We investigated the effect of the weighting between surface electromyography (sEMG) and interaction force for user intention extraction for the control of an upper-limb assistive device. A point-to-point vertical arm lifting task was performed by two age-segregated cohorts including 10 young subjects and 10 elderly subjects. The performance in terms of muscular effort and repeatability was analyzed. The elderly and young group showed age-dependent differences in terms of muscular effort with distinct sEMG weights.