An adjustable strap, detachably installed atop a mattress to serve as an aid for exercise, without having to alter the construction of the mattress or bed frame. The apparatus is utilized for sit-up exercise by an individual reclining upon the mattress. Each end of the strap forms a loop by means of a Velcro fastener of sufficient dimensions that the size of the loop can be varied. The loops are secured to the bed frame. A padded central portion is provided upon the strap, which is wide enough to accommodate a pair of ankles. Means are associated with the strap for spacing the strap from the mattress to provide a space into which the feet and ankles of the user can extend. A flat, semi rigid pad is provided, for the individual to sit atop the mattress without compressing the mattress during exercise.