Purpose:
To investigate how the activation of gluteus maximus and gluteus medius interacts with different frontal plane projection angles (FPPA) in healthy young women presenting dynamic knee valgus in one limb while performing the step-down test.

Methods:
The total of 18 young women presenting FPPA > 15° during the step-down test in one limb (dynamic knee valgus) were assessed. The other limb should present less than 15° for comparisons. The amplitude of muscle activation was assessed through surface electromyography of gluteus medius and gluteus maximus during 8 subsequent weight bearing step-down tests.

Results:
FPPA was positively correlated with gluteus maximus activation and with the assessed side showing FPPA > 15°, which also revealed increased activation of gluteus maximus. No differences were noted for gluteus medius. The principal component analysis explained 73% of the variance in 2 components, with gluteus maximus explaining 48% of the variance on the 1st component. Gluteus medius explained 25% on the 2nd component.

Conclusions:
Gluteus maximus seems to be a major component to explain dynamic knee valgus in women without symptoms of patellofemoral pain, probably owing to weakness and lack of stabilization of other proximal muscles of the hip-knee complex during the task.