Human performance seems to be neglected in the design of assembly and disassembly of a product despite the increasing awareness among people on green environmental issues. Incorporating an ergonomics evaluation can be considered as an alternative empirical solution to tackle the issue efficiently. Therefore, the objective of this study is to investigate human performance with reference to muscle activity while performing assembly and disassembly of manual tasks focusing on load variability. Upper limb muscle activities were investigated in this study. Twenty participants were asked to perform assembly and disassembly of selected tasks at two load-levels. The results of this study indicated that load tasks affect human performance. The findings from this study highlighted that the muscle activity for the medium load task was prone to fatigue faster than the low load task (28.19% in average). Thus the study has revealed the importance of incorporating the ergonomics principle solution in the design of assembly and disassembly tasks to increase human performance.