| dc.description.abstract | Maintenance of muscle mass, strength and quality across the lifespan is of vital importance. Periods of muscle disuse can occur due to a number of reasons including illness where a period of hospitalisation and thus bed rest may be required and musculoskeletal injuries where a period of limb immobilisation may be required. Musculoskeletal injuries necessitate periods of disuse (i.e. limb immobilization) during which rapid skeletal muscle atrophy occurs. The relative susceptibility of different muscles of the thigh to disuse atrophy remains uninvestigated. Furthermore, short-term muscle disuse has also been reported to lower both post-absorptive and post-prandial myofibrillar protein synthesis rates which contribute to muscle atrophy, with little being known about the role of dietary protein intake at potentially reducing muscle atrophy during shorter term disuse. Accordingly, the purpose of this thesis was to characterise muscle disuse atrophy of individual thigh muscles throughout one week of unilateral knee immobilization, alongside this we also assessed the impact of disuse on daily myofibrillar protein synthesis rates following short-term (2 and 7 days) muscle disuse under free living conditions. In a third study we assessed the influence of dietary protein intake on daily myofibrillar protein synthesis rates and loss of muscle mass and function during 3 days of unilateral leg immobilization. In the first experimental study of the thesis in contrast to the control leg total thigh muscle volume had decreased by 1.7 (P<0.01) and 5.5 % (P<0.001) in the immobilized leg after 2 and 7 days of disuse, respectively. Muscle loss was significantly greater in the M. quadriceps (day 2; 1.7 (P<0.05) and day 7; 6.7 %) when compared to the M. hamstrings (day 2; 1.4 % (P<0.01) and day 7; 3.5 %) following 7 days of disuse (P<0.001). Individual muscles of the thigh exhibited different atrophy rates with the M. vastus lateralis aCSA showing the greater, and the M. gracilis the lesser decline following 2 and 7 days of immobilization, respectively (P<0.01). In the second experimental study of the thesis we found that over the one week immobilization period myofibrillar protein synthesis rates were 36 % lower in the immobilized compared with the control leg (P<0.001). We report the novel finding that Myofibrillar protein synthesis rates in the control leg did not change over time, but in the immobilized leg were numerically lower during the 0-2 day period (16 %, P=0.153) and were significantly lower during the 2-7 day period (44 %, P<0.001) when compared with the control leg. In the third experimental study of the thesis we report the novel finding that graded protein intakes had no effect on daily myofibrillar protein synthesis rates during immobilization, and were 30 (HIGH protein intake), 26 (LOW protein intake) and 27 % (NO protein intake) lower in the immobilized compared with control leg, with no differences between groups (P>0.05). This was also reflected by changes in muscle mass where there was no difference between groups, M. quadriceps volume reduced by 2.3, 2.7 and 2.0 % in the HIGH, LOW and NO protein intake groups respectively. In conclusion thigh muscle disuse atrophy occurs rapidly and is
already evident within 2 days of leg immobilization and progresses at a similar rate over the next 5 days (~0.8% muscle loss per day). M. quadriceps muscle shows more atrophy when compared with the M. hamstrings. One week of muscle disuse induced a rapid and sustained decline in daily myofibrillar protein synthesis rates in healthy young men. Three days of muscle disuse induces considerable loss of muscle mass and function, and reductions in daily myofibrillar muscle protein synthesis rates; however, these responses are not modulated by daily dietary protein intake. Collectively the work contained within this thesis has successfully provided new knowledge and a clearer understanding of leg muscle atrophy during short term immobilisation and to what extent muscle protein synthesis rates are altered over time during short term immobilisation. Our understanding of the role of dietary protein intake on muscle atrophy and muscle protein synthesis rates during short term disuse has also been expanded. | en_GB |
