SummarySynopsis:The determinants of endurance effort vary, depending upon the extent of the muscle mass that is activated. Large muscle work, such as treadmill running, is halted by impending circulatory failure; lack of venous return may compound the bask problem of an excessive cardiac work-load. If the task calls for use of a smaller muscle mass, there is ultimately difficulty in perfusing the active muscles, and glycolysis is halted by an accumulation of acid metabolites.Simple field tests of endurance, such as Cooper’s 12-minute run and the Canadian Home Fitness Test, have some value in the rapid screening of large populations, but like other submaximal tests of human performance they lack the precision needed to advise the individual. The directly measured maximum oxygen intake (V̇O2max) varies with the type of exercise. The highest values are obtained during uphill treadmill running, but well trained athletes often approach these values during performance of sport-specific tasks.Limitations of methodology and wide interindividual variations of constitutional potential limit the interpretation of maximum oxygen intake data in terms of personal fitness, exercise prescription and the monitoring of training responses. The main practical value of V̇O2maxmeasurement is in the functional assessment of patients with cardiorespiratory disease, since changes are then large relative to the precision of the test.Determinants of Endurance Effort:During treadmill exercise, maximum effort is halted by central circulatory failure. It is less clear whether the limitation is imposed by a deficiency of venous return or an inability of the heart to develop an adequate stroke volume in the face of a rising peripheral resistance. Activities that depend upon the use of a relatively small muscle group (e.g. cycle ergometry, and especially arm ergometry) undoubtedly have some peripheral limitation, but this reflects difficulty in perfusing vigorously contracting muscles rather than any problem with utilisation of oxygen by the active fibres. Evidence of a cellular limitation of effort offered by Kaijser (1970) is disputed.V̇O2maxvaries somewhat with the type of exercise, but this reflects the difficulty in perfusing small muscles rather than an intracellular enzyme deficiency. Variations of ambient pressure cause the changes of V̇O2maxpredicted for a circulatory limitation, if due allowance is made for possible limitation of exercise by CO2poisoning. While there is some specificity of training when conditioning is based upon small muscle groups, treadmill training apparently improves cardiac performance, and thus V̇O2max, in many other types of endurance activity.Changes of maximum cardiac output induced by heat or (β-blocking drugs do not modify oxygen transport, but this is because a varying proportion of the total blood flow is directed to tissue other than muscle. Cooling reduces V̇O2max, but this could reflect a circulatory change rather than a lowering of tissue enzyme activity. Training increases the aerobic enzyme content of muscles, but the cellular response does not develop in parallel with V̇O2max; its main ’purpose’ is to encourage the utilisation of fat.Impending loss of consciousness undoubtedly reflects failing blood flow to the brain, but muscle weakness is due to an inhibition of glycolysis. Factors contributing to the latter include deficient oxygen transport, a slow diffusion oflactate and, to a lesser extent, an accumulation of hydrogen ions within the working tissue.Assessment of Endurance Performance:Simple field tests of endurance may help in screening large populations for the constitutionally well-endowed. Cooper’s 12-minute run is the best known ofthese procedures, and works reasonably well in highly motivated young adults. If the intent is to discover potential athletes, tests need to be event-specific. Moreover, in many activities (such as swimming) skill has a major impact on score.The Canadian Home Fitness Test is a simple stepping procedure, paced by a longplaying gramophone record. The safety of subjects is assured by preliminary questioning, an adequate warm-up and conservative submaximal pulse ceilings. Originally devised as a motivational tool, it can also be used to test fitness in the community, having about the same accuracy as other submaximal tests.The basic assumptions of most submaximal predictions of V̇O2maxinclude a linear heart rate/oxygen consumption relationship, a known maximum heart rate, and a known mechanical efficiency. Systematic errors of up to 10%, and random errors of ± 10% limit the value of the information that can be obtained through such prediction procedures.Direct measurements of V̇O2maxdepend upon careful technique (both preparation of the patient and biological calibration of the equipment). Increasing use is now made of sport-specific techniques, e.g. a flume for swimmers, rowing ergometers or rowing tanks for oarsmen, and roller-mounted racing cycles for cycling enthusiasts. Well-trained performers usually approach their treadmill V̇O2maxduring performance of the sport-specific task. The treadmill is the most popular of laboratory testing devices. The cycle ergometer generally yields a smaller and peripherally limited V̇O2max. although it has the advantage that it is easier to make ancillary measurements on a seated subject. A maximum effort step test remains a useful possibility for field locations, while various forms of arm ergometer are available for the testing of wheelchair athletes.Given one preliminary attempt, most adult volunteers can reach a classical plateau of oxygen consumption. Attempts to characterise the quality of an individual’s effort on subsidiary maximal criteria remain problematical. If good technical assistance is not available, fitness can be judged from the endurance of a progressive treadmill protocol, although such tests must be carried to complete exhaustion of the subject.Details of test protocol are less important than might be imagined. Factors contributing to a large V̇O2maxscore include a preliminary warm-up, running rather than walking, and possibly a schedule of effort which is intermittent rather than continuous.Interpretation:For most purposes, data are best expressed per kg of body mass, although absolute units are preferred for some weight-supported sports. Unusual results call for a critical review of both the technique and the population sampling procedures.Wide interindividual variations of constitutional potential and limitations of methodology limit the value of fitness assessment in the individual. Test scores give some guidance in work classification but in many sports the element of skill is more important than a large maximum oxygen intake. At best, exercise prescriptions based on V̇O2maxare crude, and they need fine tuning by the individual or the coach. Likewise, the training response is usually small relative to methodological errors, so that there is limited scope for V̇O2maxas a means of monitoring conditioning. The greatest potential of endurance testing probably lies in the overall functional assessment of cardiorespiratory disease, since many clinical conditions give rise to a gross impairment of V̇O2max.