Performing a trunk curl with your feet pressed flat against a wall allows you to engage the entire abdominal wall in an ultra-targeted manner, working the upper, lower, obliques, and deep transverse abdominis simultaneously.

noli's read

Placing the feet against a wall is an excellent positioning technique that, by flexing the hips, limits the involvement of the hip flexor muscles to better recruit the core. However, the idea that one can precisely isolate the upper and lower abdominals is largely nuanced by research. Observational studies measuring the electrical activity of muscles (EMG), such as that of Clark et al. (2003), show that the rectus abdominis activates globally as a single, unified functional unit during this type of movement. Furthermore, the deep transverse abdominis is a stabilizer muscle that is primarily engaged through breathing exercises (such as the vacuum) or bracing, not through standard trunk curls. Finally, the obliques require rotational or lateral flexion movements to be stimulated effectively. This exercise remains excellent for general strengthening, but the promise of surgically targeting each compartment is overstated.

the sources

1. 1Wang XQ, Zheng JJ, Yu ZW, et al. 2012