This observational study aims to investigate the relationship between vitamin D deficiency, postural stability, and neuromuscular control mechanisms in adults. The study will compare individuals with vitamin D deficiency and healthy controls.

Participants will undergo a single-session evaluation including surface electromyography and postural stability assessments during standing tasks under different loading and vibration conditions. No invasive procedures, medications, or blood sampling will be performed. Previously obtained routine laboratory results will be used for vitamin D status.

The study is designed to improve understanding of neuromuscular responses associated with vitamin D deficiency and to contribute to future rehabilitation strategies.

This single-center observational study is designed to evaluate postural stability and load-sensitive neuromuscular control mechanisms in adults with vitamin D deficiency.

Participants will be divided into two groups based on their vitamin D status: individuals with vitamin D deficiency and healthy controls. Vitamin D levels will be determined using previously obtained routine laboratory test results. No additional blood sampling will be performed for this study.

All participants will undergo a standardized assessment protocol in a single session. Surface electromyography will be used to record muscle activity during standing tasks under different postural and loading conditions. Measurements will be performed during single-leg and double-leg stance, with and without vibration stimulation. The procedures are non-invasive and do not involve any pharmacological intervention.

Postural stability parameters and neuromuscular response variables will be analyzed and compared between groups. The primary objective is to identify potential differences in reflex modulation and postural control associated with vitamin D deficiency.

The study aims to provide objective data on neuromuscular adaptations related to vitamin D status and to support future research in rehabilitation and balance training.

Inclusion Criteria:

• Age 20 to 40 years.
• Ability to provide written informed consent.
• Vitamin D deficiency group: serum 25-hydroxyvitamin D level <10 ng/mL measured within the last 3 months (routine clinical testing).
• Healthy control group: normal vitamin D status based on routine clinical testing within the last 3 months.
• No use of vitamin D or calcium supplementation within the last 3 months.

Exclusion Criteria:

• Known metabolic bone disease other than vitamin D deficiency.
• Neurological disorder (central or peripheral) or neuromuscular disease.
• History of lower-limb surgery, fracture, or prosthesis affecting standing balance.
• Vestibular disorder.
• BMI >30 kg/m2.
• Active malignancy.
• Severe psychiatric disorder.
• Skin condition preventing placement of surface EMG electrodes.

Adults aged 20 to 40 years recruited from the outpatient clinics of Istanbul Physical Medicine and Rehabilitation Training and Research Hospital. Participants will include individuals with laboratory-confirmed vitamin D deficiency and healthy volunteers with normal vitamin D status.

De-identified individual participant data will be shared upon reasonable request for academic and research purposes, after publication of the main results. Requests will be reviewed by the principal investigator and data will be provided under a data use agreement.

Supporting documents and de-identified individual participant data will be made available upon reasonable request after publication of the main results. Requests will be reviewed by the principal investigator and shared for academic purposes under a data use agreement.

Access to de-identified individual participant data and supporting documents (study protocol, statistical analysis plan, and informed consent form) will be granted to qualified researchers upon reasonable request for academic and non-commercial purposes. Requests will be reviewed by the principal investigator. Data will be shared through secure electronic transfer under a data use agreement.