The participants in the experimental group will undertake supervised aquatic high-intensity interval exercise 3×/week for 8 weeks (Tabata method). Each session consists of a 10-min warm-up, Tabata sets (20s maximal-effort aquatic exercises-e.g., water running, jumping jacks, resistance with foam dumbbells/paddles-followed by 10s rest, 8 cycles/set; total set=4 min), and a 5-min cool-down.

Intensity monitored using Borg RPE scale (6-20), target 15-17 ("hard" to "very hard") during work phases to ensure HIIT delivery. RPE recorded at end of each set with real-time feedback (e.g., "push harder" if <15). Validated in PD trials; RPE 15-17 ≈80-90% max HR in water (HR optional).

Pool depth waist-to-chest (≈1.2-1.4 m), temp 32-34 °C. Delivered in indoor therapeutic pool of Pardis Institute by certified specialists. Standard medical care continues. Adherence via attendance logs; >20% missed = non-adherent.

Patients allocated to the control group will receive their usual clinical management for Parkinson's disease, including pharmacological therapy as prescribed by their neurologist, and will not participate in any structured exercise program during the 8-week study period. Low-intensity daily activities and usual physical routines are permitted. Control participants will be advised to maintain their standard treatment regimen and customary activity patterns during the trial.

The participants in the behavioral component will receive the same supervised aquatic HIIT program described above plus structured behavioral/support measures to maximize adherence and safety. These include: an initial individual education session explaining benefits/risks and protocol; supervised real-time feedback during each session from exercise specialists; flexible scheduling options to accommodate participants; weekly review of attendance and brief motivational follow-up (phone call or in-person) for participants with missed sessions; individualized adjustments to intensity/duration based on participant feedback and safety; and documentation of reasons for non-attendance. All behavioral/support activities are delivered by trained research staff and exercise physiologists and are recorded in session logs. (These measures are drawn from the protocol's adherence and retention strategies.)

Evaluating the effect of aquatic HIIT on balance using the Berg Balance Scale (BBS), a 14-item performance-based tool assessing static and dynamic balance through functional tasks like sitting to standing, reaching forward, and turning 360 degrees. Items are scored on a 5-point ordinal scale (0-4) based on ability and time to complete, with a total score ranging from 0 (worst) to 56 (best); scores of 0-20 indicate wheelchair-bound, 21-40 indicate walking with assistance, and 41-56 indicate independent. In PD, scores <45 suggest high fall risk, and the BBS has excellent intra-rater reliability (ICC=0.98), inter-rater reliability (ICC=0.97), and criterion validity, correlating with other PD-specific measures like UPDRS.

Evaluating the effect of aquatic HIIT on physical function using the Short Physical Performance Battery (SPPB), a composite test including three subcomponents: balance (holding side-by-side, semi-tandem, and tandem stands for up to 10 seconds each, scored 0-4), gait speed (timed 4-meter walk at usual pace, scored 0-4 based on time quartiles), and chair stand (time to complete five rises from a chair without arms, scored 0-4). Total score ranges from 0 (worst) to 12 (best), with scores <10 indicating high risk for mobility limitations and falls in older adults, including PD patients. The SPPB demonstrates high test-retest reliability (ICC=0.92), intra-rater reliability, and validity in predicting functional decline and executive function associations in PD.

Evaluating the effect of aquatic HIIT on sarcopenia-related muscle mass using Bioelectrical Impedance Analysis (BIA), a non-invasive technique that estimates body composition by measuring resistance and reactance to a low-level electrical current, calculating skeletal muscle mass (ASM) and phase angle (PhA). ASM is indexed to height squared (ASM/height² in kg/m²), with sarcopenia diagnosed per EWGSOP2 criteria (<7.0 kg/m² for men, <5.5 kg/m² for women); PhA cut-offs for sarcopenia are ≤4.05° (92.9% sensitivity, 53.8% specificity) for men and ≤3.75° (78.9% sensitivity, 51.1% specificity) for women. In PD, BIA is reliable for detecting sarcopenia (prevalence 40-55%) with high correlation to dual-energy X-ray absorptiometry (DXA) as gold standard (r=0.85-0.90 for ASM), predictive validity (adjOR=0.147 for PhA), and good test-retest reliability (ICC=0.88-0.95), though wide diagnostic variations exist across criteria.

Evaluating improvements in quality of life using the Parkinson's Disease Questionnaire-39 (PDQ-39), a disease-specific 39-item self-report instrument covering eight domains (mobility, activities of daily living, emotional well-being, stigma, social support, cognitions, communication, bodily discomfort). Items are rated on a 5-point Likert scale (0=never to 4=always), with domain scores transformed to 0-100 (higher indicating worse quality of life) and a summary index as the mean. The PDQ-39 has strong convergent validity (correlates with EQ-5D), discriminant validity, internal consistency (Cronbach's alpha=0.51-0.96), and test-retest reliability (0.56-0.90 per domain) in PD.