The Tanner or Straight Leg Sit Up is a Sit Up variation that many find preferable to the traditional, bent-knee version. Though they condition almost the exact same set of abdominal muscles, the classic bent-knee Sit Up that most people think of has a unique disadvantage in that you may find your body struggling to find equilibrium when performing it.
This is because since your legs are bent with your feet on the floor, your center of gravity actually shifts to your upper body leading many practitioners needing a partner to hold their feet down to maintain their balance.
Not so with the Tanner Sit Up. With your legs extended straight out in front of you, you’ll find this position shifts the center of gravity to your midsection and makes for an easier exercise overall.
Lie down on your back on a comfortable mat with your legs extended straight out and your heels touching the ground. Keep your lower back planted firmly against the floor.
Extend your arms above your head (so they are parallel with the floor), and then bring them forward to use as momentum at the same time you start to raise your torso upwards. Keep your neck and back straight as you start to sit up and your heels remain on the floor.
Continue to raise your torso until you reach an upright sitting position and your arms are out in front of you. Pause for a second and then lower your upper body back down to its original lying position.
Your arms descend along with your body until they drop backward behind your head, priming you for the next sit up. This counts as one rep.