Coaching a "neutral spine" - a spine in proper alignment - is critical to any form of plank, squat, deadlift, swing, or other standing lift.
Coaching it can be a little tricky. Mike Robertson has a great post on exactly how to do that.
Coaching Neutral Spine
大会結果：5/21 JBJJF 第8回中国柔術選手権 | アダルト黒帯で白木大輔がWゴールド
7 minutes ago