Hold for 30 seconds, or as long as you can. Mountain climbers involve the same type of lower-body motion as the reverse crunch. Begin in plank position. Bend one knee, bringing it toward your chest.
Keeping your body in the same position (feet on ... off the floor – hold for a few seconds and then lower back down. Go for 12 crunches on one side, then repeat on the other.
To do a standard crunch, begin by lying on your back ... lifting your shoulders and head off the ground as you do so. Lower your hands and upper body back down. This is one rep.
The exercise is a variation of an ab crunch, where one leg flexes while the other extends. At the same time, there is rotation at the lumbar spine – the part of the spine in your lower back ...