In a similar way, only a small movement is required to feel the abs contraction when performing the crunch sit up. (mystrengthtraining.com)
Of the equipment studied, performing a crunch on an exercise ball and a using a vertical knee raise machine showed the most muscle activation. (verywellfit.com)
To avoid the need to tuck your feet under anything, perform a crunch style sit up instead of a full sit up. (mystrengthtraining.com)