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Abstract: Skeletal muscle is an adaptable, multi-faceted tissue that is essential for whole body movement and metabolism. When a production animal is slaughtered, its muscle goes through complex physical and biochemical changes including a shift from aerobic to anaerobic metabolism resulting in the breakdown of glycogen. Glycogen breakdown and consequent lactate accumulation lead to a decline in muscle pH which influences meat quality traits such as colour and tenderness. Ante-mortem conditions such as physical activity, physiological stress and an animal’s genotype can all affect meat quality. In conventional laying hen cages (CC), lack of physical space and inability to perform highly motivated behaviours leads to stress and inactivity. Due to the concern for hen welfare in this system, CC have been banned or are being phased out in various parts of the world. Furnished cages (FC) are an alternative to CC as they permit the expression of highly motivated behaviours and load-bearing activity which has been shown to improve hen humeral bone strength and reduce aggressive behavioural expression as compared to hens housed in CC. However, FC typically house larger group sizes than CC, thereby contributing to social stress. In the first study presented in this thesis, the objective was to evaluate the effects of CC and FC laying hen housing environments and strain differences on meat quality of 80 to 81 week old birds. Pectoralis major meat quality was assessed for two flocks of Shaver White (SH), Lohmann Lite (LL) and Lohmann Brown (LB) hens housed in either 5-hen CC or 40-hen FC. Between 80 and 81 weeks, muscle samples were collected from randomly selected hens and analyzed for muscle pH, colour and shear force (SF) using established methods. In both flocks, the combined treatment body weights (BW) were higher for CC than FC hens and the combined strain BWs were higher for LB than LL and SH hens. Flock 1 LB had lower initial and ultimate pH than SH and LL, and greater pH decline than SH. Muscle redness (a*) was higher for CC SH than FC SH in both flocks. Muscle a* was higher for LL than SH and LB in flock 1, and higher than SH in flock 2. Housing differences in muscle SF were absent. In CC, SF was higher for SH than LL and LB in flock 1, and higher than LB in flock 2. Lack of housing differences suggest that environmental stressors present in both housing systems similarly affected meat quality. Strain differences for muscle pH, a* and SF suggest increased stress experienced by SH and LL hens. The absence of flock 2 strain differences are consistent with a cannibalism outbreak that occurred in this flock and most severely impacted LB hens. Post-mortem muscle pH decline has traditionally been attributed to glycogenolysis-induced lactate accumulation. However, muscle pH ([H+]) is controlled by complex physicochemical relationships encapsulated in the Stewart Model of acid-base chemistry (Can. J. Physiol. Pharm. 61: 1444-61, 1983), and is determined by three systems-independent variables – strong ion difference([SID]), total concentration of weak acids([Atot]) and partial pressure of CO2 (PCO2). A second study therefore investigated the three systems-independent variables within Pectoralis major muscles of flock 1 hens, and evaluated the Model by comparing measured [H+] with calculated [H+]. The Model proved exceptional, accounting for 99.7% of the variation in measured muscle [H+]. Differences in [SID] accounted for most or all of the variation in [H+] between strains. Greater PCO2–induced [H+] in FC compared with CC was counteracted by greater sequestration of strong base cations. The results demonstrate the accuracy and utility of the Stewart Model for investigating determinants of meat [H+]. Additionally, the housing differences identified in this study suggested that hens housed in FC have improved muscle function and overall health due to the increased opportunity for movement. These findings, which were not apparent from the traditional meat quality measures conducted, support past studies showing improved animal welfare for hens housed in FC compared to CC. Therefore, the Stewart model has been identified as an exceptional method to assess changes in the muscle at a cellular level that affect meat quality and reveal differences in the welfare status of the research subjects.