This experimentaimed to evaluate the effect of environmental temperature upon the morphometriccharacteristics of the fiber types in the flexor hallucis longus muscle of broilersin pair-feeding, thus isolating the effect of voluntary feed intake. Sixteenmale broiler, Ross, were allotted to a completely randomized design with twotreatments (heat stress and cold stress), and seven and nine replications, respectively.Forty-three-day-old heat stressed broilers weighed 1.255 g and those in coldstress, pair-fed, weighed 1.086 g. The birds subjected to heat and cold environmentaltemperatures showed 22.82 and 27.93% white (FG - Fast Glycolytic), 52.76 and47.67% intermediate (FOG - Fast Oxidative Glycolytic) and 24.42 and 24.40% red(SO - Slow Oxidative) fibers in the flexor hallucis longus muscle, respectively.The diameter of FG fiber was higher in the muscle of heat stressed birds (48.69mm), comparing to cold stressed pair-fed ones (37.74 mm). Temperature had noeffect on the number and frequency of fibers. The cold stress associated withrestricted feed intake depressed growth and reduced the size of FG fibers inthe flexor hallucis longus muscle of broilers, but did not affect the size ofFOG and SO type fibers, which have higher oxidative metabolism which is associatedwith heat production.
