When you lower a weight slowly (like 4 seconds), your inner thigh muscle (vastus medialis) grows more than the other thigh muscles — the others don’t seem to care as much about the slow lowering.

The study found that slowing down the lowering part of the leg exercise made the inner thigh muscle (vastus medialis) grow more than the others, but didn’t help the outer or front thigh muscles any more than a normal speed—exactly what the claim says.