REVIEW ARTICLE Factors limiting maximal performance in humans Accepted: 4 July 2003/Published online: 9 August 2003 �� Springer-Verlag 2003 Abstract Theoretical best performance times (ttheor) in track running are calculated as follows. Maximal met- abolic power (Emax) _ is a known function of maximal oxygen uptake (VO2max), _ of maximal anaerobic capacity (AnS) and of effort duration to exhaustion (te): Emax=f_ (te). Metabolic power requirement (Er) _ to cover the distance (d) in the performance time tp is the product of the energy cost of locomotion per unit distance (C) and the speed: Er=C��d/tp. _ The time values for which Emax _ (te)=Er _ (tp), assumed to yield ttheor, can be obtained for any given subject and distance provided that VO2max, _ AnS and C are known, and compared with actual best performances (tact). For 15 min���te���100 s, the overall ratio tact/ttheor was rather close to 1.0. To estimate the relative role of the different factors limiting VO2max, _ several resistances to O2 transport are identified, in- versely proportional to: alveolar ventilation (RV*), O2 transport by the circulation (RQ), O2 diffusion from capillary blood to mitochondria (Rt), mitochondrial capacity (Rm). Observed changes of VO2max _ are accompanied by measured changes of several resis- tances. The ratio of each resistance to the overall resis- tance can therefore be calculated by means of the O2 conductance equation. In exercise with large muscle groups (two legs), RQ is the major (75%) limiting factor downstream of the lung, its role being reduced to 50% during exercise with small muscle groups (one leg). Rt and Rm account for the remaining fractions. In normoxia RV* is negligible at high altitude it increases progressively, together with Rt and Rm, at the expense of RQ. Keywords Maximal anaerobic capacity �� Maximal metabolic power �� Maximal performances �� Running �� VO2max _ limits Introduction The present article is devoted to a brief review of the factors setting maximal performances in track running. The problem will be addressed considering the energy requirement for covering a given distance, on the one hand, and the maximal rate of metabolic energy output from aerobic and anaerobic sources on the other. In- deed, the best performance time over a given distance is equal to the minimum time allowing the subject to ob- tain (from the energy-yielding mechanisms) the amount of energy necessary and su���cient for covering the dis- tance at stake. The analysis will be applied to track running over distances (and times) such that the rate of O2 consumption can be maintained at its maximal level (VO2max) _ throughout the effort duration. However, the model can be applied to any other forms of locomotion, such as cycling or swimming, the energy cost of which is known as a function of the speed and/or for distances such that the rate of O2 consumption maintained throughout the effort is 100% VO2max._ The first section of this article will be devoted to show that the best performance times, for any given distance and subject, can be predicted rather accurately, once his/ her energy cost of running, together with his/her VO2max_ and maximal anaerobic capacity are known. This section will also show that VO2max _ plays a central role among the energy-yielding mechanisms. Therefore, the second part of the study will be devoted to an analysis of the factors limiting VO2max. _ In this section it will be shown that the commonly held view that VO2max _ is set essen- tially by the O2 transport system (maximal cardiac output and O2 carrying capacity) is fundamentally cor- rect for exercises with large muscle groups at sea level. However, at high altitude or for exercises with small Eur J Appl Physiol (2003) 90: 420���429 DOI 10.1007/s00421-003-0926-z Pietro Enrico di Prampero P. E. di Prampero Department of Biomedical Sciences and Technology MATI (Microgravity Ageing Training Immobility) Centre, University of Udine, P. le M. Kolbe 4, 33100 Udine, Italy E-mail: pprampero@makek.dstb.uniud.it Tel.: +39-0432-494330 Fax: +39-0432-494301

