Introduction:
General Findings in Patients with Knee OA:
Altered Muscle Activation Patterns in OA Patients
Patients with knee OA often exhibit altered muscle activation patterns, such as increased co-contraction of the quadriceps and hamstrings.2 These compensatory mechanisms aim to stabilize the knee but can lead to inefficient movement and increased joint stress. EMG studies have highlighted these patterns, providing valuable insights into the biomechanical challenges faced by these patients.
Use of EMG Data in Bracing
EMG data has shown that effective knee braces can normalize muscle activation patterns.3 For instance, a well-designed brace can reduce abnormal co-contractions, thereby decreasing joint stress and improving overall knee function—this normalization is critical in managing OA symptoms.
EMG data has revealed that effective knee braces can significantly alter muscle activation in key muscle groups such as the quadriceps and hamstrings muscles.4,5 These improved muscle activation levels indicate that braces can effectively offload the knee joint by reducing the force exerted by certain muscle groups and, consequently, the stress on the joint. This reduction is crucial for alleviating pain and preventing further joint damage, as it demonstrates that the brace is successfully redistributing forces away from the affected areas.
In summary, by analyzing EMG data, clinicians can confirm that a knee brace is decreasing joint forces by assessing the muscle activity of relevant muscle groups before and after the application of a brace.
Outcomes From the Steadman Clinic Study on the Ascender
- When the Ascender knee brace was worn with tension, there was a significant reduction in EMG activity in the quadriceps muscle group. This is illustrated in the data, where EMG activity was noticeably lower in the vastus medialis, vastus lateralis, and rectus femoris in all three trials.
- Skiers achieved much higher knee flexion angles during the trials with the Ascender’s mechanism on, even with the decrease in EMG activity.
Reduced EMG Activity in the Quadriceps
The stress in the patellofemoral joint is largely defined by the forces generated by the quadriceps muscles.6 This makes sense based on our understanding of the forces influencing the patella (see last section of this paper).7 A reduction in the quadriceps’ EMG activity, which is representative of a reduction in its pulling force, is an indicator for reduced patellofemoral joint pressure.
This reduced joint pressure is a game-changer for patellofemoral OA patients, and cannot be accomplished in the same way by traditional offloaders because they lack an extension-assisting force.
Increased Knee Flexion
Notably, the tensioned Ascender brace allowed for greater knee flexion angles in all 3 trials—an indicator of improved knee function and real-world mobility.
This is great news as a standalone outcome of the study—however, it’s important to note that this occurred in conjunction with a reduction in EMG activity. Typically, increased knee flexion angles have been known to increase the quadriceps’ pulling force.8 The contrary was observed in this study with the Ascender knee brace.
In summary, despite the increased knee flexion, the quadriceps activation did not increase—suggesting that the Ascender can improve knee mobility while simultaneously reducing patellofemoral compartment pressure.
Biomechanical Explanation: The Forces Involved in the Patellofemoral Compartment
The patella acts as a pulley for the extensor mechanism, pulled upwards by the force of the quadriceps (Fq) and pulled in the opposite direction by the force of the patella tendon (Fp).
The combination of these forces generates the resultant force that pulls the patella directly into the femur (R). The magnitude of the forces in this compartment increases substantially in deep flexion. This is in part explained by the video linked below—in deep flexion, the quadriceps tendon would pull the patella more directly into the femur.
In patients with patellofemoral osteoarthritis, the application of the Ascender knee brace’s extension-assisting forces (labeled Fb,1 and Fb,2) can reduce the quadriceps’ pulling force—resulting in a significant reduction of the compressive forces in the patellofemoral compartment.
Bibliography:
- Reaz, M. B. I., Hussain, M. S. & Mohd-Yasin, F. Techniques of EMG signal analysis: detection, processing, classification and applications. Biol. Proced. Online 8, 11–35 (2006).
- Zeni, J. A., Rudolph, K. & Higginson, J. S. Alterations in quadriceps and hamstrings coordination in persons with medial compartment knee osteoarthritis. J. Electromyogr. Kinesiol. 20, 148–154 (2010).
- Ramsey, D. K., Briem, K., Axe, M. J. & Snyder-Mackler, L. A mechanical theory for the effectiveness of bracing for medial compartment osteoarthritis of the knee. J. Bone Joint Surg. Am. 89, 2398–2407 (2007).
- Gulling, L. K., Lephart, S. M., Stone, D. A., Irrgang, J. J. & Pincivero, D. M. The effects of patellar bracing on quadriceps EMG activity during isokinetic exercise. Isokinet. Exerc. Sci. 6, 133–138 (1996).
- Miller, J. P., Vailas, J. C., Croce, R. V., Confessore, R. & Catlaw, K. Dynamic Analysis of Custom-Fitted Functional Knee Braces: EMG and Brace Migration during Physical Activity. (1999) doi:10.1123/jsr.8.2.109.
- Besier, T. F., Gold, G. E., Beaupré, G. S. & Delp, S. L. A modeling framework to estimate patellofemoral joint cartilage stress in vivo. Med. Sci. Sports Exerc. 37, 1924–1930 (2005).
- The Math Behind Patellofemoral Joint-Unloading: Force Vectors Explained -. Icarus Medical https://icarusmedical.com/technical-briefings/the-math-behind-patellofemoral-joint-unloading-force-vectors-explained/.
- Becker, R. & Awiszus, F. Physiological alterations of maximal voluntary quadriceps activation by changes of knee joint angle. Muscle Nerve 24, 667–672 (2001).
- Patellofemoral Joint Motion and Patellar Tracking. (2009). https://www.youtube.com/watch?v=Q-80Qi5cx9o
