The static plank (holding in a plank position without moving) is a typical gym exercise, which I have used in the past but of late I’ve more or less stayed clear of them unless it’s incorporated with some other movement. While there are specific reasons to opt for a static plank such as to improve your handstand (totally popular right now and quite fun) generally there’s more reasons to stay away from planks and choose other core exercises.
Static planks lock the pelvis and thorax together, and you’ll note that your core is capable of more than just maintaining rigidity – your core also flexes, extends, rotates and co-contracts (like when you brace for impact).
If you have ‘weak’ abs teaching rigidity through static planks is likely to cause more problems as it will only encourage the superficial muscles to lock in more tightly when the problem often lies with the deep abs not switching on. By activating the deep abs, first in isolation then integrating with other muscles you’re more likely to have less lower back pain.
Excessive planking can lead to several issues – the continued squeezing means you’re also squeezing your pelvic floor which interferes with its function. It also limits proper breathing patterns – maintaining that stiffness means you’re less likely to be able to breathe right into your belly. This in turn interferes with your digestion and elimination functions as proper diaphragmatic breathing massages your internal organs and helps digestion.
The reality is we use the core in many gym exercises (and throughout the day), especially full body movements with rotations, push and pull patterns that unless there are particular reasons to hold a static plank, your core will be getting a good workout anyway.