Maintain solid spinal alignment. Most people who are new to this will want to cheat the single-arm plank by creating cervical hyper-extension and tilting their heads up. This strains the neck and places excessive tension on the lumbar spine by causing the hips to sag.
Focus on keeping a tall and elongated neck throughout rather than tilting the head up or down.
What happens to the after-40 lifter? Does this milestone mean you have to train like a grandpa? Not necessarily. But here's what will lengthen your lifting life.