A delivery employee in E-Commerce industry carrying bulky logistics backpack on their shoulder to deliver the item to the right destination in scheduled duration. Their body muscles are subjected to constant strain when executing last-mile delivery. The objective of this study is to investigate Erector spinae muscles subjected to high loading while walking over a flat surface.The impacts of the varying weight of the logistics backpack were studied for activation by muscle. Walking speed at 1.11 m/s with no load, 10kg, 20 kg and 30kg was maintained by twelve healthy adult male participants. Response of Erector spinae (ES) muscles, longissimus (LO), and iliocostalis (IC) have been measured for sEMG. With increasing load conditions determined by one-way ANOVA. The mean surface electromyography (sEMG) for ES muscle activation was found not statistically significant while walking over the flat surface on a higher load. When comparing the muscles of both sides of the erector spinae, LO and IC, a statistically significant change was observed while walking with a weight of 10kg (p = 0.0001), 20kg (p = 0.0001), and 30kg (p = 0.00003).Among last-mile delivery employees, normalized sEMG activation in the ES muscles is found statically significantly within muscle groups while carrying a backpack load condition.