The loaded, standing oblique crunch is way too easy to butcher and can easily become a risky component of a core development program. The average gym goer has no idea what neutral spine entails, let alone being able to execute an acute range of motion targeting deep abdominal musculature. Before we start loading up movements and working towards intervertebral disc trauma one ugly rep at a time, how about we first master the basics?
If you're going to do a side bend, make sure you're unilaterally loaded, meaning you only have a weight in one hand, not both (as you've seen practiced at Planet Fitness).
Keeping your hips and knees in line with your spine, contract your core and turn on all the muscles that surround it to keep from slumping over to the side. Only when you can master this isolation hold using pristine mechanics and form do you dare to progress into a side crunch!
The tempo side-plank crunch can produce loads of tension without the actual load:
During the side plank, core muscles – including the obliques and transverse abdominis – turn on to stabilize the spine. The other primary stabilizer of this pillar, the gluteus medius, also activates to link the hips to the ab region.
Using a strong and maximally tensioned muscular contraction, own your side plank position for 20 seconds while squeezing maximally. After you earn the right to progress this movement, incorporate precise side-bending crunches with a slow descent of your bottom hip towards the ground and an explosive side bend back up into neutral.