Limit to steady-state aerobic power of skeletal muscles

Abstract

Like any other kind of cell, muscle cells produce energy by oxidizing the fuel substrate that they absorb together with the needed oxygen from the surroundings. Oxidation occurs entirely within the cell. It means that the reactants and products of reaction must at some time be dissolved in the cell's cytosol. If a cell operates at steady state, its cytosol composition remains constant. Therefore, the cytosol in a muscle that produces work at steady state must contain a constant amount of fuel, oxygen, and product of reaction dissolved in it. The greater the power produced, the higher the concentration of these solutes. There is a limit, however, to the maximum amount of solutes that the cytosol can contain without damaging the cell. General thermodynamic arguments, which are reviewed in this paper, help relate this limit to the dehydration and overhydration limits of the cell. The present analysis shows that the same limits entail a limit to the maximum power that a muscle can produce at steady state. This limit depends on the composition of the fuel mixture used by the muscle. The analysis also determines the number of fuel carbon atoms that must be oxidized in parallel within a cell to produce a given power. It may well happen that a muscle cannot reach the maximum attainable power because it cannot activate all the parallel oxidation paths that are needed to produce it. This may be due to a series of reasons ranging from health issues to a lack of training. The paper shows how the methods of indirect calorimetry can provide all the experimental data needed to determine the actual number of parallel oxidation paths that at steady state must be active in a muscle in a given exercise. A diagram relating muscle power to the number of parallel oxidation paths and fuel composition is finally presented. It provides a means to assess the power capacity of animal muscles and can be applied to evaluate their fitness, stamina, margins for improvement, and athletic potential.

Keywords: Aerobic power; Animal power limit; Cell energy production; Muscle power; Steady-state animal power.

Fig. 1

Free energy, Δg_C, liberated upon oxidation of one mole of carbon atoms of fuel, cf. Eq. (63). Power, Δg_C/Δτ, produced at steady state per mole of carbon atoms, as given by Eq. (65)

Fig. 2

Representation of a typical muscle cell’s admissible range in the ${\overline{n}}_{\mathit{eq}}$-axis. Variable ${\overline{n}}_{\mathrm{eq}}$ denotes the total moles of cytosol solutes per mole of cytosolic water; see Eq. (28). If the cell absorbs water, the rest state moves to the left of the figure, thus increasing ${\left.\Delta {\overline{n}}_{\mathit{eq}}\right|}_{\max }$. The converse occurs if the cell loses water

Fig. 3

Values of κ_C|_max vs. fuel composition (represented in the figure by x_FFA or x_CHO). All diagrams refer to a muscle cell with the admissible range of Fig. 2. The bold curve, marked “normal”, is a plot of Eq. (77). Absorption or loss of cytosolic water moves this curve into the light curves, as specified in the figure

Fig. 4

Total power, p_t, produced at steady state per kilogram of wet muscle as a function of fuel composition. The diagrams refer to muscle cells with the admissible range and rest state described in Fig. 2. The bold curve gives the muscle’s total power when the maximum number of parallel oxidation paths are active, Eq. (79). The other curves correspond to the activation of different fractions of parallel oxidation paths