L-ornithine is an amino acid involved in the urea cycle, which helps remove ammonia from the body, a byproduct of protein metabolism. Its use for supporting the muscular system is primarily based on its potential to reduce exercise-induced fatigue and improve recovery. Several small studies have suggested that L-ornithine supplementation may help decrease perceived exertion and fatigue during prolonged exercise, possibly by enhancing ammonia clearance. For example, a 2008 study published in Nutrition Research found that L-ornithine supplementation reduced fatigue in healthy volunteers after exercise. Another study in Journal of Clinical Biochemistry and Nutrition (2010) showed decreased physical fatigue in healthy office workers given L-ornithine.

However, the current evidence base is limited, as most studies are small, short-term, and often involve healthy, non-athlete populations. The effects observed are modest, and there is little direct evidence that L-ornithine significantly enhances muscle strength, growth, or repair. Most clinical guidelines and position papers do not recommend L-ornithine as an ergogenic (performance-enhancing) aid due to insufficient evidence. Thus, while there is some scientific research supporting its role in reducing fatigue and aiding recovery, the overall strength of evidence for supporting the muscular system specifically remains weak to moderate.