A pinnacle trainer is a stair climber that has a biplane exercise trajectory and an adjustable pedal stance width (PSW). A pinnacle trainer integrated with a body weight support (BWS) system can help overweight individuals or individuals with poor balance exercise safely by reducing excessive or improper joint loads, preventing training-related injuries. However, few studies have investigated the biomechanical features of the lower extremities during pinnacle trainer exercise with and without partial BWS for various PSWs.We aimed to investigate the effects of partial BWS and PSW on the joint loading of the lower extremities during stepping on a pinnacle trainer.Seventeen healthy adults exercised on the pinnacle trainer with or without BWS using various PSWs. The joint resultant forces and joint moments of the lower extremities were calculated according to the kinematic and kinetic data measured via a motion capture system and force transducers on the pedals, respectively.The joint resultant forces and joint moments of the lower extremities significantly decreased with increasing percentage of BWS. The internal knee adduction moment and internal hip abduction moment significantly increased with increasing PSW. For every kilogram of BWS, the joint loading of the lower extremities decreased by approximately 1% of the joint resultant forces of body weight during exercise with the pinnacle trainer.Exercise on the pinnacle trainer with partial BWS significantly reduced joint loading. Exercise with a wider pedal stance may be helpful for knee osteoarthritis rehabilitation as it produces greater internal hip abduction and internal knee adduction moments.