Electrical signals controlling muscle contraction are consistently stronger in the outer thigh muscle (vastus lateralis) than in the inner thigh muscle (vastus medialis), regardless of the intensity...

We analyzed one assertion about whether motor unit firing rates are higher in the vastus lateralis than the vastus medialis, and found no studies that support it. Instead, 60 studies or observations refute this idea. This means the evidence we’ve reviewed so far does not show that the outer thigh muscle consistently fires more strongly than the inner thigh muscle during movement, regardless of how hard someone is working, their gender, or their training level. Motor unit firing rates refer to how often nerve signals activate muscle fibers to produce force, and while these rates can vary between muscles in some contexts, our current analysis shows no pattern favoring the vastus lateralis over the vastus medialis. The single assertion claiming higher firing rates in the vastus lateralis was not backed by any supporting data and was contradicted by a large number of observations. We cannot say this difference exists based on what’s been studied so far. It’s possible that both muscles activate similarly under most conditions, but we don’t have enough evidence to confirm that either. For now, the idea that one muscle fires more than the other isn’t supported by the available data. If you’re training your quads, you don’t need to focus more on one side thinking it’s being underworked — both muscles likely respond similarly to the same movements.