Possible Clues:

• 'Tis this, notwithstanding
• Abs-working exercise
• Partner of ''take notice''
• Partner of "take notice"
• One's back put out - by this exercise?
• "Don't slouch!"
• 'Tis, we deduce, to keep going beyond 15?
• Backing places around island to exercise
• Strengthener of abs
• Exercise that might be done with crossed arms