Can You Isolate VMO During Exercise?

The Concept of Muscle Isolation

Muscle isolation refers to activating a specific muscle independently of others during an exercise. In the context of the quadriceps, this means engaging the VMO without significantly activating other quadriceps muscles, such as the vastus lateralis, rectus femoris, or vastus intermedius. This concept is central to rehabilitation settings where targeting the VMO might be desired to correct imbalances or address specific patellar tracking issues⁽¹⁾. Now, while most practitioners can agree that there is no way to activate only one muscle, they continue to use “isolate” when discussing exercise prescriptions with athletes. While they may argue that it is simply semantics around “isolating” and “enhancing” activation, athletes and the general public may not be able to differentiate and apply these everyday words in the context of rehabilitation. They may create unhealthy thoughts around “weakness” and the etiology of their pain. It facilitates a biomedical approach to what is a complex biopsychosocial interplay.

Clinicians’ words can profoundly influence patients’ perceptions, potentially aiding or hindering their rehabilitation. Psychological factors, often overlooked in favor of biomedical ones, can more accurately predict pain and disability levels. Misunderstood medical terms can cause unnecessary fear, so clinicians must communicate clearly and reassuringly. A holistic approach, integrating both physical and psychological aspects of patient care, fosters better patient outcomes and helps them manage their conditions more effectively⁽⁷⁾.

Can the VMO be Isolated?

Researchers at the Norfolk and Norwich University Hospital conducted a systematic review in 2009 to answer this question⁽¹⁾. The review included 387 subjects from 20 studies and found that altering lower limb joint orientation or co-contraction does not preferentially isolate VMO over VL (see table 1)⁽¹⁾. Three papers demonstrated that co-contraction may enhance VMO activation, but the researchers warrant caution due to limitations in the study design and reporting. The limitations include:
1.    Gaps in the exact placement of the EMG electrodes, creating doubt on the data reliability, 
2.    Poorly described subject characteristics,
3.    Poorly described participant inclusion methods,
4.    Small sample sizes, and
5.    Poor reporting of how pain influenced participants with PFPS. Pain influences muscle contraction and strength through arthrogenic muscle inhibition⁽⁸⁾.

The authors conclude that physiotherapists should not attempt to focus primarily on the VMO. The anatomical and functional interconnectedness of the quadriceps muscles means that VMO weakness is more likely indicative of complete quadriceps weakness. Therefore, improving quadriceps strength will improve the function of all the muscles associated with knee extension and patellar stability.

Table 1: Summary of findings in systematic review(1)

EMG Evidence

Patellofemoral pain syndrome (PFPS) causes anterior or retro patellar pain and is prevalent in athletic and non-athletic populations. The incidence of PFPS is notably high, affecting about one in four individuals. Weakness in the VMO may contribute to PFPS, leading to treatments focused on strengthening the VMO. Researchers at the Grand Valley State University in the USA aimed to examine the activation of the VMO and VL muscles during rehabilitative exercises to determine effective treatment methods for PFPS⁽⁹⁾.

They used EMG to measure the activation levels of the VMO and VL muscles during four rehabilitative exercises. The study examined the effects of different lower limb joint positions and muscle co-contractions on VMO activation to see if these variables could preferentially enhance VMO activity over VL. The exercises evaluated included open-chain knee extension and other lower extremity movements across various knee angles⁽⁹⁾.

The findings revealed inconsistent results. While some exercises and specific joint positions showed potential for enhancing VMO activation, the results did not consistently support one method over another. The study highlighted the complexity of achieving selective VMO activation. The authors suggest that no single exercise or joint position could universally preferentially strengthen the VMO⁽⁹⁾.  Therefore, clinicians should be cautious in recommending particular rehabilitative movements solely for this purpose. Instead, a more comprehensive approach to strengthening the entire quadriceps complex may be necessary.

To further highlight the interconnectedness of the quadriceps complex, researchers at the University of São Paulo aimed to challenge the hypothesis that hip abduction for PFPS facilitates the vastus lateralis longus (VLL) muscle activation, leading to a VLL and VMO imbalance⁽¹⁰⁾. The results showed no selective EMG activation occurred between the VMO, VLL, and VLO during the different exercises, indicating that hip abduction exercises may not exacerbate VMO and VL muscle imbalances in healthy individuals⁽¹⁰⁾.

Practical Implications in Rehabilitation

Holistic Quadriceps Strengthening

• Focus on strengthening the entire quadriceps muscle group rather than attempting to isolate the VMO.
• Use exercises that engage all quadriceps muscles, such as squats, leg presses, and lunges, to improve overall knee stability and function.

Exercise Selection

• Avoid promising that specific exercises will isolate or preferentially strengthen the VMO.
• Incorporate a variety of exercises that promote balanced muscle activation.
• Use open-chain and closed-chain exercises at different knee angles to ensure comprehensive management.

Patient Communication

• Use clear, positive, and non-threatening language when discussing muscle activation and rehabilitation goals.
• Educate patients on the importance of overall quadriceps strength and its impact on knee health rather than focusing solely on the VMO.

Addressing Psychological Factors

• Consider psychological factors and their impact on pain and recovery.
• Adopt a biopsychosocial approach to rehabilitation.
• Ensure athletes understand that various factors beyond muscle strength alone influence knee pain and function.

Monitoring and Adaptation

• Regularly assess progress and adapt exercise programs.
• If available, use objective measures to monitor muscle strength.

Clinicians must base exercise prescriptions and rehabilitation strategies on current research and evidence rather than prevailing myths. Furthermore, they must be cautious about claims that specific positions or co-contractions can isolate the VMO, as evidence does not consistently support these methods.

Conclusion

Substantial evidence supports the hypothesis that the VMO cannot be isolated during knee extensions or any knee exercise. The anatomy and innervation of VMO means that it cannot be completely isolated from other quadriceps muscles. Practical implications for rehabilitation should focus on comprehensive quadriceps strengthening and neuromuscular training to enhance overall knee stability and function.

References

1.    Physiotherapy Theory and Practice, 25(2):69–98, 2009
2.    Physical Therapy, 75: 672–683
3.    Physical Therapy in Sport 7: 87–92
4.    Journal of Sport Rehabilitation 11: 120–126
5.    www.clinbiomech.com/article/S0268-0033(99)00089-3/abstract#articleInformation
6.    J Funct Morphol Kinesiol. 2020 Dec; 5(4): 98
7.    J Orthop Sports Phys Ther 2018;48(7):519–522
8.    J Sport Rehabil. 2021 Sep 1;31(6):707-716
9.    The Open Rehabilitation Journal, 2012, 5, 1-7 
10.    Journal of NeuroEngineering and Rehabilitation 2006, 3:13