Hip OA Alters Brain Function and Muscle Activation

“Focused and functional exercises may be more effective in this group than focusing on strength.”

It may be that impairments to the neurophysiological drive of a muscle could actually be the problem, as voluntary muscle drive strongly predicts muscle strength⁽⁶⁾. Thus, maximal contraction efforts in people with knee OA and anterior cruciate ligament rupture do not recruit all motor units, resulting in lower force being production by the muscle.

Both voluntary activation and maximal strength are strongly associated with an excessive level of intracortical inhibition from the brain^(7-9). This essentially means that the brain is working very hard to reduce the amount of descending neural drive to muscles. However, despite extensive research in knee osteoarthritis, it was previously unknown whether these impairments are present in other body regions, such as hip OA. 

Objective and methods 

Finding an answer to this gap in the literature is important because there are clear clinical implications if people with hip OA also have impairments in their maximal voluntary activation. Therefore, the primary aim of our study was to evaluate quadriceps voluntary activation, corticospinal excitability, and intracortical excitability in people with and without hip OA. We also evaluated how maximal voluntary activations were associated with intracortical excitability and hip-related pain.

We performed a cross-sectional case-control study to answer our research question. Our study included 41 participants, of which 17 had hip OA (76% female sex) and 24 did not (92% female sex). The participants had a mean (standard deviation) of 58.7 (7.9) years of age, and all participants were Australian residents. We used supramaximal electrical stimulation of the femoral nerve to evaluate voluntary activation of the quadriceps, transcranial magnetic stimulation to evaluate intracortical excitability (both inhibition and facilitation), and the pain subscale of the hip and groin outcome score (HAGOS) to quantify hip related pain. We performed this experiment in the quadriceps as a muscle group essential to optimal lower limb function. While we also recognize the importance of gluteal muscle function in hip OA, it was impossible to evaluate the voluntary activation of the gluteal muscles due to positioning and device set-up.

Results

Our study showed that people with hip OA demonstrated lower quadriceps voluntary activation than controls, which may be partly driven by greater intracortical inhibition (but not facilitation) due to an association between these two variables.

Interestingly, people with hip OA demonstrated higher motor cortex facilitation than controls, which was directly associated with higher hip-related pain (as measured using validated patient-reported outcome measures). Therefore, this research suggests that hip OA may influence the intracortical network excitability of the motor system.

Based on prior meta-analyses, the deficit in voluntary quadriceps activation is not as severe as in knee OA or anterior cruciate ligament reconstruction but worse than controls (see figure 2). Overall, participants with hip OA demonstrated a poorer ability to activate quadriceps. However, the observed group deficit was not as large as that seen with knee OA⁽⁴⁾. 

“…motor cortex excitability is altered in these patients.”

Clinical Implications

Our findings of lower quadriceps voluntary activation and higher motor cortex facilitation in people with hip OA indicate that motor cortex excitability is altered in these patients. This may reduce the efficacy of an exercise program in producing meaningful changes in strength, function, and pain. Our findings are particularly alarming considering that the quadricep muscle group is not a prime mover of the hip, with only rectus femoris crossing the joint.

Traditionally, clinicians designing an exercise program for people with hip OA may not consider targeting the quadricep muscle group. Our findings indicate that addressing quadriceps strength and voluntary activation may be an important component to include in rehabilitation. Focused and functional exercises may be more effective in this group than focusing on strength. There may be novel ways of harnessing neuroscience that we can incorporate into rehabilitation to overcome this automatic muscle inhibition and more effectively rehabilitate our patients.

References

1. Journal of Science and Medicine in Sport 2023;26(4-5):253-60.
2. Osteoarthritis and Cartilage 2020;28(12):1501-13. 
3. Journal of Orthopaedic Research 2022;40(5):1135-42. 
4. EFORT Open Rev 2023;8(12):883-94.
5. Sports Health 2019;11(2):163-79. 
6. The Knee 2012;19(6):939-43.
7. Experimental Brain Research 2014;232(12):3991-9. 
8. Sports Medicine and Health Science 2024; Available online first 
9. Scientific Reports 2021;11(1):24011.