An Investigation into the Effect of Skeletal Muscle Metabolic Function & Cardio-Respiratory Fitness on Exercise Capacity in the Presence and Absence of Disease

It is uncommon for cardio-respiratory fitness and skeletal muscle metabolic function to be assessed independently within an exercise test. Exercise capacity may require adaptation of one or other of these systems to a greater or lesser extent and the presence of disease may affect them to different extents. The physiological mechanisms underpinning the decline in exercise tolerance with age/disease and the benefits of exercise-training as a preventative therapy for some diseases are not fully understood.

In this thesis functional capacity was assessed in terms of oxidative capacity. Near-Infrared Spectroscopy (NIRS) measures microvascular changes in oxygenated and deoxygenated haemoglobin and can be used to estimate oxidative capacity in skeletal muscle when combined with arterial occlusions. Oxidative capacity of skeletal muscle was determined as part of 3 studies; (1) in a group of older adults (>65 years old) with or without type 2 diabetes (T2D), (2) in a tri-ethnic group of older adults from the same cohort with the objective of determining ethnic difference in oxidative capacity independently of T2D and (3) in a group of young adults before and after a period of endurance training in preparation for their first marathon.

NIRS measurements of muscle oxidative capacity revealed poorer function in older adults with diabetes (57.5±6.8 versus 38.7±2.6 s, p=0.02) and poorer oxidative capacity in South Asian older adults independently of T2D (difference (95%CI): 10.1 (2.3, 17.9) s, p=0.011). In young healthy men and women, skeletal muscle oxygen consumption post-exercise increased with endurance training (p < 0.01) despite no improvement in cardio-pulmonary peakV̇O₂ (p=0.81). Faster marathon completion time correlated with cardio-pulmonary peakV̇O₂ (r_partial=-0.55, p < 0.01) but not oxidative capacity.

Skeletal muscle oxidative capacity can be measured in old and young adults using NIRS combined with arterial occlusion performed immediately following exercise testing. People with T2D have poorer oxidative capacity compared to people without and South Asians have poorer oxidative capacity compared to Europeans which African Caribbean’s and Europeans had similar skeletal muscle oxidative capacity. Skeletal muscle metabolic adaptions occur following 6 months of endurance training. Although the cardio-pulmonary system is limiting for running performance, skeletal muscle changes can be detected despite no significant improvement in cardio-pulmonary function.