Use Isometric Holds for Abs
The plank, an isometric pose, was a movement that functional training gurus flocked to as soon as there were signs pointing to spinal flexion as a culprit for shear and compressional forces through the lower lumbar spine. But it's time to think outside the traditional plank. There's more than one way to use isometric contractions to make ab gains.
You can use isometric properties to safely train spinal flexion. And you ought to if you want a stronger core. Isometrics activate the targeted muscle by increasing muscular recruitment in a static position in order not to deviate from that central setup.
Here's What To Do
Add isometric holds into your direct spinal flexion core training. Try holds at any degree in the range of motion of the movement. It'll increase the recruitment of the rectus abdominis and, to some extent, hit the internal and external obliques and other deeper stabilizers of the spine and pelvis.
Decline Sit-Up, Iso-Hold at Top and Bottom
Start with adding a 1-3 second isometric hold at the top of the sit-up, tapping into your mind-muscle connection and squeezing as hard as you can. This small use of isometrics during an otherwise dynamic exercise is a game changer and something that you'll "feel" instantaneously.
Then try placing an isometric hold at the bottom portion of the range of motion. Why? Because many times in the sit-up, people will lose their tension and core positioning at the bottom, resting but not bracing with their back on the bench or ground. Then they try to re-recruit tension and stability before every single rep.
It's hard enough to create proper pillar tension once. You don't want to be doing it repeatedly, especially when you're trying to time it with a dynamic contraction into flexion of the spine.
By using an isometric hold of 1-2 seconds at the bottom range of the movement, you'll get the bonus of continuous tension and core stability throughout all the reps in a set. You'll also increase your total time under tension of the set, which is great for strength, hypertrophy, and looking sexy.