In this study, the degree of contractile and metabolic development of myofibers in porcine LM, rectus femoris (RF), and dark and light portions of the semitendinosus (STD and STL, respectively) was determined, and their impact on meat quality was compared at the same age but different BW (trial 1) or at a given BW but different age (trial 2) in 48 Swiss Large White barrows from 12 litters after the growing and finishing period. The barrows had ad libitum (A) or restricted (R, 80% of A) feed access. In trial 1, at 113 and 154 d of age, 6 barrows in treatment A (62.1 and 99.5 kg of BW, respectively) and 6 siblings in treatment R (51.0 and 86.6 kg of BW, respectively) were slaughtered. In trial 2, a similar protocol was used except that the barrows were slaughtered at 61.3 (104 or 119 d of age, respectively) or 101.3 kg of BW (145 or 167 d of age, respectively). Muscle fibers were stained and classified as slow oxidative (SO), fast oxidative-glycolytic (FOG), or fast glycolytic (FG), and fiber area and distribution were determined. At 113 and 154 d of age, R barrows had smaller (P < or = 0.04) SO fibers in the LM, STD, and STL, smaller (P < 0.01) FOG fibers in the STL, smaller (P = 0.03) FG fibers in the LM, and smaller (P < or = 0.04) overall mean area in the LM, STD, and STL. In the STL and RF, R barrows had fewer (P < or = 0.06) FG and more (P < or = 0.08) FOG fibers than A barrows at 113 and 154 d of age. Except for smaller FOG fibers in the STD of R compared with A barrows slaughtered at the same BW, the myofiber size did not differ (P > or = 0.11). However, the LM tended to have fewer (P = 0.06) SO and more (P < 0.01) FG fibers, and the STD had more (P < 0.01) FOG fibers in R barrows. Regardless of whether R barrows were slaughtered at the same age or the same BW as the A barrows, shear force values and cooking losses were greater (P < or = 0.08) in the STD and STL of R barrows. These findings revealed that myofiber hypertrophy was impaired by feed restriction in barrows compared at the same age, but differences in myofiber size vanished at the same BW. By contrast, restricted nutrient supply affected myofiber maturation depending on the age and BW, but the impact differed between muscles. The absence of changes in myofiber type distribution among the younger-lighter and older-heavier barrows indicated that myofiber maturation was already completed in the younger-lighter barrows. Although changes in meat quality traits were affected by the feeding regimen, they were not related to myofiber characteristics.