The Study of Muscle Metabolism in Young People using 31P-Magnetic Resonance Spectroscopy
Barker, Alan Robert
Thesis or dissertation
University of Exeter
The purpose of this thesis is to extend understanding of the muscle metabolic responses of children and adolescents during exercise using the non-invasive technique of 31P-magnetic resonance spectroscopy (31P-MRS). The first experimental paper examined the reliability of measuring parameters of muscle metabolism in 11-12 year old children over three exhaustive incremental tests using a single-legged quadriceps ergometer. Exercise performance (peak power: ~ 10% coefficient of variation [CV]) and metabolic variables (muscle phosphate and pH intracellular thresholds [IT]: ~ 10% CV, and pH: ~ 1% CV at exhaustion) demonstrated good reliability, whereas the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) at exhaustion had poor reproducibility (~ 50% CV). The second paper examined the influence of age and sex on the muscle metabolic responses during incremental exercise in 9-12 year old children and young adults. The Pi/PCr and pH responses before and at the ITs were independent of age and sex, although during exercise above the ITs, the anaerobic energy contribution (increase in Pi/PCr, fall in pH) was higher in adults than children and in females compared with males, indicating an intensity dependence on age- and sex-related differences in muscle energetics. The third paper examined the relationship between the dynamics of muscle PCr, a putative controller of muscle respiration, and pulmonary oxygen uptake (pVO2) in 9-10 year old children during moderate intensity quadriceps and cycling exercise respectively. No differences were found between the PCr and phase II VO2 time constants at the onset (PCr 23 s [SD 5] vs. pVO2 23 s [SD 4]; P=1.000) or offset (PCr 28 s [SD 5] vs. pVO2 29 s [SD 5]; P=1.000) of exercise, suggesting an age-related slowing of the phosphate linked controller(s) of mitochondrial oxidative phosphorylation may underlie the faster pVO2 kinetics found in children compared to adults. The final experimental chapter tested this hypothesis, but no age or sex related differences were found in the PCr kinetic time constant at the onset (boys: 21 s [SD 4]; girls: 24 s [SD 5]; men: 26 s [SD 9]; women: 24 s [SD 7], P>0.200) or offset (boys: 26 s [SD 5]; girls: 29 s [SD 7]; men: 23 s [SD 9]; women: 29 s [SD 7], P>0.070) of exercise. In conclusion, this thesis has demonstrated that muscle metabolic parameters determined by 31P-MRS are suitable for the study of developmental exercise metabolism. During exercise below the metabolic ITs, the phosphate-linked regulation of muscle respiration is comparable between children and adults, although during exercise above the ITs children are characterised by a lower ‘anaerobic’ energy turnover than adults, indicating an age-related modulation of metabolic control during high intensity exercise.
PhD in Sport and Health Sciences