Individuals With Chronic Neck Pain Have Lower Neck Strength Than Healthy Controls

??Chronic pain has serious effects on people’s lives, reducing the quality of life, impairing performance in daily activities, causing psychological illness, and even impairing performance at work. For this reason, it is crucial to identify the functional changes that chronic neck pain can cause and to develop better treatments and intervention actions.

??Despite the interference of methodological variability related to neck strength assessment, individuals with chronic neck pain have lower neck strength than healthy controls for flexion, extension, and lateral flexion.

??Considering the main forces involved during the neck strength assessment, and considering the isometric assessment for the extension movement, for example, there must be an equality between the extensor moment

(Me), produced by the neck extensor muscles, and the flexor moment, produced by the resistance of the instrument, for example, by a handheld dynamometer (F) and by the weight of the head (W).

??Subsequently, if the moment arm of the force decreases as the dynamometer approaches the rotational axis, our equality between the moments will only be achieved if the value of the force F increases to maintain the isometry (because W is constant); consequently, we will acquire a greater measure of force than recorded by the dynamometer.

?The greatest neck strength is shown in the extension movement. This greater strength can be attributed to the greater internal moment generated by the extensor muscles,

resulting from a greater moment arm, in a neutral head position, and greater physiological cross-sectional area of the extensor compared with the flexor muscles.

??Moreover, the greatest difference between the chronic neck pain and healthy control groups was found for flexion and extension movements. The reduced neck strength of the chronic neck pain group is similar for all movements; clinically, strength gains can improve the symptom pain and, consequently, neck exercises may also improve the quality of life and disability level of these individuals.

??In this way, rehabilitation professionals should be aware of the need to increase neck strength for all planes of movement of the neck because the reduction of neck strength is associated with individuals with chronic neck pain.

??Several mechanisms may explain this reduction of neck strength for individuals with chronic neck pain compared to healthy control participants.

The 1??st mechanism can be explained with a behavioral analysis of pain. There is evidence that, because of the mechanisms of fear of

movement (kinesiophobia), the presence of chronic neck pain may cause individuals to avoid daily activities, which may lead to a decrease in neck strength due to disuse of the neck musculature. Chronic pain could then lead to a vicious cycle where a catastrophizing pain would lead to kinesiophobia, hypervigilance of the region, disuse, and ultimately more pain and more chronicity.

??Another possible mechanism is due to morphological changes in which there is a reduction of the physiological cross-sectional area of the neck musculature in individuals with chronic neck pain, perhaps for the same reasons of disuse owing to kinesiophobia.

??This reduction of the physiological cross sectional area of the neck musculature can cause lower neck strength, since the cross-sectional area is one of the determining factors of muscle strength.

??In addition, the reduction in the cross sectional area is associated with a reduction in the number of parallel sarcomeres, the structures responsible for the capacity to produce muscle strength.

??Another possible biomechanical mechanism for this lower neck strength can be credited to changes in the electromyographic activity of the neck muscles.

??Although these electromyographic changes do not directly represent muscle strength, they may represent muscle imbalance and consequently, lower neck strength.

??The reduction of the neck strength may also be related to neurophysiological aspects. Because the nociception caused by dysfunctions in the neck region alter the excitatory threshold of the mechanoreceptors, and the neck area of the spine contains a high concentration of mechanoreceptors, it is possible to affirm that changes in the mechanoreceptors owing to the nociception can lead to a change in the sensorimotor integration and, consequently, lower neck strength.

??Regardless of the mechanisms that may have caused the reduction of neck strength, recently, reseachers encourage the evaluation of strength in clinical practice because it might have a reduction of strength. Therefore, considering this possibility, it is fundamental to evaluate and manage these deficits, regardless of which muscle group is affected.