Association between neck muscle coactivation, pain, and strength in women with neck pain
From: Man Ther. 2011 Feb;16(1):80-6
Chronic neck pain is a common musculoskeletal disorder. Epidemiological studies show a lifetime prevalence of neck pain between 43% and 66.7%, a one-year prevalence rate which ranges between 17.9% and 64%, and a point prevalence around 20%. Neck pain is also associated with a high recurrence rate and, subsequently, high economic costs.
Altered activation of the neck muscles is a well-known feature of neck pain. Patients with neck pain show increased antagonistic activity of their superficial neck muscles. Reduced specificity of sternocleidomastoid muscle activation was observed in patients with neck pain when performing isometric contractions with continuous change in force direction in the range 0–360°, resulting in increased activation of the muscle when acting as an antagonist. This result supports the consistent finding of augmented activity of the superficial neck muscles in patients with neck pain. These observations are also in agreement with experimental pain studies which show a pain induced reorganization of the motor strategy characterized by reduced activity of the agonist muscle and increased activity of the antagonist muscle. Possible explanations for these findings include the direct effects of nociception on motor neuron output, effects of pain on sympathetic activity, and changes in motor planning.
Although increased coactivation of the neck muscles may be beneficial in the presence of acute pain to enhance cervical stability by reducing velocity and range of movement, it may reduce neck strength and contribute to recurrent pain by altering the load distribution on the spine and irritating pain sensitive structures. However, the relationship between neck muscle coactivation, strength and pain intensity is unknown. Therefore, the purpose of this study is to investigate the relationship between the extent of neck muscle coactivation, the maximum amount of neck strength and perceived pain and disability in women with persistent neck pain.
This study showed that patients with neck pain have higher levels of coactivation of the sternocleidomastoid and splenius capitis muscles compared to control subjects. Furthermore, increased coactivation of the splenius capitis muscle was associated with lower neck strength and higher levels of pain and associated disability.
Patients with neck pain showed an overall reduction of neck strength. Maximum strength was 31.7%, 22.6%, 33.2%, and 32.2% less for the patients than controls for flexion, extension, right lateral flexion and left lateral flexion, respectively, resulting in a total neck strength which was 29.2% lower for the patient group. The strength loss found in this study is similar to the results of several previous studies. However, there is a large variability in the results reported in the literature and other studies showed greater reductions. Pearson et al. found a 52% reduction in force in patients with neck pain for flexion, 60% for left lateral flexion, 62% for right lateral flexion, and 66% for extension and Prushansky et al. reported a 90% reduction of neck force in all directions. Variability of neck strength in patients is presumably due to differences in patient populations (pain intensity, duration, cause of neck pain), but may also reflect varying degrees of neck muscle coactivation, which was investigated in this study. Furthermore the general physical activity level of the patients may influence strength measurements and may partially account for the reduced force output compared to control subjects.
The sternocleidomastoid and most extensor muscles have well-defined preferred directions of activation in healthy subjects, which was confirmed in this study. It has been shown that the directional specificity of sternocleidomastoid muscle activity is reduced in patients with neck pain and is associated with reduced modulation of sternocleidomastoid motor unit discharge rate with multidirectional force contractions. The results of the current study show that reduced specificity of activity is not unique to the sternocleidomastoid, since similar observations were made for the splenius capitis muscle. Reduced specificity of muscle activity results mainly in increased activation of the muscle when acting as an antagonist and is consistent with the increased antagonist activation of the extensors during the ramped contraction. This suggests that increased antagonist activity is a common feature associated with neck pain. Furthermore, the results of this study show that increased levels of neck extensor antagonistic activity are associated with impaired neck strength.
Increased neck muscle coactivation likely reflects reorganization of the motor control strategy potentially to enhance cervical spine stability. Coactivation of the neck flexor and extensor muscles is considered a normal strategy to increase stiffness of the spine, for example when a postural perturbation is applied to the trunk. Up to 80% of cervical spine stability is provided by the surrounding muscles, especially the deeper muscles with their smaller moment arms and attachments to adjacent vertebrae. Weakness of the deep cervical flexor muscles is known to be present in patients with neck pain and is associated with increased activity of superficial synergists. Increased coactivation of neck muscles may therefore indicate an attempt to enhance the stability of the neck. Fear of pain during maximal contractions (kinesiophobia) may also induce a strategy of increased neck muscle coactivation. Fear of pain during neck extension is thought to limit extensor force in patients with whiplash-induced neck pain who often have injured posterior cervical spine structures. However, a non-significant correlation between kinesiophobia and neck strength, and between pain catastrophizing and neck strength was shown in patients with chronic whiplash. Increased sternocleidomastoid and splenius muscle activity for the patient group may also be due to an increased sympatho-adrenal outflow as a consequence of pain. Increased activity of the sternocleidomastoid and splenius muscle has been observed in healthy volunteers following physiological sympathetic activation elicited by the cold pressor test.
Although increased coactivation of the neck muscles may be beneficial in the presence of pain to increase cervical stability, as observed in this study, it is associated with functional consequences, i.e. reduced neck strength. Furthermore, increased neck muscle coactivation may contribute to recurrent pain by altering the load distribution on the spine and subsequently aggravating the patients’ condition. Coactivation of agonist/antagonist muscles significantly increases spinal stiffness and spinal compression which is considered sufficient to induce lumbar spine injuries and consequently low-back pain and may also be relevant in persistent neck pain disorders. Unique to this study, the authorswe showed that the degree of coactivation of the splenius capitis muscle is positively correlated with the patients’ reported pain and perceived disability which supports this premise. Surprisingly, a similar relation was not observed for the sternocleidomastoid muscle despite reduced specificity of sternocleidomastoid activity and increased activation of the sternocleidomastoid muscle during the ramped cervical extension contraction in the patient group. This finding may be attributed to the greater reduction in neck flexion strength for the patient group (31.7% less than controls) compared to extension (22.6% less than controls). That is, increased muscle coactivation may particularly occur in the directions of least strength.
Patients with neck pain have higher levels of coactivation of the sternocleidomastoid and splenius capitis muscles. Furthermore, increased coactivation is associated with reduced neck strength and higher levels of pain and associated disability. These observations indicate a relation between alterations in neuromuscular control of the cervical spine in patients with neck pain and functional consequences including impaired motor performance and increased levels of perceived disability.