Lactate production and the redox state of muscle
In this thesis, the wrist flexors of a single forearm (representing a small muscle group) carried out a series of flexion exercises at easy, moderate, and maximal intensities at first alone, then repeated superimposed on top of bilateral supine cycling (representing a large muscle group). Blood flow was measured in the excising and resting forearms. Arterialised venous blood, and venous blood effluxing the exercising forearm were also sampled to give an indication of net lactate movement across the working muscle. There was only non-significant evidence of a reduction in the forearm blood flow as the leg exercise intensity increased. This was attributed to the supine position of the exercise causing greater perfusion of the forearm muscles than would be achieved in an upright subject. The blood lactate concentrations showed a net lactate output from the forearm when exercising alone. As leg exercise intensity increased arterial blood lactate concentration rose and the exercising forearm changed to net lactate uptake. The exercising forearm muscles exhibited net uptake, or no net lactate movement, even when working maximally. This was when the arterial lactate concentration was above 7 mmol.1-1, presumably causing the blood-muscle lactate concentration gradient to be towards the muscle. The fact that lactate could go into the muscle in conditions of maximum metabolic challenge when the blood flow may have been compromised seemed to refute any possibility of lactate production being due to oxygen starvation. This then led to the question of whether the muscle ever becomes oxygen-limited in exercise. The technique of Near Infrared Spectroscopy (NIRS) which can monitor the oxidation state of haemoglobin and cytochrome oxidase non-invasively in tissue was applied to the problem. Cytochrome oxidase is the only point in aerobic metabolism that makes direct use of oxygen, so will be the point at which any shortfall will be most apparent. NIRS of the forearm muscle was carried out during isometric handgrip exercise in conditions of normoxia, hyperoxia, and hypoxia, with free blood flow to the forearm muscle or with flow occluded using a brachial cuff. No effect was seen on the cytochrome oxidase in the non-occluded bouts of exercise.