The sartorius is a difficult muscle to isolate but it can be done (or at least you can get close to it as posible). To isolate a muscle we first need to find out what kind of things it does.
The sartorius is a long mucsle, the longest in the body as a matter of fact. It crosses the hip and stretshes down and attaches medially below the knee. It's functions are knee flexion, hip flexion, hip abduction and hip external rotaion. So now it is simple to find a way to isolate this muscle.
As stated before an abducion exercises will work however I don;t ever want to see some one in the gym sitting on those damn abduction machines, they are worthless. Why?? Because they don;t work what they say they are working. Becasue you are in a seated position you are putting yourself in hip flexion. In hip flexion your hip flexors and abductors are slacking and inactive, this is why so many people with desk jobs complain about lower back pain, becuase their hip flexors are always tight from sitting all day. If your hip flexors are slacking then they are not going to be effective (like they would be if you were standing). So what is being worked with this excercise?? Your piriformis, a small muscle, sometimes know as the wallet muscle becuase it lies right by your back pocket were your wallet goes. Your piriformis is an external rotator of the hip. Why is it bad to work this muscle?? Becuase the piriformis lies right next to the siatic nerve if you build that muscle it may start to push against the nerve and in time can result in siatica (don;t know if that was spelled right), some of you may have experienced a feeling of numbeness down your leg form sitting for extended periods of time with your wallet in you back picket becuase your wallet is pushig your piriformis up against the siatic nerve (hence the name the wallet muscle).
So if you want to isolate the sartorius try going to the cable machine and putting the straps around your ankles and do hip abduction standing up, that way the sartorius is more active. Also, because the sartorius is an external rotator you can perform hip extension standing up with the cable straps on your ankles and instead of going straigh back, although this wont isolate the sartorius completely it will be effextive, turn your foot outwards slightly which will cause some hip external rotaion (which the sartorius does).
hope that helps, sorry for the long winded response, it is a difficult muscle.
