Metabolic Power and Oxygen Consumption in Soccer: Facts and Theories

Abstract

After a brief overview of the principles underlying the assessment of the energy cost of accelerated/decelerated running on flat terrain, as obtained from the biomechanical equivalence between this last and uphill/downhill running at constant speed, metabolic power and actual O₂ consumption (${\dot{\text{V}}}{\text{O}_2}$) will be estimated during a typical training drill and during a brief period of a soccer match. This exercise will show that: (i) ${\dot{\text{V}}}{\text{O}_2}$, as estimated from metabolic power assuming mono-exponential ${\dot{\text{V}}}{\text{O}_2}$ on/off responses with a time constant of 20 s at the muscle level, is essentially equal to that directly measured by means of a metabolic cart. Hence, (ii) the fractions of the overall energy expenditure derived from aerobic and anaerobic sources can also be estimated, provided that the individual ${\dot{\text{V}}}{\text{O}_2}$max is also known. Furthermore, the metabolic power approach will be updated to make it possible to estimate energy cost and metabolic power also during walking episodes. Indeed, whereas for any given incline the energy cost of running is independent of the speed, the energy cost of walking reaches a minimum at an optimal speed above and below which it increases. With increasing the incline of the terrain, the curve relating energy cost to walking speed retains this general U shape, but is shifted to a higher level, the optimum speed decreasing towards lower values. In addition, for any given incline there exists a “transition speed” at which the energy cost of walking becomes greater than that of running, and which corresponds to the speed at which the subject spontaneously adopts the running gait. Hence the updated algorithms detect this transition speed, as a function of the equivalent slope, and utilise the appropriate energy cost of walking or running in the subsequent calculations. The concluding paragraphs are devoted to a critical discussion of the principal assumptions underlying the metabolic power approach. Specifically, we will stress that the approach as such can be meaningfully applied only to forward walking or running. Thus, whereas other specific sport activities such as jumping, moving backwards, laterally, or with the ball, may be taken into account, at least in a broad sense, assuming appropriate correction factors (possibly on the bases of actual experimental data), other activities, such as those occurring in rugby, of American football, need entirely different approaches, a fact that, all too often, does not seem to be appropriately considered.