'Upper' and 'lower' abs aren't separate muscles
Fitness culture has long split the abs into "upper" and "lower," complete with dedicated exercises for each — crunches for the top, leg raises for the bottom. Anatomically that's not quite right: the rectus abdominis is one continuous muscle running from the ribs down to the pelvis, divided into visible segments by tendinous bands (the linea alba and tendinous inscriptions) — that's literally where the visible "six-pack" squares come from — not into functionally independent upper and lower halves you can isolate.
What EMG studies actually show
Electromyography (EMG) research, which measures electrical activity in a muscle during exercise, does find that different ab exercises shift activation slightly along the rectus abdominis. Exercises usually labeled "upper-ab" work, like crunches, show marginally higher activation toward the top; "lower-ab" work, like leg raises or reverse crunches, shows marginally more activation lower down. But because it's one continuous sheet of muscle fiber, activating any part of it recruits the whole structure to some degree — the difference between exercises is a few percentage points in emphasis, not a clean, isolatable split.
Why exercise selection isn't the lever people think it is
Because the rectus abdominis works as a single functional unit, you can't meaningfully starve one "half" of stimulus while overloading the other — heavily training crunches for months doesn't leave the lower portion undeveloped the way training only biceps leaves triceps undeveloped. What actually differs across ab exercises has more to do with which movement pattern is involved — spinal flexion, hip flexion, or anti-rotation — and which other muscles get recruited alongside (obliques, hip flexors, transverse abdominis), not which "half" of the rectus abdominis gets built.
The real driver of visible abs
A bigger factor in whether abs are visible at all isn't exercise selection — it's overall body-fat percentage. The rectus abdominis doesn't have a separate fat depot specifically layered "up top" or "down low"; fat distribution across the abdomen is driven by broader genetic and hormonal factors that no amount of targeted ab work can override. This limitation is usually called "spot reduction," and it doesn't hold up under controlled research. Someone at 25% body fat with an extremely well-developed rectus abdominis still won't have visible abs, while someone at 10% body fat with minimal direct ab training often will.
Practical takeaway
Training the abs with a mix of movement patterns — flexion (crunches), anti-extension (planks), anti-rotation (Pallof presses), and hip flexion (leg raises) — is worthwhile for core strength and stability across different demands, not because it targets separate "upper" and "lower" muscles. If visible definition is the actual goal, body-fat percentage does far more work than any specific ab exercise or rep scheme.