Locomotor Aptitude Is Associated with Differences in Protein-Bound and Free Amino Acid Pools in Breast and Thigh Muscles of Organic Chickens

Organic poultry production requires genotypes suited to outdoor rearing, yet associations between previously characterised locomotor aptitude and meat amino acid composition remain poorly understood. This study examined breast and thigh muscles from 60 organically reared chickens representing six genetic lines (three classified as active and three as sedentary; ten carcasses per line), slaughtered at 81 days. Proximate composition and total and free amino acids were determined. Breast muscle contained more crude protein and total essential amino acids (EAAs) than thigh muscle. Genotypes classified as active had higher total EAA concentrations in breast (7.80 vs. 7.20 g/100 g meat), but not in thigh. After false discovery rate correction, the overall locomotor-group difference in crude protein was not significant. Free amino acid differences were concentrated in thigh: Bonferroni-adjusted comparisons showed lower total free amino acids, free EAAs, and individual free valine, isoleucine, and leucine in active-classified lines, whereas breast concentrations were comparable. The group × muscle interaction was significant for free EAAs (Q = 0.013), but not for total free amino acids. These muscle-specific patterns demonstrate the complementary value of total and free amino acid measurements for characterising poultry meat composition, without establishing an independent effect of locomotion or in vivo metabolic mechanisms.