This study examined the relationship between riding posture and muscle activities during passive exercise on horseback-riding simulation equipment. The effects of passive training on the prototypes were also determined. Three prototypes with tilted seat (A110, A130, and A140) were developed with an attempt to change the angle between trunk and leg to 110, 130, and 140 degrees, respectively. Twelve female aged 42.7±2.3 years performed passive exercise on the three prototypes and isometric maximal voluntary contraction (MVC) tests. Electromyogram of nine muscles in the trunk and lower limb were recorded. On A110, back muscle showed the largest activity (22%MVC ; p<0.01 ; repeated measures ANOVA). Contrastingly on A140, abdominal muscle and knee extensor showed the largest activity (40%MVC and 26%MVC ; p<0.01). Passive training on the prototypes for 30 minutes/day, 4 times/week, 8 weeks produced enhancement of muscle strength in trunk and hip. Riding posture is an effective factor to control physical effects without increasing the velocity on horseback-riding simulation equipment.