Particularly as you place the ball of your foot down and transfer weight onto it.
From this image I believe it's #3, and #18-20. These correspond with Sartorius, Gracilis, Adductor Longus, and Pectineus, respectively.
They've become deactivated after my PCL tear, and due to 'not having faith in them' I've developed bad form in my one-leg squats.
Preferably something that I can do at home. I'm attempting some sit-ups/leg-lifts next, but I'd really like to pack on improve my strength in this area.