This study was performed to investigate the effect of the length of backpack shoulder straps on upper trapezius muscle pain threshold and craniovertebral angle. There were 25 participants, with ages from 15 to 23 years old. Upper trapezius pain threshold and craniovertebral angle were measured for all subjects without the backpack then re-measured after walking on a treadmill for 15 min under 2 conditions: 1) wearing a backpack with short straps; and 2) wearing a backpack with long straps. there was a significant reduction in upper trapezius pain threshold and craniovertebral angle while carrying a backpack with long shoulder straps, compared to use of a backpack with short shoulder straps or no backpack. A backpack with short straps is less harmful than a backpack with long straps. This result should be considered in ergonomic design of backpacks to reduce the incidence of various physiological and biomechanical disorders.