The Quadriceps consists of 4 muscles that include 3 different VASTI muscles and the RECTUS FEMORIS.
The 3 vasti muscles consist of the vastus medialis, vastus intermedius, and vastus lateralis, which are single joint muscles (they only cross the knee joint) originating on the proximal aspect of the femur and insert onto the tibial tuberosity. The rectus femoris is a 2-joint muscle (crossing the hip and the knee) originating on the anterior-inferior portion on the iliac crest/pelvis and inserting onto the tibia in the same place as the vasti muscles.
All 4 of these muscles will extend the knee joint, however the rectus femoris will ALSO flex the hip.
The first 2 repetitions shown here will target the 3 vasti due to SLACKENING the rectus femoris. This slackening is referred to as an “active insufficient” position, meaning that there is excessive overlap of this muscle to the point that the shortened length makes it difficult to contract.
Active insufficiency is also the reason why when you flex your wrist, it is very difficult to make a tight fist.
The next two repetitions are performed with the hip in LESS hip flexion, which will put the rectus femoris slightly on stretch, allowing it to be at a more optimal length for the muscle to be recruited.
In order to maximally target rectus femoris, remove hip flexion all together. However, the seated knee extension machine makes this difficult with the backrest.
The take home message here is that every muscle has an optimal length-tension relationship and by putting the muscle on slack or shortening the muscle, you are targeting it at different intervals along the length-tension curve.