Stair ascent is a challenging daily task that requires fine muscle coordination to complete the task while avoiding falls. However, there is little information on training paradigms that could improve stair ascent. This study investigates how external forces applied on the pelvis in different directions and different magnitudes affect the muscle coordination during stairmill ascent. We used a Tethered Pelvic Assist Device (TPAD) to apply external forces on the pelvis during continuous ascent on a stairmill. Ten young healthy subjects participated in sessions where external forces were applied on the pelvis laterally, vertically, forward, and along the incline of the stairmill. Each session was over a duration of two minutes with force magnitudes starting from 0% body weight (BW). The applied force was held constant and increased by 5% BW every 30 seconds up to 15% BW. Muscle activation signals from fifteen muscles were measured and the structure of muscle synergy was calculated under these altered forces. The results show five muscle synergies during the stair ascent motion. We found that vertical and forward external forces on the pelvis affected the quadriceps group during knee extension. Additionally, the muscle activation timings of the thigh and back shank muscles were significantly different during different force conditions. These findings provide insights into muscle responses when external forces are applied on the pelvis in different directions and magnitudes. These results may help clinicians and physical therapists to design customized rehabilitation strategies to suit patient needs to improve performance during stair climbing.