Measuring a client’s progress against a previous trial is always useful when there isn’t an “industry standard.”
It seems to me like a one-minute sit-up test is very different than a planking for maximum time test. Why the switch? I’m NOT being critical with this question. I’m honestly curious and wanting to know why you’ve decided to use plank for time as a standard.
The deeper reason that I ask your reasoning is because I have a client who loves to plank for time. She’s up to over five minutes and very proud of that. But is it functional for her? I don’t think it’s necessarily that useful. When I put her in a frontal plane (i.e. side plank) or ask her to rotate, she’s nowhere near as strong. So when I work with her, we don’t plank (smilie).