A study characterizing the functional role of the RCP muscles is relevant because FI of the RCP muscles has been associated with chronic headache.
6 Osteopathic physicians often target the RCP muscles when using cervical techniques, such as suboccipital release, to treat patients presenting with head and neck pain.
Activation patterns of RCPM muscles on EMG were found to be similar to SC muscles, implying that RCPM muscles function as extensors. No significant difference was seen between EMG activity of RCPm muscles during flexion and extension of the head when an external force was applied to the forehead or the back of the head. This finding suggests that RCPm muscles cannot be classified as a flexor or as an extensor. Therefore, the RCPm muscles may function as a stabilizer of the OA joint. However, these findings do not rule out the possibility that RCPm muscles may also function to maintain normal levels of tension on the dura mater to prevent infolding both during flexion and extension movements of the head and neck.
5,23
The myodural bridge is a connective link between the RCPm muscles and the dura mater.
24-26 One study
27 reported the existence of this connection and added there is also a similar connection between the RCPM muscles and the dura mater. The RCP muscles may function to prevent or limit infolding of the pain-sensitive dura mater during normal movements of the head.
5,25 Fatty infiltration of RCPm muscles on magnetic resonance imaging has been reported in patients who experience chronic headache, with the progression of FI over time directly related to pain and disability.
6 In general, FI results in a decrease in RCPm muscle mass accompanied by a decrease in patients’ ability to generate normal levels of force, and it would compromise their ability to maintain normal levels of tension within the myodural bridge.
27 The FI could result in clinical manifestations, such as head and neck pain, as a result of dural infolding during daily activities
5 and could explain the positive effects that some osteopathic manipulative treatment techniques have in managing craniocervical disorders.
26 The fiber orientation of the myodural bridge is perpendicular to the spinal dura and the RCPm muscles, and it results in a posterior displacement of the dura mater with static loading of the RCPm muscles.
5 Our study showed that the RCPm muscles are active during both flexion and extension movements of the head and neck, which implies that a functional role of RCPm muscles may be to ensure that appropriate levels of tension are maintained within the myodural bridge to prevent infolding of the pain-sensitive dura mater.
5,23,25
A loss of RCPm muscle mass due to FI would also decrease the ability of these muscles to generate normal levels of proprioceptive feedback to the central nervous system. In the upper cervical spine, this would compromise the ability to precisely coordinate eye and head movements and respond protectively to sudden external forces.
28 The incongruence between motor output and sensory feedback could result in an exacerbation of symptoms in patients with whiplash-associated disorders.
29
Our study had limitations, such as the absence of data from rectus capitis anterior (RCA) muscles. The EMG voltages from RCA muscles might mirror the activation levels of RCPm muscles if their functional role was to increase the stability of the OA joint by helping maintain congruency of joint surfaces. Access to the RCA muscles with fine-wire electrodes was not considered to be feasible because of either an unacceptable level of risk related to hitting a critical vascular structure or because the protocol would not be tolerated by the participants. Another limitation was the size of the final data set; 25% of the data were discarded because of dislodged or noisy electrodes and implausible values.