Why focus on the Vastus Medialis?

The clinical relevance of the vastus medialis is already evident in the fact that it is one of the few muscles explicitly mentioned by referring doctors. Especially in cases of patellofemoral pain syndrome, lateral patellar dislocation, or jumper’s knee, the vastus medialis repeatedly appears as a potential problem area. Although scientific literature presents mixed results – with some studies confirming significantly reduced vastus medialis activity in certain pathologies and others disputing it – the experience of physiotherapists and myoact co-founders Simon Roth and Philipp Piroth suggests that knee pain is frequently accompanied by decreased activation of the vastus medialis.

(Im)balance between Vastus Medialis and Vastus Lateralis

What matters here is not necessarily the difference between the left and right sides of the body, but rather the relationship between the activation of the vastus medialis and the vastus lateralis. This ratio can be objectively evaluated using EMG mapping. The myoact app then displays the results using a traffic-light system: a difference of up to 15–20% is considered “green” and unproblematic. If values fall into the “yellow” or “red” range, this may indicate muscular imbalance. While imbalances are not always the cause of pain, and not all pain is the result of imbalances, they are nevertheless an important contributing factor that should always be considered in cases of knee discomfort. The fact that visible swelling around the knee almost always coincides with an arthrogenic muscle inhibition (AMI) of the vastus medialis reinforces this point. In such cases, a reflexive inhibition of muscle activity occurs even if the vastus medialis itself is not injured. 

EMG Mapping for Detecting Imbalances

If swelling of the knee persists or this compensatory pattern remains despite a reduction in swelling, a persistent imbalance develops which, in combination with other imbalances, can lead to symptoms. Whether this is the case can be checked using EMG mapping of the vastus medialis.
The myoact app can be used to map a muscle in 3 minutes. This means that the activity of a muscle is analysed functionally. Resting activity, voluntary and involuntary activity are checked using predefined exercises and then presented in an individual report. For 36 superficially measurable muscle groups, results can be provided that show imbalances, reduced muscle activity or even inefficient muscle activity (i.e. overload). In the case of the vastus medialis, in addition to the resting position while standing, the maximum voluntary activity when extending the knee in various positions and the two-legged and one-legged squat (involuntary activity) are tested.

Case Study: Jumper’s Knee

This case focuses on a professional volleyball player who developed jumper’s knee after returning to sport following a pregnancy-related break. EMG mapping of the vastus medialis revealed significant differences in balance scores between the affected right side and the healthy left side. The left leg scored a balance of 92%, while the right side reached only 63%. The comparison between the left and right sides showed that there was no significant difference in the activity of the vastus lateralis, while the vastus medialis on the affected right side was significantly less active than on the healthy left side, both in maximum voluntary activity and in squat exercises. Accordingly, the patient’s training was adjusted to include exercises aimed at dominant recruitment of the vastus medialis.

Retest after 5 Weeks

Following five weeks of targeted training, a retest showed marked improvements. While activity in the vastus lateralis remained stable, the vastus medialis demonstrated increased activation. This brought the muscle pair into better balance. The improvement was reflected in the balance scores, which rose for example, during squats from 75% to 91%. Most importantly, the patient’s symptoms also improved, achieving the overarching therapeutic goal.

The Key to Progress: Dominant Recruitment of the Vastus Medialis

The dominant recruitment that led to improvements in both the balance score and the patient’s symptoms can be achieved through various exercises and validated using EMG biofeedback. Unlike palpating muscles or merely observing exercise performance, EMG biofeedback provides objective insights into whether an exercise primarily activates the vastus medialis or whether the vastus lateralis is taking over. Two separate bars, one for the vastus lateralis and one for the vastus medialis, display muscle activity during training. This visual feedback helps patients monitor which muscle they are actually activating.

Focusing on Leg Alignment

The function of the knee heavily depends on the movement quality of surrounding joints, particularly the foot and hip. If compensatory patterns develop in these areas, they can be addressed with EMG even before symptoms arise. Compensations and imbalances along the leg axis can be identified through EMG mapping, for example by assessing the peroneus longus, gluteus medius, or of course the vastus medialis. Potential imbalances can then be specifically treated using biofeedback-based training.

Conclusion

The vastus medialis is often a central factor in knee complaints. With EMG mapping, its activity can be objectively measured and compared to that of the vastus lateralis. Targeted, dominant recruitment supported by EMG biofeedback training not only improves muscular balance, but also leads to measurable improvements in symptoms.