Titled "Influence of Pneumonia on the Physiological Cost Index (PCI) of Walking" investigates how pneumonia impacts energy expenditure during ambulation. Pneumonia, a serious respiratory infection caused by bacteria, viruses, or fungi, leads to inflammation of the alveoli and compromises respiratory efficiency. It remains a major public health issue in India, particularly affecting the elderly and individuals with weakened immune systems. The condition often results in reduced exercise tolerance, fatigue, and impaired functional mobility. While conventional tools such as spirometry are effective for assessing lung function, their reliance on specialized equipment and trained personnel makes them less feasible in resource-limited settings. The Physiological Cost Index (PCI), a simple and non-invasive method, offers an alternative by estimating energy expenditure through heart rate changes in relation to walking speed. This cross-sectional study will recruit adults aged 40–70 years with clinically diagnosed pneumonia who are medically stable and able to walk independently. Participants will undergo a 6-minute walk test (6MWT) during which key physiological parameters heart rate, respiratory rate, oxygen saturation, and blood pressure, will be measured pre- and post-test. PCI will be calculated using MacGregor’s formula: (Post-walking Heart Rate – Resting Heart Rate) / Speed (m/min). Additional outcome measures include the Borg Scale for perceived exertion, CURB-65 pneumonia severity score, Fatigue Severity Scale (FSS), and other clinical markers. Individuals with unstable cardiovascular conditions, neuromuscular impairments, ongoing fever, or chronic respiratory disorders will be excluded. This study is distinctive in applying PCI to pneumonia patients—a population not commonly assessed for ambulatory energy cost. It aims to reveal elevated PCI values in this group, reflecting the increased physiological demand associated with pneumonia. The findings are expected to guide physiotherapists in developing tailored rehabilitation programs based on functional limitations. Moreover, it supports the integration of PCI into routine physiotherapy assessments as a cost-effective and accessible tool for tracking functional recovery in both hospital and community settings.