Two meat-type broiler strains, strain A and strain B, were reared in floor pens (25 birds/pen; 45 pens/strain) for pectoralis (P) major collagen and mixed muscle protein turnover (
PT
) study from 0–56 D using primary breeder nutrition and husbandry guidelines. Forty broilers (n = 10/strain for collagen PT; n = 10/strain for mixed muscle PT) were selected at each sampling age at day 21, 28, 35, 42, and 56 and infused with 1-
13
C proline (
Pro
) and
15
N-phenylalanine (
Phe
) which are used as amino acid tracers for collagen and mixed muscle PT measurements, respectively. Muscle and plasma samples were collected, and enrichments of 1-
13
C Pro and
15
N-Phe were determined using mass spectrometry. Fractional synthesis rate (
FSR
) and fractional degradation rate (
FDR
) were measured for collagen and mixed muscle using precursor-product principle. At day 42, after separating the sampled broilers as myopathy (woody breast [
WB
] score > 1) and nonmyopathy (WB = 0), plasma metabolites were screened for differential 3-methyhistidine (
3-MH
) expression for both strains. Data were analyzed using one-way ANOVA using
t
test. Results showed that collagen and mixed muscle FSR and FDR in pectoralis major decreased (
P
< 0.05) for both strains as the broilers aged. FSR for collagen and FDR for mixed muscle were higher for strain B than those for strain A (
P
< 0.05). Total collagen was higher (
P
< 0.05) for strain B. Differentially expressed 3-MH in plasma was higher (
P
< 0.05) for myopathy-affected broilers indicating greater muscle degradation occurring in myopathy-affected broiler types for both strains. 3-MH Expression in plasma was higher for strain B than for strain A. The research findings showing an increased collagen content per unit muscle weight in pectoralis major in strain B (than in strain A) could be due to higher mixed muscle FDR and increased collagen FSR occurring during the grow-out period. The increased degradation of muscle fibers and probable replacement of muscle-specific protein with connective tissue, mainly collagen, was an evident pathophysiological phenomenon occurring in myopathy-affected broilers.