Does pumping up right before stage make muscles look bigger?

Short answer

Muscle really does get thicker: a pump made it 7-20% thicker. But the gain was water: muscle swollen by 1 set was back to normal in 30 minutes, and even after light sets to exhaustion it was still there only at 60 minutes.

Does pumping up right before stage make muscles look bigger?
'Pump → water and blood rush into muscle → it looks bigger' — is that right?
Backstage at bodybuilding shows, competitors pump up with bands or dumbbells. We checked studies that measured muscle thickness by ultrasound: does muscle really get thicker, by how much, and for how long?
Muscle really got thicker right after exercise
About 7% from just 1 set
When 41 people did 1 set of leg presses to exhaustion, thigh muscle thickness rose about 7%. When 22 people did 3 sets of knee extensions, it rose 8.3% right after.
Light weights to exhaustion swelled it more
Arm muscle 1.2 times thicker
When 10 people did 4 sets of dumbbell curls to exhaustion at 20% of their max, arm muscle thickness grew 1.2 times. It was no different from the arm trained with a pressure band (1.21 times), and the extra thickness was still there 60 minutes later.
But it didn't last
Muscle pumped by 1 set was back to normal in 30 minutes
Thigh muscle thickness gained from 1 set of leg presses was back to normal in 30 minutes. In a trial where 13 people did 7, 14 or 21 sets for the lower body, thickness returned to normal within 24 hours no matter the number of sets.
The gain was water, not muscle
About 3 times the muscle grown in 6 weeks
In 22 people, real muscle growth after 6 weeks of training was 2.9%, while swelling right after the first workout was 8.3% — about 3 times as much. In a study of 18 people, muscle thickness rose in step with the share of water outside the muscle cells. A pump is water briefly rushing into muscle.
Compared with carb loading in peak week
Loading added about 3% thickness
In a case study that measured 1 person throughout peak week, cutting and then refilling carbs raised total muscle thickness about 3% above its lowest point. Over the same period, the fat layer under the skin at the back of the arm got 43% thinner. That is small next to the 7-20% from a pump.
Bottom line: a pump really thickens muscle
It lasts 30-60 minutes, which is why it's done right before stage
Pumping made muscle 7-20% thicker. Since the gain is water, it fades within 30-60 minutes. Pumping with light weights for many reps within 30 minutes of walking on stage fits the research.