Effects of Cervical Flexion on the Flexion-relaxation Ratio during Smartphone Use.

[Purpose] The purpose of this study was to measure the cervical flexion-relaxation ratio (FRR) and intensity of neck pain and identify the differences according to postures adopted while using smartphones. [Subjects] Fifteen healthy adults with no neck pain, spinal trauma, or history cervical surgery participated in this study. [Methods] The activity of the cervical erector spinae muscle was recorded while performing a standardized cervical flexion-extension movement in three phases (flexion, sustained full flexion, extension). And neck pain intensity was recorded using a visual analog scale (VAS) with values between 0 and 10. Postures held while using a smartphone are distinguished between desk postures and lap postures. The FRR was calculated by dividing the maximal muscle activation during the extension phase by average activation during the complete flexion phase. [Results] No significant differences were found in the FRR between desk posture, lap posture, and baseline, though the intensity of the neck pain increased in the lap posture. [Conclusion] The FRR could be a significant criterion of neuromuscular impairment in chronic neck pain or lumbar pain patients, but it is impossible to distinguish neck pain that is caused by performing task for a short time. Prolonged lap posture might cause neck pain, so the use of smartphones for a long time in this posture should be avoided.