The effect of strain, breeding system and sex on body weight, carcass andleg yield, and morphologic aspects of skeletal muscle fibers of flexor hallucislongus muscle of broiler chickens was evaluated in this study. The experimentconsisted of a 4x2x2 factorial arrangement of treatments, with four strains(Ross-308, Naked Neck Label Rouge, Caipirinha and Paraiso Pedrês), twobreeding systems (confinement and semi-confinement) and two sexes, with tworeplicates per treatment, considering each bird selected randomly at 56 daysof age an experimental unit, with a total of 64 birds. Ross strain had the highestvalues of body weight, weights of legs and meat of legs and weight of flexorhallucis longus muscle and highest values of carcass yield and meat of leg yield.The higher muscle mass of birds selected for high growth is related to an increasein the area of the three types of muscle fibers evaluated (SO, FOG and FG).Males presented higher muscle mass and a musculature higher in fast-glycolityc-typefibers than females. Semi-confined system modified the composition of skeletalmuscle fibers of males, increasing the oxidative-type fibers. However this effectwas not observed in females.
