Final answer:
The synergist to the gluteus maximus and minimus during extension is the tensor fascia latae muscle.
Step-by-step explanation:
The synergist to the gluteus maximus and minimus during extension is the tensor fascia latae muscle. It is located in the superior aspect of the lateral thigh and acts in flexing and abducting the thigh. It also helps stabilize the lateral aspect of the knee by pulling on the iliotibial tract.