Straight-arm plank can be a bigger challenge on your upper body compared to other types of planks. Activate core muscles and tilt pelvis up, rather than down. Pelvis should be at the same level of as your shoulders. Squeeze glutes. Keep back flat and not rounded and push through your arms to hollow out shoulders. Align your neck with the rest of your spine, and have the top of your head lengthening towards the front of the room.
If you suffer from a lower back issue or weak core, come down to the knees for a modified full plank pose. Continue to squeeze core and glutes, lengthen neck, and push up through your shoulders.
If you want to focus a bit more on core strength and less on upper body, try forearm plank. For forearm plank, separate hands with forearms parallel to each other. Place feet hip-width apart. Look in a mirror to make sure, like the straight-arm plank, that your head, spine, and hips are all in line. There should be no mountains or valleys.
Just like straight-armed plank, forearm plank can be modified for back issues by dropping to your knees.
Little movements in plank such as shifting weight forwards and backwards and dipping hips side to side can really help build core strength. Challenge yourself to a 1 minute isometric hold in either straight-arm or forearm plank. Each day increase your time to see yourself get stronger!