kinematics were measured via accelerometers. Statistical analysis using repeated measures ANOVA was used to determine differences between loading conditions. Post-hoc testing of Tukey’s Pairwise Comparison was used to determine differences between pairs of loading conditions. Descriptive analysis looked at changes in load perception. Unexpected changes in load did result in changes in lifting biomechanics. Average EMG showed significant increases between conditions in the RA (HL vs KL: p=0.0012, LL vs KL: p=0.0076, HL vs LL: p=0.024), TES (KL vs HL: p=0.0001 and HL vs LL: p<0.0001), and LES (HL vs LL: p<0.0001). Peak EMG also showed significant increases in the TES (HL vs KL: p=0.015) and decreases in LES (LL vs HL: p=0.024). Delays in time to peak EMG were found in the EO (HL vs LL: p=0.024). Average acceleration showed significant increases in the trunk (LL vs HL: p=0.018 and LL vs KL: p=0.0028) and box (LL vs HL: p<0.0001). Alterations in perception were found for all participants (82% in exertion, 88.2% in load magnitude, and 29.4% in box size). Although changes in biomechanics were identified, under these lifting conditions, they are not likely to result in injury. Future research should examine changes in lifting biomechanics and load perception under more extreme changes in load. Future research should continue to include the temporal characteristics of muscle excitation and jerk when investigating lifting tasks with changes in load knowledge.