Abdominal muscle visibility is determined by the combination of subcutaneous fat thickness below a critical threshold and a sufficient rectus abdominis muscle cross-sectional area.

Study: Predictive efficacy of rectus abdominis muscle and psoas major muscle thickness for postoperative morbidity in patients with endometrial cancer

This study found that people with thicker stomach muscles had fewer health problems after surgery, which supports the idea that having strong, thick abdominal muscles matters — even if it didn’t measure body fat.

Study: Effects of Pilates on inter-recti distance, thickness of rectus abdominis, waist circumference and abdominal muscle endurance in primiparous women

This study found that doing Pilates made women’s stomach muscles thicker and their waists smaller, which means they’re likely losing fat and building muscle — two things needed to see abs.

Study: The Effects of Functional Magnetic Stimulation on Rectus Abdominis Muscle Size and Abdominal Subcutaneous Adipose Tissue Thickness

This study showed that when people used a special machine to make their abs contract, their stomach muscles got bigger and their belly fat got thinner—exactly what you need to see your abs.