Reframing Nutraceuticals in Knee Osteoarthritis with Sarcopenia: A Muscle–Joint-Centered Narrative Review

Background/Objectives: Knee osteoarthritis (KOA) is increasingly recognized as a function-limiting condition in which pain, neuromuscular impairment, and reduced physical activity interact with sarcopenic vulnerability to accelerate functional decline. This review reappraises commonly used oral nutraceuticals through a muscle–joint framework and examines whether they can be conservatively positioned as adjuncts that reduce symptom-related barriers to exercise-based care rather than as disease-modifying therapies. Methods: This review was conducted as a structured narrative synthesis informed by SANRA principles, using a structured and transparent search process and dual-independent study selection, without quantitative meta-analysis or formal certainty-of-evidence grading. PubMed/MEDLINE, Embase, and the Cochrane Library were searched for English-language studies published from January 2000 to March 2026, supplemented by reference screening of key reviews and international guidelines. Results: Mechanistic and clinical evidence supports a plausible pathway linking KOA pain, arthrogenic muscle inhibition, reduced loading, physical inactivity, and sarcopenic vulnerability. Across glucosamine/chondroitin, collagen peptides, omega-3 fatty acids, curcumin, and Boswellia, symptomatic benefits were modest, heterogeneous, and formulation-dependent, with no consistent evidence of structural disease modification. Direct evidence that nutraceuticals improve exercise adherence or long-term physical activity remains limited; however, selected exercise-integrated or function-oriented studies show participation-relevant signals in gait speed, activity volume, and performance-based outcomes. Conclusions: Nutraceuticals should be interpreted as optional, time-limited adjuncts within exercise-centered KOA management. Their potential value lies in modest symptom support that may facilitate rehabilitation participation in selected patients, not in stand-alone treatment of KOA or sarcopenia.