For most elite athletes winning an Olympic gold medal is the ultimatedream. To make this dream come true, in the first place one needs sufficienttalent. However next to this talent, several years of training with largeamounts of strenuous work is necessary. It is therefore not remarkable thatthe time required for adequate recovery may easily be compromised.Doing so, an athlete is often challenging the optimal balance betweenexercise and recovery.The general purpose of this study was to get more understanding in thephysiological adaptation processes of highly trained elite enduranceathletes during periods of strenuous training.This thesis is divided into three parts. The first part (chapter 2, 3 and 4)describes the results of a three-year longitudinal study in professionalOlympic distance (OD) triathletes in preparation for Sydney 2000 Olympicgames. Chapter 2 gives a description of some measured performancerelated physiological- and anthropometric variables in OD triathletesduring the different phases of a year. Despite a clear difference in theamount of training between the several phases during the year, nosignificant differences were found between the measured performancerelated variables during submaximal and maximal exercise intensities.In spite of large amounts of training elite OD triathletes have still a lowermaximal oxygen uptake (VO2max) compared to elite distance runners andelite cyclists. Chapter 3 describes the changes in the hematological bloodprofile, as a result of endurance- and altitude training. As red blood cellsplay an important role in the transport of oxygen to the working muscles,it is important to get insight into the dynamics of the red blood profileand its possible changes in athletes, for instance with very intensivetraining and altitude training. Over a three-year period, in a group of elitemale and female OD triathletes' blood samples were collected duringseveral moments throughout the year. It was found that intensiveendurance training had not much influence on the red blood cell profile inmale and female OD triathletes. However, it was found that most of theathletes examined (female even more than male athletes), hadhematological values (particularly hemoglobin and ferritin) close to or insome cases below the lower limit of the normal range. Altitude trainingdid alter the red blood profile significantly only when altitude training wasperformed above 2000 meters above sea level. In chapter 4, the effects ofendurance training on the endocrine system throughout a year in ODtriathletes are described. Changes in hormonal plasma values are oftenmentioned as good indicators for the balance between training andrecovery. Despite the significant differences in the amount of trainingbetween the different phases of a competitive season, no significantchanges were found in the measured hormones throughout the year inelite male and female triathletes.In the second part (chapter 5 and 6) some exercise protocols that aregenerally used to evaluate training and performance are addressed. Inchapter 5 (cycling) and chapter 6 (running), the effect of stage durationon physiological variables in an incremental exercise test was studied.For measuring reliable lactate values during an incremental cyclingexercise tests a stage duration of at least 5 minutes has to be taken intoaccount. In a running protocol, the stage duration for obtaining bloodlactate values that match exercise intensity, should be at least 6 minutes.When measuring maximal values such as peak VO2, peak heart rates ormaximal workload incremental protocols with increments every minuteare sufficient.In the third part (chapter 7 to 9) training intervention studies are described.The purpose of these studies was to get more insight into the physiologicalchanges of the human body during periods of imbalance between trainingload and recovery. Chapter 7 describes the effects of a 50% reduction intraining load over a period of 21 days on some performance-relatedvariables. Many trainers and athletes are afraid to reduce the training asthey think this will cause a loss in performance. However, the resultsdescribed in this chapter show that this fear is not supported by evidence.Furthermore, it was found that an intermittent training program had nobenefit above a long slow distance training program. In chapter 8, theresults of a training intervention study (overreaching) are described.The main purpose of this study was to investigate whether the generallyaccepted overtraining markers are useful, and can be used as early markersfor overreaching. The main outcome of this study was that simple psychologicalquestionnaires appear to be a more indicative and sensitive markersfor detecting overreaching/overtraining in an early stage compared to themeasured biochemical- endocrinological- and physiological variables.Furthermore, it was found that overreaching is associated with earlychanges in cognitive performance, which can be detected by simplecognitive performance tests. In chapter 9, possible differences in bloodchemical, neuro-endocrine and psychological parameters in response to anintensified training period in runners and cyclists were investigated. Theresults of this study indicate that runners experienced a period ofstrenuous training as heavier compared to the cyclists. This difference ofsubjective feeling was not reflected in the measured blood chemical andneuro-endocrine variables.In conclusion, the most important findings in this thesis are:'' OD triathletes show hardly any variation in generally in sport measuredbiochemical-, hormonal-, or performance related variables.'' Regular control of the red blood profile is useful in well-trained enduranceathletes as many of these athletes have blood values close to or below thelower limit of the normal range'' The results of this study indicate that there exists an 'altitude threshold'above which hematological alterations occur.'' The endocrine system is (with the exception of cortisol) not a good toolto prevent overtraining.'' Cognitive performance tests appear to be useful to detect overreaching inits earliest stage.'' Endurance athletes are able to maintain their performance level for someweeks in spite of reducing the training volume.'' The subjective perception of very intense training is different betweencyclist and runners'' To ensure a reliable lactate level in incremental exercise tests, a stageduration of at least 5 minutes is required.