The present study examined the cervical CES FRR in patients with cervical lateral spinal stenosis compared with asymptomatic controls.
Of note, FRR is a quantitative measure for evaluating FRP in ES muscles as well as a practical and reliable method for reflecting on the functional pattern of these muscles in patients with neck pain (16). Accordingly, the study results showed that cervical FRR in patients with cervical lateral spinal stenosis was significantly lower than that in asymptomatic controls, which was consistent with the findings of previous studies on neck pain (16, 17, 25). Murphy et al. had further observed a significant drop in mean FRR in patients with chronic neck pain compared with controls, and had reported the high reliability of the results and differences between groups, suggesting that FRR was a good indicator of the changes in neuromuscular function in patients with neck pain (16). Shamsi et al., comparing the FRR of the CES muscles and the upper trapezius of patients with non-specific neck pain and healthy individuals, had similarly observed that the FRR in patients was lower than that in healthy individuals, but there was no significant difference in the upper trapezius muscles (25). In fact, the fall in FRR may occur due to the elevated activity of ES muscles in the state of full flexion or a rise in the activity of these muscles during re-extension of the full flexion. According to Maroufi et al., boosting the activity of the CES muscles in the full flexion phase could have a significant effect on reducing FRR in patients with chronic neck pain compared with controls (17). Decreased FRR in patients with cervical stenosis in this study may also indicate the changes in neuromuscular function and muscle activity pattern. One possible hypothesis to explain the reduced FRR in these patients is no relaxation of the cervical superficial ES muscles in the full flexion phase, attributed to the increased activity of these muscles to stabilize the neck following a decline in the deep contractile units of muscle fiber after increased fat infiltration in the state of nerve root compression (14, 26). To the best of the authors' knowledge, this study was one of the first attempts to investigate FRR in patients with cervical lateral spinal stenosis, so further research is needed to evaluate other features of FRP.
As recommended in the study by Dulcina et al., ∆FRR was used to compare the asymmetry of the cervical FRR between the two groups (20) and the results showed that the asymmetry of CES FRR in patients with cervical lateral spinal stenosis was significantly higher than that in asymptomatic controls. Dulcina et al. had similarly measured asymmetric FRR in the ES muscles and superficial lumbar multifidus in patients with non-specific low back pain, and had observed that the ∆FFR of the ES muscles in patients was higher than that in controls, and stated that asymmetric FRR could cause unilateral overactivity of the lumbar ES (20). Previous studies had also reported muscle atrophy associated with cervical myelopathy and radiculopathy (11, 13, 14, 27, 28). Besides, Fortin et al. had reported more asymmetry in the cross-sectional area of the paraspinal muscles below the compression level in patients with degenerative cervical myelopathy, and had reported that elevated asymmetry was associated with reduced functional scales (29). Therefore, the changes in muscle morphology could shape muscle function in patients with nerve compression syndrome in the cervical region (14, 29). In other words, the asymmetry in atrophy can be associated with that in muscle activity since muscle atrophy is related to a reduction in the basic contractile units of muscle fiber. In addition to the inter-group comparison, an intra-group comparison was also performed in the patient group between the FRR of the involved and non-involved sides, and it was revealed that the FRR of the involved side was significantly lower than that of the non-involved one. In line with the study results, Shamsi et al. had reported significant differences between FRR on the left and right sides in a group with non-specific neck pain, and had suggested that asymmetric FRR could induce unilateral overactivity of the CES muscle, and consequently, persistent pain (25). In another study, Park et al. had demonstrated that the pattern of muscle activity on the painful side could change more in patients with unilateral posterior neck pain, and the asymmetry in pain could affect that of muscle activity (30). An imbalance in bilateral paracervical muscle activity had been further reported during neck endurance testing in patients with cervical radiculopathy (31). Therefore, the existence of asymmetry in the pattern of muscle activity and FRR are made possible due to the one-sided symptoms and pain in the patients participating in this study. However, it is not yet clear whether this asymmetry comes from the effect of unilateral symptoms or it is the cause. In general, it may be effective in perpetuating or reversing pain due to unilateral muscle overactivity.
The neck ROM, as one of the components studied in patients with neck pain (32, 33), was also examined in this study, and the results showed that the neck extension ROM was significantly lower in patients with cervical lateral spinal stenosis than that in the controls. Other studies had further reported significant differences in the cervical total ROM of the sagittal plate in patients with cervical myelopathy (34, 35). The changes in the neck ROM can be thus assumed as one of the disruptive factors in neck biomechanics. In this respect, Yeo et al. had detected a descending trend in FRR in a group with forward head posture, and had concluded that fatigue in the ES muscles may be a factor in changing FRR (22). Mousavi-Khatir et al. had also studied the effect of static neck flexion on FRP in healthy individuals, and had observed that the FRP parameters such as FRR, neck flexion angle, and amplitude of ES muscle activity had changed after 10 minutes of keeping the head in the flexion position (19). Of note, patients with spinal stenosis avoid extension movements to prevent pain, irritation, and exacerbation of symptoms, and typically tend to maintain the flexion posture (1, 8). As suggested by Mousavi-Khatir et al., maintaining neck flexion in the long run may change the length-tension characteristics, reduce the stiffness of passive viscoelastic tissues, and ultimately give rise to compensatory cervical muscle activity (19). Accordingly, it can be assumed that the changes in the neck ROM in patients with cervical lateral spinal stenosis can be a factor in changing their muscle activity pattern.
This study had several limitations. Although there was no significant difference between the contextual variables, due to the attrition in the number of volunteers for their reluctance to participate in the study following the pandemic, coronavirus disease 2019, some interfering conditions such as occupation and socioeconomic status of the participants were not taken into account. As well, only patients with mild-to-moderate spinal stenosis were evaluated in this study, and the cases with severe spinal stenosis were not included for ethical considerations. Therefore, more studies are needed to clarify the relationship between spinal stenosis severity and FRR.
The study findings also revealed that FRR and its asymmetry were different in patients with cervical lateral spinal stenosis compared with those in asymptomatic individuals, indicating the changes in FRP in these patients, so further research is required to investigate other features of FRP as well as the presence or absence of FRP in patients with cervical lateral spinal stenosis. In addition, knowing about the changes in the pattern of cervical muscle function in patients with cervical spinal stenosis is effective in planning rehabilitation practices and treatments to maintain the correct function pattern of the muscles in this area and prevent pain reversibility, along with other procedures to relieve pressure on the spinal cord and the nerve roots. Therefore, more studies are needed to determine the relationship between cervical spinal stenosis, the changes in muscle activity pattern, and the effect of rehabilitation exercises on it in patients with cervical lateral spinal stenosis.