The normal position for leg extensions — toes pointing straight up — emphasizes the rectus femoris, for greater front-quad sweep. Pointing your toes inward places more emphasis on the vastus lateralis, which builds more outer-quad sweep. Pointing your toes outward places more emphasis on the vastus medialis (the teardrop muscle). However, there’s another method to use during leg extensions to shift emphasis to diferent quad muscles: partial range of motion. 

Researchers from the University of Miami (Coral Gables, Florida) tested the muscle activity of the quad muscles through different ranges of motion during leg extensions. For reference, when you do a leg extension, you start with your knees bent at about 90 degrees and finish with your legs fully extended at 180 degrees. The Miami researchers found that when the subjects performed the leg extension in the middle of the ROM (about 120-150 degrees), the vastus lateralis received the majority of the focus; in the last 30 degrees of the ROM (about 150-180 degrees), the vastus medialis was the major focus. So, for better outer-quad sweep, start leg extensions by doing two or three sets through just the first half of the ROM and finish with two or three sets at full ROM. To develop bigger teardrops, start leg extensions with two or three sets going through just the top half of the ROM, and then finish with two or three sets at full ROM.