Most studies find that the vastus medialis cannot be emphasized over the vastus lateralis and vice versa. These usually rely on EMG though, which can be misleading. My view is that muscle length can be changed through valgus and varus positions, so a slight emphasis is possible.
Bodybuilders have historically aimed for balance among bodyparts, especially when striving for an aesthetic physique over sheer size.
The quadriceps has four muscles. The vastus medialis forms the teardrop above and inside the knee. The vastus lateralis is the largest and creates outer sweep. The vastus intermedius is a deep, non-visible muscle within the middle of the thigh. The rectus femoris is a superficial, two-headed muscle also within the middle of the thigh, that attaches to both the knee and hip.
Since these muscles spread throughout the thigh, it makes sense that they should develop in proportion, if possible, for bodybuilding purposes. Though they all seem innervated by the femoral nerve, it may be more complex due to different nerve supplies among other factors.
Physical therapists and exercise scientists have also been interested in balanced quads, albeit for a different reason.
Patellofemoral pain syndrome (PFPS), sometimes known as runner’s knee, may occur in part due to a poor balance of the quadriceps muscles surrounding the patella or kneecap, though other muscles definitely play a role too. They are concerned with the balance between the vastus lateralis and the vastus medialis. They each pull laterally and medially respectively to fix the kneecap within its groove called the femoral trochlea.
Most studies relevant to physical therapy have analyzed possible vastus medial oblique (VMO) activation, a division of the vastus medialis. Some experts believe these fibers run horizontally and therefore stabilize the knee.
This is contentious. Most studies have found no EMG variations between various exercises to selectively recruit the vastus medialis, specifically the vastus medialis oblique, over the vastus lateralis.
Some have noticed that vastus medialis paralysis inhibits terminal knee extension. This may just be a loss of mechanical advantage with the final degrees of knee extension, not specific muscle weakness. The fiber orientation of the vastus medialis may also compensate for less strength and size compared to the vastus lateralis.
Outside of EMG, Per Tesch found a difference via MRI. He confirmed that the vastus medialis and vastus lateralis can be emphasized based on toes out and toes in respectively for the leg extension.
Let’s take a look at what the studies and bodybuilders have tried to accentuate each muscle.
The vastus medialis, according to some studies, is comprised of both the vastus medialis longus (VML) and the vastus medialis oblique (VMO). The VMO is supposed to focus on medial patella tracking, with more horizontal fibers, and the VML on knee extension with relatively more vertical fibers.
Some anatomists have noted that this distinction is impossible if the fibers are arranged in-series. They would all pull on one another. For our purposes then, we will treat this as a single muscle, the vastus medialis (VM).
Bodybuilders, and some research, have found these factors to possibly matter:
- Toes pointed out during knee extension, both with closed-chain (squats & leg presses) and open-chain (leg extension) exercises, with the tibia externally rotated.
- A wider stance or feet placed further apart.
- Emphasizing the endpoints for the range of motion, with the knee both near full extension (final 15-30°) and near full flexion such as with deep squats, perhaps due to longer sarcomeres.
- Instability may recruit the VM more, like squatting on unstable surfaces, and including unilateral exercises such as split squats.
- Knee extension simultaneously occurring with hip adduction may stabilize the origin of the VMO for greater contraction strength, and the VMO fibers partially attach to the adductor magnus. This seems only the case for closed chain exercises like squats and leg presses though. This means techniques like pinching the knees. Hip abduction has also been tried with less success.
- Higher reps, since most studies report the VM as more slow-twitch than the vastus lateralis. This may partially explain the higher EMG from the vastus lateralis on most resistance training exercises. Cycling also likely hits Type 1 fibers more so than lifting weights, and many note that it seems to develop the teardrop muscle preferentially.
- Visualization may help, imagining the muscle working during the exercise, as EMG-based biofeedback has had some success.
Many are quick to criticize that these actions seem irrelevant, but I disagree.
Anecdotally, I notice that by fully extending the knee and pointing the toes inward, the VM cramps, representing an overly shortened state. This has me realize that its length is changing, albeit slightly, from this action. It seems this may occur due to varus (bow-legged) or valgus (knock-knee) positions during knee extension.
My view is that all of this may lead to a valgus or knock-kneed position that slightly stretches the vastus medialis to emphasize it.
We find these factors for the vastus lateralis (VL):
- Toes pointed forward or slightly in during knee extension, both with closed-chain (squats & leg presses) and open-chain exercises (leg extension), with the tibia internally rotated.
- A closer stance or feet placed closer apart.
- Emphasizing the midpoint for the range of motion, with the knee at about 90° of flexion.
- Instability may recruit the VL more, perhaps when testing it laterally, or perhaps it does better with stability.
- Knee extension with hip abduction may stabilize the origin of the VL for greater contraction strength or may not matter since the VL is not as complicated with its pennation angles as the VM.
- Lower reps, since most studies report the VM as more slow-twitch than the VL.
- Visualization may help.
My view is that all of this may lead to a varus or bow-legged position that slightly stretches the vastus lateralis to emphasize it.
How to Target the Vastus Medialis vs. Vastus Lateralis
Taking into account these factors, here are my recommendations to emphasize each muscle:
- Remain stable by using a bilateral exercise, to best express strength.
- Include mostly moderate reps of 6-12 to balance all growth stimuli.
- The best exercise for these single-joint muscles, regardless of muscle emphasis, allows the deepest possible range of motion. This includes maximum passive tension due to the stretch, in addition to active tension from contracting. This is not just for the vastus medialis but for the vastus lateralis too.
- The leg extension could work in theory, but most machines fail to allow enough range of motion at the knee not have an ascending strength curve. The rectus femoris also stretches to prevent knee extension as the hip extends. Therefore, some sort of deep squat or leg press is best to maximize knee flexion.
- A hack squat or a leg press with feet low on the platform and hips forward will remove most hip extension from the exercise. You can also also put your feet in front on a Smith machine squat for an ideal strength curve. This overloads the quads best in the middle of the movement but will also involve more hip extension, so bring in the glutes more (classic bodybuilders tend to minimize hip size).
- Experiment with the toes pointed out for the vastus medialis, with the stance slightly wider, and the toes pointed in or forward, with the stance slightly closer, for the vastus lateralis. Realize that this will not allow for isolation, and both muscles will still develop. It may slightly emphasize each muscle. Be careful not to irritate the knee though with too extreme of a position either way. Consider this more an option to rebalance the quads slightly.
- A good program for the quads as a whole would involve leg extensions or sissy squats targeting the rectus femoris. You would also perform a knee-dominant squat or leg press, for the same set and rep scheme, once weekly but during a separate session. This targets both the vastus medialis and vastus lateralis, modifying position as described to suit your goals.