The purpose of this cross-sectional and comparative study is to investigate the impact of smartphone addiction on the cervical posture, musculoskeletal system, balance, and tongue pressure in individuals diagnosed with Rheumatoid Arthritis (RA) and Ankylosing Spondylitis (AS), and to compare these findings with a healthy control group. Smartphone addiction has become a major contributing factor to postural alterations such as Text Neck Syndrome. This study aims to evaluate and compare the Craniovertebral Angle (CVA), neck extensor muscle strength, Pressure Pain Thresholds (PPT) of the neck and shoulder, Single-Leg Stance (SLS) static balance, and tongue pressure among three distinct groups. A healthy control group will be included as a reference baseline to differentiate the postural and functional deterioration caused by rheumatological diseases from the additional mechanical load imposed by smartphone dependency. To eliminate the misleading effects of acute pain, joint swelling, and severe functional limitations during flare-ups, only patients with stable disease activity (DAS28 < 3.2 for RA and BASDAI < 4 for AS) will be enrolled. This approach aims to examine the pure biomechanical relationship between technological dependency and cervical functions, independent of active systemic inflammation.

This study is designed as a cross-sectional, comparative, and correlational investigation conducted at the Necmettin Erbakan University Faculty of Medicine Hospital, Rheumatology Outpatient Clinic. The study population will consist of three groups, each comprising 37 participants (Total N = 111): the Rheumatoid Arthritis (RA) group, the Ankylosing Spondylitis (AS) group, and a Healthy Control (HC) group. Participants will be recruited using consecutive sampling according to their eligibility.

Following the written informed consent process in accordance with the Declaration of Helsinki, a single-session comprehensive assessment protocol will be conducted by an experienced physiotherapist. No therapeutic interventions or follow-up procedures will be performed.

The structured evaluation protocol will be executed in the following chronological order:

1. Sociodemographic and Descriptive Information Form: Participants will be questioned regarding age, sex, height, body weight, marital status, educational level, smoking habits, medical history, disease duration, active medications, sports participation, daily smartphone/computer/tablet usage duration, and habitual posture/ergonomics during technology use.
2. Neck Disability Index (NDI): Symptoms and functional limitations will be evaluated using the Turkish validated version of the NDI (Aslan & Karaduman, 2009). Scoring ranges from 0 to 50, where higher scores indicate greater functional disability.
3. Smartphone Addiction Scale-Short Version (SAS-SV): Technological dependency will be assessed using the Turkish validated version of the SAS-SV (Noyan et al., 2015). Cut-off values of 31 for males and 33 for females will be utilized to determine addiction status.
4. Craniovertebral Angle (CVA) Measurement: To objectively assess forward head posture, goniometric measurement will be performed. Anatomical reference points (the spinous process of the C7 vertebra and the tragus of the ear) will be marked. The goniometer center will be aligned with C7, with the stationary arm perpendicular to the ground and the movable arm aligned with the tragus. The average of three measurements with 2-minute rest intervals will be recorded. A CVA below 50° indicates Text Neck/forward head posture.
5. Neck Extensor Muscle Strength Test: While the participant sits upright, a digital handheld dynamometer probe will be placed at the occipital protuberance. Participants will perform a maximal isometric contraction pushing backward against the probe. The average of three trials with 30-60 second rest intervals will be recorded in Newtons (N).
6. Single-Leg Stance Test (SLST): Static balance performance will be evaluated on the non-dominant leg (stabilizing limb). The test will be performed three times with eyes open and three times with eyes closed, with a ceiling effect set at 60 seconds. The best scores will be recorded in seconds.
7. Pressure Pain Threshold (PPT) via Algometry: A digital algometer will be applied perpendicularly to the midpoint of the upper trapezius muscle and the suboccipital region. Pressure will be increased until the participant experiences the first sensation of pain. Two or three measurements will be averaged with 1-minute intervals to ensure reliability.
8. Tongue Pressure Assessment via IOPI: To evaluate the motor control of swallowing and suprahyoid muscle activity, the Iowa Oral Pressure Instrument (IOPI) will be utilized. An air-filled tongue bulb will be placed on the midline of the hard palate. Participants will press the bulb maximally against the palate for 5 seconds. The maximum value of three trials with 2-minute rest intervals will be recorded in Kilopascals (kPa).

Statistical analyses will be conducted to evaluate the negative correlations between smartphone addiction and functional parameters (CVA, muscle strength, PPT, balance, and tongue pressure), positive correlations with NDI, and to compare the physical deterioration between the clinical groups and healthy controls.

Inclusion Criteria:

• Aged 18 to 65 years.
• Using a smartphone for at least 2 hours daily.
• Able to understand and follow postural commands during clinical measurements.
• For the Rheumatoid Arthritis (RA) Group: Diagnosed with RA by a rheumatologist, with a stable disease activity score (DAS28 < 3.2).
• For the Ankylosing Spondylitis (AS) Group: Diagnosed with AS by a rheumatologist, with a stable disease activity score (BASDAI < 4).

Exclusion Criteria:

• History of prior surgical intervention in the cervical region.
• Presence of neurological diseases (e.g., Parkinson's disease, ALS).
• Severe visual or hearing impairment.
• Vestibular system disorders.
• Severe limitation in jaw opening.
• Presence of acute neck trauma or malignancy.
• Lower extremity amputation or advanced arthritis that prevents performing the balance test.
• Active use of muscle relaxants or advanced sedative medications.

The study population consists of adult patients diagnosed with Rheumatoid Arthritis (RA) and Ankylosing Spondylitis (AS) who are actively followed at the Rheumatology Outpatient Clinic of Necmettin Erbakan University Faculty of Medicine Hospital. Additionally, age-matched healthy volunteers recruited from the community with no systemic, neurological, or chronic musculoskeletal disorders will serve as the control group. All participants must fulfill the specific age criteria (18 to 65 years) and technological dependency baseline (using a smartphone for at least 2 hours daily).

To ensure participant confidentiality and in accordance with the informed consent approved by the institutional ethics committee, individual participant data will not be shared with third parties or made publicly available outside of the primary research team.