Perpustakaan Digital ITB

Tote bag is among the most common everyday bags, yet it places the entire load on one shoulder. For backpacks, which spread the weight across both shoulders, a limit of 10% of body weight is widely accepted, but no comparable limit has been established for single-shoulder loading. Previous work has measured shoulder movement and muscle activity separately rather than in the same participants, so how the two respond to the same load is unknown. This study therefore measured both responses together across four load levels. Twenty healthy young adult females (18-25 years, all right-hand dominant) walked on a treadmill carrying a tote bag on the right shoulder at 0, 5, 10, and 15% of body weight. Upper trapezius activation was recorded on both sides using surface electromyography and normalized to each participant's maximum voluntary contraction (MVC), while bilateral shoulder movement was obtained from optical motion capture and inverse kinematics. Each outcome was evaluated using two-way repeated measures analysis of variance with side and load as within-subject factors. On the loaded side, activation rose from 11.14 to 58.23 %MVC and exceeded the 15 %MVC fatigue threshold at every loaded condition, with the between-side difference widening fourteen-fold. The movement asymmetry behaved differently: it favored the unloaded shoulder, was present before any load was added, and did not scale with load in any of the nine kinematic outcomes. Muscular demand therefore rises with load while movement kinematics do not, and the backpack limit cannot be applied to tote bags, since at 10% body weight the loaded muscle was already working at 40.03 %MVC.