You could do a 'stir the pot' exercise in a similar position to this but with your feet on the ground and arms straight up. Hold a resistance band that is tied to something sturdy behind you and while bracing the core move the band in circles, slowly, and with intention. This position will protect the spine and engage the core muscles.
The idea is that any exercise which puts the lower back into flexion (rounding) should be avoided as much as possible.
If you notice, the lower back will still round, in the reverse crunch, when the hips are lifted from the floor.
You could do a 'stir the pot' exercise in a similar position to this but with your feet on the ground and arms straight up. Hold a resistance band that is tied to something sturdy behind you and while bracing the core move the band in circles, slowly, and with intention. This position will protect the spine and engage the core muscles.