Metabolic alterations in skeletal muscle following eccentric exercise induced damage : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Massey University, Palmerston North, New Zealand

Abstract

Eccentric exercise-induced muscle damage (EEIMD) is experienced following unaccustomed eccentric-biased exercise. Gaps in knowledge on aspects of the metabolic response to EEIMD exist, particularly on in vivo metabolism. The aim of this thesis is to provide empirical evidence to advance the scientific knowledge and understanding of EEIMD by investigating the metabolic responses following acute and adaptive bouts of eccentric exercise. Eccentric exercise causes changes to the ultrastructure of skeletal muscle and may alter the ability of the muscle to store and utilise intracellular substrates such as glycogen and intramyocellular lipid (IMCL). Using expired respiratory gases collected during one legged cycling to estimate whole body substrate utilisation, the first study showed that acute bouts of eccentric exercise alter the pattern of substrate selection. The effect of EEIMD on substrate utilisation during one legged cycling revealed significantly higher rates of CHO oxidation in EEIMD and that the CHO oxidation further increased during one legged cycling at 48 hours. This is suggestive of greater reliance upon muscle glycogen during subsequent bouts of exercise. The utilisation of nuclear magnetic resonance (NMR) spectroscopy to measure phosphate compounds and IMCL content of the vastus lateralis allowed for examination of changes in substrate storage following exposure to an acute bout of eccentric exercise. The second study showed that, following EEIMD, using proton spectroscopy (1H-MRS), alterations occur in the IMCL pool within skeletal muscle with

a higher concentration evident in the eccentric leg at 24 hours but the trend had been reversed at 48 hours with higher concentrations of IMCL in the concentric leg at 48 hours. Using phosphorous spectroscopy (31P-MRS) there was also a significant alteration for resting phosphate stores with increases in inorganic phosphate concentration ([Pi]) in EEIMD. Eccentric exercise also alters the physiological response to normal levels of insulin and can be defined as "transient insulin resistance". Repeated eccentric exercise training initiates a protective adaptation so that recovery results in reduced symptoms of damage in the repeat bout compared to the initial bout. The third study investigated; via a standard 75g oral glucose tolerance test (OGTT), whether disruptions to glucose and insulin responses following eccentric exercise could be attenuated after a repeated bout of eccentric exercise. There was no change in the insulin response, in comparison to a control trial, 48 hours after a bout of 100 squats of

30% body mass; this formed the eccentric exercise for the study. A subsequent bout of the same eccentric exercise did not attenuate the insulin response. It is not known if repeated exposure to eccentric exercise can attenuate increases in indirect measures of intracellular metabolism (Pi / PCr) following EEIMD, as seen in study two. Study four utilised 31P-MRS to examine the effect of EEIMD on intramyocellular phosphate stores in skeletal muscle, which had been concentrically or eccentrically trained. The data revealed that increases in skeletal muscle phosphate metabolism were not attenuated following exposure to repeated bouts of eccentric exercise and decrements in force generating capacity of muscle following EEIMD must be mediated by central factors. The four studies have provided novel insights into the influence of eccentric, muscle-damaging exercise on the metabolic response of skeletal muscle.