Limitations of maximal oxygen uptake during whole-body exercise

Oxygen uptake increases linearly with increased running speed or rate of work and finally reaches a maximum beyond which no effort can drive it further. This phenomenon is referred to as the plateau in oxygen uptake . The classical and generally accepted view is that the maximal oxygen uptake plateau is limited by the central circulation s inability to distribute sufficient oxygen to all the working muscles. Throughout history several different theories have been proposed as limitation causes to maximal oxygen uptake, principally Central vs. Peripheral limitations. Even though there is more than 80 years of scientific investigation of maximal oxygen uptake, there is still no unarguable agreement regarding the fundamental question: What sets the upper limit for maximal oxygen uptake during exercise involving a large muscle mass? More than a decade ago a new theory named Central Governor was proposed to challenge the classical concept of maximal oxygen uptake. It suggested that the absence of an oxygen uptake plateau in a few studies was enough evidence to support that the oxygen delivery system was not a limiting factor for maximal oxygen uptake. This theory has been criticized by many physiologists who have provided important evidence for the classical concept of maximal oxygen uptake. The theory also states that the governor reduces the neuronal output to the heart and active muscles in order to protect these tissues from becoming ischemic. Therefore, the main purposes of this Licentiate thesis were: 1) To determine and investigate the importance of the work of the heart as a limiting factor in the attainment of maximal oxygen uptake during maximal or supramaximal workloads, 2) To elucidate the relationship between maximal oxygen uptake during maximal and supramaximal workloads and the neuronal output to active skeletal muscles in order to evaluate the model of regulation of the Central Governor theory. The main hypothesis was that after different combined arm and leg exercises with different maximal work rates, the mechanical work of the heart and the active skeletal muscles would not be hindered by a command from the central nervous system. Sixteen subjects, healthy, endurance trained males participated in the studies. Three different maximal combined arm and leg exercises were used to determine maximal oxygen uptake. Oxygen uptake, pulmonary ventilation and heart rate were measured continuously. Electrocardiography signals were recorded in the end of every maximal exercise. Blood samples were taken and electromyography activities were measured during exercise at various time points. The main findings were: 1) During all the maximal combined arm and leg exercises, despite large variations in maximal work rates there was a clear plateau in oxygen uptake both with rate of work and with time for all participants. 2) During the plateau phase both the electromyography activity of the activated muscles and the work of the heart increased with increased rate of work without any increase in oxygen uptake. These studies support the classical view of a limitation of maximal oxygen uptake in the central circulation. This limitation refers to the hearts inability to pump more blood at increasing exercise levels and not the mechanical work of the heart or the heart muscle energetic insufficiency. Furthermore, these data do not support a maximal oxygen uptake limitation based on the Central Governor theory

Unexpected cardiovascular response during ultra-endurance exercise.

During prolonged exercise at fixed work rate heart rate (HR) increases slowly with concomitant decrease in stroke volume (SV) in order to maintain cardiac output. Simultaneously, an increased oxygen uptake (VO2) occurs. In this paper we report an unexpected and previously not observed cardiovascular response to ultra-endurance exercise. Nine well-trained male athletes performed 24-h exercise in a controlled laboratory setting, with altering blocks of kayaking, running and cycling. Each block consisted of 110 min of exercise and 10 min of rest. Measurements (HR, VO2 and blood samples) were conducted during cycling at fixed work rate every 6th hour. The average work intensity was approximately 55 % of respective VO2peak. HR was increased at 6 h with 15 beats/min (13 %) compared to pre-exercise (Pre-Ex), but thereafter unexpectedly returned towards initial values. VO2 on the other hand was increased with 0.22 l/min (10 %) at 6 h and 0.37 l/min (17 %) at 12 h compared to Pre-Ex, and thereafter remained stable. This implies an increased oxygen pulse (VO2/HR) with approximately 10 % compared to Pre-Ex at the later half of the exercise. The cardiovascular drift did not progress continuously, but instead changed drastically when duration exceeded 6 hours. The changes in HR and VO2 might have different and complex explanations. HR drift might be explained mainly by central circulatory adaptation (e.g. desensitisation of cardiac adrenergic receptors) whereas drift in VO2 may depend upon peripheral changes (e.g. decreased mitochondrial efficiency). Consequently, using solely HR for determining exercise intensity and energy expenditure becomes invalid during ultra-endurance exercise, if the cardiovascular drift is not measured and taken into account.  Physiology of Adenture Racin