This prospective, single-blind, randomized controlled trial investigates whether the addition of spinal mobilization to a structured home exercise program improves pain, flexibility, lower-extremity strength, dynamic balance, and jump performance in recreational tennis players (aged 18-45 years; BMI 18-30 kg/m²) with non-radiating mechanical low back pain. Participants were allocated to two parallel groups: a Control Group performing a 21-session home exercise program for 3 weeks, and an Experimental Group performing the same program plus four physiotherapist-administered spinal mobilization sessions (one at baseline and one weekly for 3 weeks). Outcomes were the Visual Analog Scale (VAS) for pain during physical activity, at rest, and at night; the Y-Balance Test; the Vertical Jump Test; the Sit-and-Reach Test; and isometric strength measured with a back-chest-leg dynamometer.

Design: prospective, randomized, single-blind, parallel-group controlled clinical trial. The study was conducted under the supervision of the Department of Physiotherapy and Rehabilitation, Chiropractic Master's Program. Recreational tennis players (aged 18-45 years; BMI 18-30 kg/m²) with mechanical low back pain of more than 1 week duration and a baseline pain intensity exceeding 4 on the Visual Analog Scale (VAS) who played tennis at least twice per week (≥2 hours/week) were enrolled after providing written informed consent. Allocation was performed using sealed opaque envelopes combined with computer-assisted randomization, with masking applied to the outcome assessor. Participants were randomized to: (1) Control Group (CG) - structured home exercise program comprising posterior pelvic tilt, erector spinae stretching (double knee-to-chest), combined hamstring/lumbar extensor stretching (long-sit toe touch), piriformis stretching, and static hamstring stretching (5 repetitions of 30-second holds for stretches; 5 repetitions of 10-second holds for posterior pelvic tilt); the program was performed daily for 21 sessions over 3 weeks and supported by written instructions, visual materials, and weekly telephone follow-up; (2) Experimental Group (EG) - the identical home exercise program plus four sessions of supervised spinal mobilization delivered by a physiotherapist (one at baseline and once weekly for 3 weeks). Mobilization was administered according to the Maitland classification (graded I to V; end-range was not forced) and included thoracic posteroanterior mobilization (T5-T12), rotational thoracic mobilization ("butterfly/screw" technique), sacroiliac mobilization, side-lying lumbar mobilization, and anteroposterior lumbar mobilization. Each technique was applied as five sets of one-minute oscillations with 20-second rest intervals across the posteroanterior, mediolateral, and transverse planes. Outcome assessments were performed at baseline (Week 0) and at the end of the intervention (Week 3) by a single blinded assessor. Pain was quantified with three 10-cm Visual Analog Scales (physical activity, rest, and night). Dynamic balance was assessed with the Y-Balance Test in three directions (anterior, posteromedial, posterolateral). Lower-extremity power was measured with the Vertical Jump Test using the validated My Jump Lab application (iPhone 15 Pro Max). Hamstring and lumbar flexibility were measured with a Baseline sit-and-reach box. Isometric strength of the back, chest, and leg extensors was measured with a calibrated Baseline back-chest-leg dynamometer (New York, USA). Statistical plan: normality was tested by Shapiro-Wilk; within-group changes were analysed by paired-samples t-test, between-group differences by independent-samples t-test, and categorical variables by Chi-square; Cohen's d was calculated for parameters with significant within-group changes. Analyses were performed in IBM SPSS 25.0. A priori sample size estimation (G*Power 3.1.9.2) with 85% power, an effect size of d = 0.8, and α = 0.05 yielded a target of 30 participants; 32 were enrolled to allow for attrition. Post hoc power analysis based on the post-intervention VAS during physical activity (d = 1.24, α = 0.05) yielded a power of 0.91. No external funding; no conflicts of interest declared.

Inclusion Criteria:

• Adults aged 18-45 years.
• Body mass index (BMI) between 18 and 30 kg/m².
• Mechanical low back pain that decreases in the supine position and increases with activity or in various postures.
• Low back pain of more than one week duration with a baseline intensity greater than 4 on the Visual Analog Scale (VAS) and unrelated to any defined medical condition.
• Playing tennis on a recreational basis at least 2 days per week, totalling at least 2 hours per week.
• Able to communicate in Turkish or English in both written and spoken form.

Exclusion Criteria:

• Pain in the lower back, hip, or groin lasting longer than 6 months.
• Pain accompanied by a sensory deficit.
• Pain radiating to the hip or lower extremity.
• History of long-term steroid use.
• Regular medication use.
• History of injury or surgery resulting in residual deficits.
• Previous interventional treatment to the lumbar region.
• Pregnancy or suspected pregnancy.

Termination Criteria During Study:

• Onset of an acute illness requiring treatment (e.g., influenza, pneumonia).
• Use of analgesic or sedative medication.
• An increase in VAS pain score greater than 3 points during exercise or treatment.
• Failure to perform the home exercise programme on three consecutive days.
• Missed manual therapy session.

Outcome assessments were performed by a single assessor blinded to group allocation.