Cradling a barbell in the crook of your elbows while splitting your stance and dropping the rear knee toward the floor adds an anti-flexion core demand on top of the usual split squat quad burn. The front-loaded position also keeps your torso more upright than a standard barbell version.
Log your barbell zercher bulgarian split squat sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.