muscle groups, other factors, such as ventilation or the mitochondrial capacity, assume a substantial role in setting VO2max. _ In concluding this Introduction, it seems crucial to point out that the factors considered in this study must be viewed as the ������energetic bottleneck������ setting actual performances. It seems a platitude to state that this bottleneck cannot be surpassed. Whether or not it is indeed reached by a given subject in a given instance depends on a set of other factors that I do not dare to discuss. They are reviewed in this same issue by Bengt Kayser. Of best performance times The overall amount of energy (Er) required to cover the distance d is the product of the energy cost of transport, per unit of distance (C) and the distance itself: Er �� C d ��1�� where C is the integrated average over the distance d. For an exercise intensity greater than VO2max _ and for an exercise duration su���cient for complete exploitation of the anaerobic energy stores (AnS), but within the range for which VO2max _ can be maintained at 100%, the metabolic energy output (Em) is given by Em �� AnS �� MAP te ��2�� where MAP is the maximal aerobic metabolic power, as obtained expressing VO2max _ in the appropriate units, and te is the exhaustion time. Thus, setting Eq. 1 equal to Eq. 2, and solving for te: te��tp����C d AnS��=MAP ��3�� the calculation of the best performance time, assumed to be equal to the exhaustion time (tp) for any given subject and distance may appear to be a straightforward task, provided that C is known. It is a melancholy fact, however, that Eq. 3 cannot be solved analytically, be- cause of the following reasons. 1. The energy cost of transport is an increasing function of the speed, so that C and t in Eq. 3 are not inde- pendent. It should also be pointed out that, from this viewpoint, track running is relatively privileged, since C increases only slightly with the speed. 2. Since the majority of the actual competitions begin from a stationary start, C must take into account the energy necessary to accelerate the body in the initial phase of the run. Thus, again, this fraction of C is dependent on both te and d. 3. In addition, the time range to which Eq. 3 can be meaningfully applied is rather restricted (from about 50 s to about 10 min), because this equation is based on the assumption that: (1) the anaerobic energy sources are fully exploited and (2) VO2max _ (MAP) is maintained at 100% throughout the duration of the run. 4. Finally, as written, Eq. 3 implies that VO2max _ (MAP) is attained at the very onset of exercise. Therefore, once more, the actual performance time must be gi- ven due consideration in assigning numerical values to AnS and MAP in Eq. 3. In spite of these drawbacks, Eq. 3 has the advantage of making explicit the role of the energy cost (C), of the capacity of the anaerobic stores (AnS) and of the max- imal aerobic power (MAP) in setting maximal perfor- mances in human locomotion. For endurance running, under which conditions the role of AnS is negligible and C is essentially independent of the speed, Eq. 3 reduces to: tp��C d=��F MAP�� ��4�� where F is the maximal fraction of VO2max _ that can be maintained throughout the run. Indeed, for marathon or semi-marathon (21 km) tp, as calculated from Eq. 4, is not significantly different from the actual running time (di Prampero et al. 1986 Brueckner et al. 1991). To extend the analysis to shorter times and distances, wherein the simplification represented by Eq. 4 does not apply, a somewhat more tortuous route will be followed. For a given distance and subject, both the power requirement to cover the distance at stake (Er) _ and the maximal metabolic power that the subject in question is able to sustain until exhaustion (Emax) _ will be repre- sented on the same time axis. The time value for which the dependent variables of the two functions [Er=f(t)_ Emax=f(t)] _ become equal will be calculated graphically, or by iterative procedures, and will be taken to represent the best performance time, the additional underlying assumption being that best performance and exhaustion time coincide (Keller 1973). It will be shown that the so calculated best performance times are remarkably close to the actual best individual performances. This approach was originally proposed by di Prampero (1984, 1989) and further developed by several other authors for running (Peronnet and Thibault 1989 di Prampero et al. 1993 Alvarez-Ramirez 2002 Arsac 2002 Arsac and Locatelli 2002) and cycling (Olds et al. 1993, 1995 Capelli et al. 1998). For a comprehensive analysis of this approach, the reader is referred to these studies as well as to van Ingen Schenau (1991) and Capelli (1999). Of maximal metabolic power When the energy requirement of the exercise is greater than can be aerobically provided by the subject���s VO2max, _ the exercising muscles must rely on anaerobic energy sources. These are: (1) net splitting of phospho- creatine (PCr) with a concomitant fall of its concentra- tion in the working muscles and (2) net production of lactate, leading to an increase in its concentration in the body fluids. So, since the ATP concentration in the working muscle is essentially unchanged, the maximal 421