
I don’t know why but I find plank difficult than straight arm plank. I can do 20 seconds of straight arm plank with ease but I can’t do a 20 secs of plank with ease
Is there a standing alternative to the plank in the warm up?

That’s actually very normal. A forearm plank shortens your base of support and increases the lever on your core, so your abs have to work harder to keep your pelvis from dropping. A straight-arm plank lets your shoulders and arms share more of the load, which is why it feels easier even though it looks “harder.”
Nothing’s wrong with you. It just means your core is being challenged more in the forearm version. Stick with shorter, clean forearm planks and build them up gradually, and use straight-arm planks as a stepping stone or warm-up. Over time, the forearm plank will catch up.
