The transversus abdominis forms the deepest layer of abdominal muscles. It covers the trunk and functions as a trunk stabilizer. The objective of this study was to examine the effect of training the transversus abdominis in healthy subjects and patients who underwent hip or knee surgery. There were 22 healthy young subjects and 11 ambulatory patients who underwent hip or knee surgery included in the study. The center of gravity postural sway trajectory length in a standing position, the length and area of the sway of the center of gravity, and quadriceps femoris strength were evaluated as measures of physical function. Healthy subjects and patients underwent trunk stabilization training and regular training（ balance training for healthy subjects and 20 minutes of physical therapy for patients） as a control. After trunk stabilization training, quadriceps femoris strength increased in both groups and the center of gravity postural sway trajectory length decreased among patients. When the order of training was compared, quadriceps femoris strength increased when trunk stabilization training was given before regular training. This is probably because trunk stabilization training increased abdominal pressure and improved trunk stability, thereby also stabilizing the pelvic girdle where many groups of leg muscles have their origins.