The bilateral deficit is a strange but well-documented quirk: two limbs working together produce less total force than the sum of what each limb can do alone. Test one leg on a leg press, then the other, and add the numbers. That total usually beats what both legs press together. The gap is neurological. Your nervous system holds a little back when driving both sides at once.
Why it matters
Say your two-leg press max is 400 pounds. You'd expect each leg to manage 200 solo, right? In practice each leg often pushes 210 or 220. Weird. And useful. It means single-leg exercises let each leg work against a relatively heavier challenge than it ever sees inside a two-leg lift, using far less total load on your spine and joints. That's a big part of why split squats and single-leg presses build strength so efficiently. The deficit also shrinks with training, so athletes in jumping and sprinting sports (mostly one leg at a time efforts) care about closing it.
How to use it in training
Keep your bilateral lifts. Squats and deadlifts still load the most total muscle. But add one or two unilateral movements per session: Bulgarian split squats, single-leg presses, step-ups, single-leg RDLs.
Don't just halve your two-leg weight for the single-leg version. Each limb can handle more than half, so work up honestly. Start each set with your weaker side, and match reps with the stronger one. That habit also keeps side-to-side gaps from quietly growing.
Related terms
Go deeper
The single-leg training evidence is all here: Single-leg training research.