Carve Your Thighs With Leg Extensions

By Stephen E. Alway, Ph.D., FACSM

Powerful and thick thighs require gut-busting exercises like squats and leg presses. However, once you have acquired adequate thigh mass and strength, you should consider adding some balance and sharpness to the muscle bellies in your thighs. Although tough to accomplish, leg extensions provide a great way to carve the separations between the muscle bellies, and to accentuate the teardrop shape of the four quadriceps muscles of the anterior thigh.

Active Muscles

The three vasti muscles comprise most of the anterior thigh.1 The vastus medialis covers the medial (inner) part of the femur bone (thigh bone). When it is well developed, it forms a teardrop-like shape over medial side of the knee joint. The vastus lateralis muscle attaches to the lateral (outer) part of the femur bone. The vastus intermedius connects to the femur bone between the vastus lateralis and the vastus medialis muscles. The fibers of all three vasti muscles come together at the quadriceps tendon, which crosses the patella (knee cap) to attach to the tibia bone just below the knee.

Together, the three vasti muscles extend the leg at the knee joint, although the vastus intermedius may be more fatigue resistant than the vastus lateralis. The vastus medialis oblique (VMO), which is a small part of the vastus medialis muscle, attaches to the medial part of the patella. It is thought to help the patella track properly during movement of the knee. Improper tracking can increase the likelihood for knee injury.

The vastus medialis and especially the VMO part of this muscle are primarily responsible for tibial rotation (rotation of the tibia bone of the lower leg on the femur) during knee extension. This rotation or “twist” has been shown to increase the activation of the VMO portion of the vastus lateralis even more than doing knee extensions with the hip adducted (thigh rotated so that the medial portion of the knee is facing mostly upwards). Dorsiflexion of the foot (moving the ankles so the toes are pointing towards your head) also increases the activation of the VMO by more than 20 percent. Likely this is because the dorsiflexor muscles stabilize the tibia during knee flexion and resist rotation of the tibia on the femur as the knee straightens.

The fourth muscle of the quadriceps group is the rectus femoris muscle. It attaches to the anterior part of the hip bone just above the hip joint.1 The largest bulk of the muscle fibers are located on the upper three-quarters of the thigh, whereas the largest belly of the vastus medialis and vastus lateralis are more inferior (i.e., closer to the knee). The distal end of the rectus femoris muscle becomes tendinous and it creates a deep valley between the lateral and medial vastus muscles as it approaches the knee.1 It assists the other quadriceps muscles by extending the leg at the knee joint, although it is less effective when the hip is flexed than if it is straight.

Knee Extensions

The three vastus muscles of the anterior thigh are strongly activated by single-leg knee extensions. The rectus femoris is not activated as strongly, but it does undergo some overload when the anterior thigh is under contractile effort, about half way up to the top of each repetition.

1. You should always warm up your knees with some stationary cycling prior to getting into knee extensions. Furthermore, the resistance on your first set should be fairly light to allow the joint to fully warm up before you get to the heavier stuff.

2. Adjust the knee extension machine so that the pivot point of the lifting arm is directly adjacent to the center of the side of your knee joint.

3. Position the ankle roller/leg pad over the lower part of the leg (above the ankle joint).

4. Take about three seconds to slowly extend both knees so that the weight is lifted upward from the stack.

5. Continue upwards until the tibia and the femur bones form a straight line and the knee angle is straight. Hold this for two seconds at the top.

6. Slowly lower the weight (about four seconds down) towards the starting position. Once the knee has reached 90 degrees, start the upwards extension phase again. Continue for 12-15 repetitions for the first set. Lower the number of repetitions, but increase the resistance for subsequent sets.

7. On the next sets, lift the weight upwards until the knee joint becomes almost straight, but just slightly short of a total knee lockout. Be careful that you do not “jam” the knee joint into a fully locked out position because this could cause knee cartilage damage, especially with heavy weights. Hold the top position for a count of three before lowering the weight.

8. Lower the weight slowly (four to five seconds) towards the starting position where your knee is flexed to 90 degrees. Just before the weight stack contacts the remaining plates at the bottom, start lifting it upward for the next repetition.

The downward movement should be slower than the upward phase because you are resisting the pull of gravity. The slow lowering of the weight stretches the muscle under a resistance and this is a great stimulus to improve muscle shape and size.

Make sure that you do not hold your breath during the lift upwards. Rather, take a breath at the bottom (start) of the lift, and exhale as you extend the knees/legs. Take another breath at the top and slowly exhale as the weight is lowered. Take another breath at the bottom and repeat the sequence.

This is a mechanically simple exercise, but it can be very challenging and blood depriving, especially if you try to control the weight as it is moving up and down. However, if you are willing to work through some discomfort, you will be soon enjoying your new shape and slabs of carved thighs.

Illustrations by William Hamilton, CMI

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