Hip OA Alters Brain Function and Muscle Activation

South Korea’s Chaeyeon Kim performs during the women’s free skating REUTERS/Tyrone Siu

Musculoskeletal injury and pain have a substantial burden to both the person living with the condition and society via the economic costs related to healthcare. However, despite thousands of publications related to musculoskeletal pain, these conditions remain incredibly difficult to manage. Osteoarthritis (OA) is a typical example of a common musculoskeletal condition that causes significant pain and loss of function for the patient and high healthcare costs. Despite the pathophysiology of OA being largely understood, it still has a massive personal and societal burden. One intervention advocated for use in OA is exercise rehabilitation, and there is a plethora of evidence to support its efficacy. One of the main mechanisms by which exercise rehabilitation is purported to work is that by increasing muscle strength, the load going through the joint is reduced.

This model of resistance training leading to improvements in strength appears feasible as we know that in healthy populations, resistance training improves muscle size, quality, and function. However, there is little evidence that strength in pathological populations improves with resistance training, and strength gains obtained from resistance training often do not explain improvements in pain and symptoms^(1-3). Therefore, the role of strength is uncertain. It may be that strength changes are actually not associated with pain improvement in people with musculoskeletal pain. Alternatively, it may be because exercise programs that have been evaluated have resulted in little to no improvement in strength in the participants, so any benefits of strength cannot be evaluated.

Some evidence that may support the latter argument is that people with pain and injury, such as knee OA or anterior cruciate ligament rupture, have substantial changes in the neurophysiological drivers of muscle strength (specifically impaired voluntary activation)^(4,5). Voluntary activation is the maximal amount of muscle a person can voluntarily activate. It is evaluated by performing a maximal voluntary isometric contraction and recording the force. Following this, a maximal voluntary isometric contraction is performed. During maximal contraction, the contraction is superimposed with a supramaximal electrical stimulation to the femoral nerve to determine ‘maximal’ non-voluntary muscle activation (see figure 1).