Measurements of muscle area expressed in square inches (or centimeters) contribute to the improvement of prediction equations used to estimate cutability grades in livestock and their carcasses. Muscle area is measured on the surface of the transverse section of a specific muscle at a specific anatomical location. For example, rib muscle area on beef carcasses is obtained on the surface of the rib muscle sectioned at the interface between the 12th and 13th ribs. For any constant weight or size of animal or carcass, the size of individual or groups of muscles is related to lean content. For a given frame size, muscles having larger areas are associated with higher muscle mass and a higher muscle/bone ratio. However, fatness is the single most important factor affecting composition. When muscling is represented by muscle area and included in a prediction equation, the estimate of composition improves.[4,5] It is obligatory to use ultrasonic measurements for livestock whereas a plastic grid is used to determine muscle areas for carcasses. The loin muscle is used as a point of reference for muscle areas of beef, pork, and lamb. This area is used in combination with fat depth and weight to predict leanness. Occasionally, areas of intermuscular (seam) fat have been assessed. However, the difficulty in standardizing anatomical locations has made such areas less practical.
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