Objective To determine the factors affecting the performance of the deep cervical flexors (DCFs) in young people using smartphones. Methods Eighty-six individuals were enrolled in this study. Joint position sense errors, forward head and protracted shoulder posture, DCFs, and the superficial cervical muscles were evaluated. Smartphone Addiction Scale, State-Trait Anxiety Inventory, and International Physical Activity Questionnaire were used. Results Regression analysis indicated that smartphone addiction and superficial cervical muscle strengths affect the performance of DCFs independently (p < 0.05). SAS was correlated with JPSE-flexion (r = 0.408), DCF activity/performance score (r = -0.453/r = -0.431), forward head posture (r = -0.412), and cervical flexor/extensor muscle strengths (r = -0.313/r = -0.319). Conclusion Smartphone addiction and cervical superficial muscles are related to the performance of the DCFs in young people. Using smartphones causes impaired joint position sense in flexion, decreased DCF activity, and forward head posture.