Lift off, legs out, hold the L.
Push your shoulders down away from your ears and lock your knees straight.
Pressing your body off the floor and holding your legs out straight makes the abs and hip flexors work flat out just to keep the L shape, while the rest of your trunk braces against gravity trying to fold you. There is no rep rhythm to hide in, only unbroken tension for as long as you hold. It earns its place as a core strength benchmark from gymnastics that also builds the compression strength behind moves like toes to bar.
Usually a mix of tight hamstrings and weak hip flexors rather than weak abs. Train seated leg lifts and pike compressions, stretch the hamstrings, and hold tucked L sits in the meantime.
Parallettes, by a lot. The raised hands give your legs room to clear the ground, on the floor you need extra shoulder depression and compression just to get airborne, so start on parallettes or push-up handles.
Accumulate time in short pieces, five holds of 5 to 8 seconds beats one shaky max effort. A crisp 20 to 30 second hold with straight legs is a strong benchmark to build toward.
