The goal of this observational study is to learn about the relationship between hamstring tightness and the lumbar lordosis angle in sedentary adults across different sitting postures. The main question it aims to answer is:

Is hamstring tightness related to the lumbar lordosis angle across three functional sitting postures (upright, slumped, and forward-leaning)? It also asks whether hamstring tightness differs between men and women, whether it differs between the dominant and non-dominant leg, and whether the lumbar lordosis angle differs between men and women across the three postures.

Participants will have their hamstring tightness measured with the Active Knee Extension test and their lumbar lordosis angle measured with a bubble inclinometer while sitting in each of the three postures.

Prolonged sitting is a defining feature of contemporary sedentary lifestyles and an established risk factor for musculoskeletal disorders of the lumbar spine. During sustained sitting, the hip is held in flexion and the hamstring muscle-tendon unit is maintained in a shortened position, which over time is associated with reduced hamstring extensibility (hamstring tightness). Because the hamstrings originate from the ischial tuberosity, a loss of hamstring extensibility exerts a posteriorly directed pull on the pelvis, promoting posterior pelvic tilt. Through regional spinopelvic coupling, this change in pelvic orientation alters the sacral slope and, consequently, the lumbar lordosis angle.

Although the relationship between hamstring tightness and lumbar lordosis has been described previously, most existing studies have evaluated it in the standing position. Sedentary individuals, however, spend the majority of their day seated, where the line of gravity, baseline muscle activity, and passive tissue demands placed on the spine differ substantially from those in standing. Evidence is therefore limited regarding how hamstring tightness relates to the lumbar lordosis angle across the functional sitting postures encountered in daily life, and regarding how this relationship may vary by sex and between the dominant and non-dominant lower limb.

This cross-sectional observational study will be conducted on 110 sedentary adults (55 male and 55 female), aged 18 to 45 years, recruited according to predefined eligibility criteria. The sample size was determined using G*Power based on a bivariate-normal correlation model. Eligible participants will be sedentary (more than six hours of sitting per day and a physical-activity level below 600 MET-minutes/week on the International Physical Activity Questionnaire), with a body mass index below 30 kg/m², bilateral hamstring tightness, and no current low back pain or other exclusionary conditions.

Following screening and informed consent, each participant's demographic and anthropometric data will be recorded, and lower-limb dominance will be determined using a three-task behavioral battery. Hamstring tightness will be assessed bilaterally using the Active Knee Extension (AKE) test, recorded with a digital goniometer, with the mean of three trials used for analysis. The lumbar lordosis angle (T12-S2) will be measured using a double bubble inclinometer in three randomized functional sitting postures-upright, slumped, and forward-leaning-with the mean of three trials per posture used for analysis.

The primary aim is to examine the correlation between hamstring tightness and the lumbar lordosis angle in each of the three sitting postures. The secondary aims are to compare hamstring tightness between male and female participants, to compare hamstring tightness between the dominant and non-dominant lower limb, and to compare the lumbar lordosis angle between male and female participants across the three postures. Data will be analyzed using Pearson correlation coefficients, independent- and paired-samples t-tests, and a mixed-design analysis of variance, with the significance level set at p < 0.05.

Inclusion Criteria:

• Healthy adults aged 18-45 years.
• Sedentary occupation requiring > 6 hours of sitting per day.
• Active Knee Extension (AKE) angle >15°.
• Bilateral hamstring tightness.
• Sedentary individual as determined by IPAQ (MET≤ 600 min/week).
• Asymptomatic for low back pain at the time of testing.

Exclusion Criteria:

• History of significant low back pain requiring medical treatment in the past 12 months.
• Previous spinal surgery or structural spinal deformities.
• Lower extremity injury or surgery within the past 6 months.
• Neurological disorders affecting sensation or movement.
• Pregnant female or get birth in the last year.
• Body mass index (BMI) ≥ 30 kg/m².
• postural deviations resulting from shortening of the iliopsoas and erector spinae or insufficiency of the core stabilizers and hip extensors

Sedentary adults of both sexes, aged 18 to 45 years, recruited from the Nasser institute hospital, and the surrounding community. Eligible participants are healthy individuals with a sedentary lifestyle (more than six hours of sitting per day and a physical-activity level below 600 MET-minutes per week on the International Physical Activity Questionnaire), a body mass index below 30 kg/m², and bilateral hamstring tightness, with no current low back pain or other excluding musculoskeletal, neurological, or spinal conditions