On floor or mat, place medicine ball between bent knees. Lie on back with arms extended out to sides. Raise bent legs so thighs are vertical and lower legs are horizontal.
Lower legs to one side until side of thigh is on floor. Raise and lower legs to opposite side. Repeat.
Maintain knee position throughout movement. For less resistance, do not use additional weight (See Bent-knee lying Twist without ball). See Spot Reduction Myth. Also see Lying Twist with exercise ball.