Why use the ball for single and double leg extensions at the barre!
Reason one: the ball actually keeps your spine in a deep C-curve spine without the ball your students will have tendency to extend the spine and use the hip flexors.
Reason two: the ball supports the lumbar spine this allows you to push up and forwards with the hands on the barre. By depressing the scapula and scooping out the abs you will access a stronger abdominal contraction. Without the ball you’re unable to do this as you have no lumbar support.
Don’t forget your musicality to help you control the movement, four counts up and down, never any faster.
Enjoy abs at the barre!