<p>Aerobic exercise can stimulate mixed muscle protein synthesis (MPS) acutely post-exercise; however, the types of proteins synthesized as a result of aerobic exercise are not known by studying changes in mixed MPS. We aimed to study the effect of aerobic exercise intensity on the 4 and 24 h post-exercise fractional synthesis rate (FSR) of myofibrillar proteins. Using a within-subject design, eight males (21 ± 1 years, VO_{2 peak}: 46.7 ± 2.0 mL kg^-1 min^-1) underwent 2 trials with a primed constant infusion of L-[<em>ring</em>-¹³C₆]phenylalanine in the fasted state for each work-matched exercise intensity (LOW: cycling for 60 min at 30% W_max and HIGH: 30 min at 60% W_max). Muscle biopsies were obtained to determine resting, 4 and 24 h post-exercise myofibrillar FSR. We also studied the phosphorylation of signaling proteins involved in protein synthesis at each time point using immunoblotting methods. Phospho-p38^{Thr180/Tyr182} was greater at 4.5 h after exercise compared to 0.5, 24 and 28 h post-exercise (<em>p</em> < 0.05). Additionally, a strong trend was present for phospho-mTOR^Ser2448 (<em>p</em> = 0.056) with 0.5 h post-exercise phosphorylation significantly higher after HIGH than after LOW exercise (<em>p </em>< 0.05). Myofibrillar protein synthesis was stimulated 1.5–fold 0.5 – 4 h post-exercise (<em>p</em> < 0.05), returning to rest in the LOW condition 24 h post-exercise, while 6 out of 8 subjects maintained increased myofibrillar FSR 24 h post HIGH exercise (<em>p</em> < 0.05). The increase in myofibrillar FSR 0.5 – 4 h post-exercise was correlated with phospho-mTOR^Ser2448 0.5 h post-exercise (r = 0.698, <em>p</em> < 0.01), indicating the role of this signaling pathway in myofibrillar protein synthesis. It is concluded that aerobic exercise has an effect on myofibrillar protein synthesis and intensity may play a role in the duration of this response.</p> / Master of Science in Kinesiology