Hips drop the instant the hands release
The core must maintain the plank with no hand support at all.
Overactive muscles
Erector spinaeHip flexors
Corrective work
Foam RollHip flexors↗StretchHalf-kneeling hip flexor stretch↗The core must maintain the plank with no hand support at all.
Build to both arms; practice the hold with one arm first then progress.