For many years I have suffered from patellofemoral syndrome. My understanding is that, in my case, the cause is the relatively under developed VMO on both legs. I do a great deal of split squats and they help a lot, but I would like to identify some more exercises. Can anyone offer any suggestions?
PFPS (patello-femoral pain syndrome) is usually caused by the knee cap tracking off to the lateral side instead of keeping centered in the femoral groove. The causes can be from muscle weakness (the VMO as you mentioned), tightness (muscle and fascia) esp. laterally, and/or faulty lower extremity alignment.
If you haven't already consulted with a sports physical therapist or doctor, that would be a good place to start esp. if you haven't tried the McConnell taping mentioned below.
To strengthen the VMO, first release any lateral muscle or hamstring tightness, and then make sure that you have good alignment while doing the exercises. Avoid pain as this will cause an inhibition of the VMO and negate your attempts to strengthen.
• Muscle Weakness - The problem of VMO weakness as compared to the VL (vastus lateralis) is that it is not just a matter of weakness, but also of sequencing or timing of the muscle contraction. You want the VMO to kick in before the larger VL muscle contracts. If the VMO is delayed, the VL over-powers the smaller, weaker VMO causing the patella to track off to the lateral side.
One training technique involves taping to control the tracking of the knee cap. (McConnell taping.) Then you can practice eccentrics ie step downs within a pain-free range. Pain needs to be controlled when attempting to strengthen the VMO because pain inhibits the muscle contraction. Strengthening of the VMO usually occurs in the latter degrees of knee extension, so you don't need to go into full flexion which places a greater strain on the patella.
• Alignment - When doing VMO exercises, you need to look at the entire leg alignment including the foot and ankle. For instance, a foot that pronates (flattening the arch) causes a shift in the knee and patellar alignment. Foot orthotics may help relieve PFPS by improving foot alignment and controlling pronation.
This video gives a good explanation about how to align your foot and tibia, and focus on the recruitment of the VMO and glutes in weight bearing or closed kinetic exercise such as your split squats.
Muscle tightness may also contribute to poor knee and patellar alignment.
• Tightness - Gently test the mobility of your knee cap. If it glides easily, especially medially (toward the inner thigh) then you are in good shape. However, often with PFPS the knee cap movement is restricted.
Tight hamstrings and gastroc muscles behind the knee can cause a slight flexion at the knee which increases pressure on the patella and inhibits the function of the VMO. Self-massage to release the hamstrings, esp. the biceps femoris on the lateral side, can improve the function on of the VMO, so release tight muscles before strengthening the VMO. Lateral muscles that may need releasing are the lateral quads, TFL/IT band and glutes to allow for neutral alignment.
The following exercises use a a foam roller to massage or release:
• VMO Strengthening - Start with simple quad sets, placing your hand on your VMO for feedback. Tapping on the muscle can also help to facilitate a better muscle contraction.
Progress to closed kinetic chain (weight bearing) exercises like split squats,
Hope that helps.