Effects of exercise during growth and alternative rearing systems on muscle fibers and collagen properties

Abstract

Muscle characteristics, and particularly fiber type frequency and collagen properties, may be a source of variation in eating meat quality. Expanded space allowance in alternative breeding systems theoretically increases animal physical activity during growth. This review deals with effects of endurance training and spontaneous exercise in large areas in- and out-doors, on muscle characteristics in rabbits and pigs, two species of agronomic interest, and rats. Endurance training induces a fast-to-slower transition in myofiber contractile characteristics, following the IIB -> IIX -> IIA -> I transformation sequence. These changes are accompanied by a greater ability to transport fatty acids intracellularly, and (or) by enhanced activities of the mitochondrial reference enzymes. Newly synthesized heat-soluble collagen may be observed in the recruited muscles after endurance training in rats. Depending on the experiments (stocking density, ambient temperature, gender, and muscles), area allowance in- or out-of-doors, does not affect fiber type frequency compared with conventional systems or results in a lower proportion of type IIB/X fibers at the benefit of slower twitch fibers. Muscle lipids and collagen content are generally not modified by expanded indoor area, however, a higher proportion of non-soluble collagen may be observed in free-range animals in doors compared with confined ones. It is impossible to state a general rule for lipid stores and collagen properties in animals reared out-of-doors. Therefore, exercise studies are unsuitable to predict adaptative responses in muscle characteristics to alternative outdoor rearing systems, and in fine meat quality.