An integrated model
Exercise scientists have found it difficult to develop an all-encompassing, integrated model of the various factors determining endurance performance. One model, developed by physiologist Edward Coyle PhD, seems to remain popular1 although others exist in the literature.2 In his hypothetical model, Coyle lists a number of morphological components that describe the structure of muscle and the cardiovascular system, all of which can differ between individuals. These components shape a performer’s functional abilities which, in turn, determine their performance velocity, defined as "the average speed an athlete maintains for a given racing distance".
Maximal oxygen consumption (a traditional indicator of endurance performance) is just one of several components in Coyle’s list of functional abilities, along with economy of movement, gross mechanical efficiency, oxygen consumption at the lactate threshold and the athlete’s power output at the lactate threshold.
While there is no question that maximal oxygen uptake is important in sustaining high intensity exercise for longer than four or five minutes,3 Coyle argues that the concept of the measurement of the blood lactate threshold provides information that is most indicative of the stress experienced by the exercising person as it relates to performance velocity, and that blood lactate concentration can be used to predict endurance performance ability. There is a substantial, and growing, body of evidence that supports this view.
According to exercise physiologist Andrew Jones PhD: "Traditionally, maximal oxygen uptake was considered to be the most important physiological measure in the assessment of potential for endurance exercise. More recently, it has been acknowledged that factors such as the lactate threshold, defined as the sub-maximal running speed that invokes a sudden and sustained increase in blood lactate concentration, and the running economy, defined as the energy cost of sub-maximal running, also contribute to endurance running performance."4
Defining the lactate threshold
Coyle, like Jones, is precise in his definition of the lactate threshold terms used in his theoretical model. Thus, lactate threshold VO2 is the VO2 (rate of bodily energy expenditure) that elicits a 1mM increase in lactic acid within the blood, and lactate threshold velocity or power is the velocity of movement or power output corresponding to the lactate threshold. These definitions are important, as are others used in the literature, since some authors may use the same terms to mean different things, potentially confusing the reader.
Physiologist Arthur Weltman PhD provides the following definitions: "Lactate threshold has been defined as the breakpoint, that is, the highest VO2 that can be attained during incremental exercise before an elevation in blood lactate is observed. This phenomenon has also been called the lactate breaking point, the onset of plasma lactate accumulation, the anaerobic threshold and the aerobic threshold."5
It has also been defined, says Weltman, as "the VO2 observed during incremental exercise associated with a blood lactate concentration that is 1.0mM (or 2.5 mM) above the baseline blood lactate concentration". In addition, he says: "The onset of blood lactate accumulation (OBLA) is used to describe the VO2 observed during incremental exercise associated with a blood lactate concentration of 4.0 mM."
Finally, Weltman defines the individual anaerobic threshold (IAT) as "the highest VO2 that can be maintained over time without a continual increase in blood lactate accumulation". This is sometimes referred to as the maximal steady state (MSS) or maximal lactate steady state (MLSS).
Economy of movement
While Coyle discusses the importance of gross mechanical efficiency of cycling at considerable length in his model, possibly the most impressive data on the topic was presented at a scientific meeting held in London.
Speaking at the conference Genes in Sport (London, 30 November 2001), Professor Bengt Saltin of the Copenhagen Muscle Research Centre, Denmark, described the physiological aspects relating to the dominance of endurance athletics by Kenyan runners. Saltin has spent many years collecting invaluable data on many of the top African distance runners. One of the major differences between the elite Kenyan distance athletes and European runners Saltin has assessed is that of running economy. In short, the Kenyans are able to run at a faster speed for the same oxygen consumption as the Europeans. Saltin believes that part of the Kenyans’ greater running economy is mechanical in origin, relating to both morphology and technique. Put simply, elite Kenyan distance runners seem to have relatively longer legs yet a smaller lower limb mass than their European counterparts. Consequently, less energy is required to accelerate and decelerate the leg during the running action. However, other factors that may influence physiological (as opposed to mechanical) efficiency, such as muscle fibre type composition, must also be considered.
Attempts to clarify the relationship between the various physiological factors determining endurance performance have resulted in several hypothetical models. These models, along with ongoing research, indicate that relying upon a single factor (eg, maximum oxygen uptake) is wrong when attempting to determine or predict an individual’s performance potential. There are other factors to be taken into account, some of which are modifiable through training. Consequently, it is important for sports performers to gain a greater understanding of the inter-relationship of the factors determining endurance performance if the most appropriate training programme is to be provided.
References available on request.
Typical values of maximal oxygen uptake in various sports (modified from Shephard, 1992, p193)
|Maximum oxygen uptake (ml/kg/min)|
|Long distance running||75-80||65-70|
|Discus, Shot Putt||40-45||35-40|
(NB: it can clearly be seen that elite performers in endurance events have high maximal oxygen uptakes).