Although robotics and automation machines are leading over modern industry, manual material handling tasks remain inevitable due to high demand in the workplace. Unsafe manual handling tasks affect many people in the workplace, even those with professional skills. To prevent an injury, a handler must know when to stop handling when the task is executed improperly;​ however, there is no definitive answer to know when to stop without an injury risk to the handler. Therefore, the primary objective of this study was to determine whether and how a correlation exists between psychophysical rate of intensity of a task and the change in patterns in kinematics of lumbar and knee joint angles as a function of repetitive material handling tasks as weight increased. To perform the experiment, kinematic of joint angle data were collected from the knees and lumbar region of participants as they lifted and lowered incremental weights repetitively or until fatigued while an intensity of activity response called Borg CR10 scale was recorded after each trial. A relationship was identified between the change in joint angles and an intensity scale as one fatigued. Additionally, significant differences were identified between male and female participants, lifting and lowering tasks, different box weights, transitions of lifting and lowering techniques as weights increased and a state of intense fatigue was noticeable. Based on the statistical model, male participants were able to lift a 30 kg box before an intense sign of fatigue, while female participants repetitively lifted and lowered a box weighing 25 kg before a sign of fatigue. With all of these variables discussed, future research should be able to further understand the intensity of tasks, fatigue and the musculoskeletal human body during manual material handling with a broader approach using biomechanical and psychophysical approaches. The current research is only a beginning of what can be used on a larger scale to help eliminate the issue of musculoskeletal disorder and reduce absence days from work due to injuries.