Prepare the ball. Lie on your back and put your legs on the ball with calves leaning against the ball at pelvis width apart. Your hands relaxed alongside your body. Lift one of the legs up in the air and keep it in that position all the time, not touching the ball.
From the starting position lift your pelvis up. As you are lifting your pelvis, simultaneously bend your leg at the knee (the leg on the ball) and curl the ball under your body. Moving the leg to your body, you get into the bridge position with one leg in the air. The knee joint is at a 90 degree angle in this position. Keep your shoulders fixed on the mat. Go down the same way from this position but do not put your glutes on the mat and stay with your body firm and stretched with one leg leaning against the ball and with your shoulders on the mat. Go to the starting position (lying down completely on the mat) once you have done the required number of leg curls below your body. Then switch legs and do the same number of repetitions. Exhale while doing the upward movement and inhale going down.
Pay attention to correct position of your pelvis to avoid excessive bending at the lower back during the movement upward.
Pay attention to the correct position of your foot and sufficient angle at the knee joint. In the upper position the entire foot is leaning against the ball. The other leg is stretched in an unchanged position.
You can begin with smaller resistance by leaning your trunk on an elevated platform. Choose the platform tilt in line with your fitness level.