60 Seconds of Straight-Leg Sit-Ups For this exercise, the goal is again to complete as many sit-ups as possible while also maintaining your form. Begin on your back with your legs straight and arms raised toward the ceiling. Roll up to sitting, focusing on the abs rounding the back. Roll back down to the mat slowly, one vertebra at a time. Do as many reps as you can in 60 seconds. Image Source: POPSUGAR Photography