Escaping Virgil's underworld: dissociating Aeneas's task from his toil

When Virgil's hero Aeneas sought to return from the underworld the task (power output) was carefully discriminated from the toil (exercise intensity) “hoc opus, hic labor est” (Fig. 1). Physiologists interested in athletic performance have recognized that exercise tolerance and the development of muscle fatigue across and within individuals does not depend as much upon the absolute power output (or metabolic rate, i.e. O₂ uptake, ${\dot{V}}_{{\mathrm{O}}_2}$) per se as on the exercise intensity domain in which that task is performed. In this regard, the ${\dot{V}}_{{\mathrm{O}}_2}$ equivalent of the lactate threshold defines the boundary between moderate (no sustained increase in blood [lactate], rapid ${\dot{V}}_{{\mathrm{O}}_2}$ kinetics) and heavy (sustained increase in [lactate], emergence of ${\dot{V}}_{{\mathrm{O}}_2}$ slow component and decreased work efficiency) (Poole & Jones, 2012). Likewise, the critical power discriminates heavy from severe exercise, with the latter characterized by an inexorably increasing profile of ${\dot{V}}_{{\mathrm{O}}_2}$ (to ${\dot{V}}_{{\mathrm{O}}_2\max }$) and [lactate] (reviewed by Poole et al. 2016). Whereas it has been long established that intermittent exercise protocols allow more work to be performed than their continuous counterparts (Astrand et al. 1960), the mechanistic bases for this effect eluded resolution.

Figure 1. Toil in the underworld.

M. Gustave Dore's depiction of toil in the underworld, ‘Punishment of the Avaricious and the Prodigal’, from Dante Alighieri's Inferno (plate 22) translated by The Rev. Henry Francis Cary. Cassell, Petter, Galpin & Co., New York, 1890.

Thus, Davies and colleagues (2017) employed an elegant sequence of intermittent and constant‐load exercise bouts combined with phosphorous magnetic resonance spectroscopy (³¹PMRS) interrogation of the intramuscular milieu and breath‐by‐breath pulmonary gas exchange (${\dot{V}}_{{\mathrm{O}}_2}$) to dissociate power output generated from muscle energetics, pH and ${\dot{V}}_{{\mathrm{O}}_2}$. In so doing, their data yield unique insights into the processes of muscular fatigue and exhaustion. Specifically, short exercise:recovery durations (16:32 s) compared with up to 64:128 s intervals and continuous protocols meant that the equivalent mean ATP turnover rate could be achieved via increased reliance on PCr breakdown and venous O₂ stores in preference to anaerobic glycolysis and pulmonary ${\dot{V}}_{{\mathrm{O}}_2}$. The short work interval largely curtailed the pronounced fall in intramyocyte pH characteristic of continuous severe intensity exercise and the slow component of the ${\dot{V}}_{{\mathrm{O}}_2}$ kinetics. Indeed, during the short exercise: recovery bout the muscle was alkalotic as PCr was falling, raising the fascinating question as to what degree this profile in‐and‐of itself helped preserve muscle functionality, preventing fatigue/exhaustion and also development of any ${\dot{V}}_{{\mathrm{O}}_2}$ slow component behaviour or related muscle inefficiency.

Effectively, the short exercise:recovery protocol converted power outputs from the non‐sustainable severe exercise intensity domain to either moderate (16:32 s) or heavy (32:64 s) where the ${\dot{V}}_{{\mathrm{O}}_2}$ profile no longer projects to ${\dot{V}}_{{\mathrm{O}}_2\max }$ signalling impending exhaustion. Indeed, exercise tolerance was increased over threefold by this strategy which, in itself, begs the query as to why the individuals would fatigue at all, at least within the course of minutes to hours. Comparison of the short versus longer exercise:recovery intervals does lead to the compelling conclusion that system stability and resisting large fluctuations in intramuscular phosphagens and pH, as well as the ${\dot{V}}_{{\mathrm{O}}_2}$ slow component, are crucial to preventing or at least delaying the processes of fatigue and exhaustion (Jones et al. 2008). Perhaps this realization means that the exercise intensity (toil) is best related to the extremes of the metabolic perturbations evoked by the exercise protocol.

By explaining the underlying phenomenon in terms of muscle energetics, this discovery offers substantial promise for improving exercise tolerance and designing effective training protocols for a range of indigent patient and healthy aged populations. For instance, exercise intolerance compromises life's quality and increases morbidity and mortality for the burgeoning cohort of chronic heart failure, emphysema (chronic obstructive pulmonary disease, COPD) and diabetic patients. These individuals, in whom the O₂ transport system enforces a low and limiting ceiling, will benefit from restructuring sustained physical tasks such as stair climbing or carrying groceries into short intervals repeated after ∼30 s rest by increasing their functionality and thus quality of life. Moreover, Davies et al.’s (2017) findings help us understand and, to a certain extent, predict, the impressive effects of high intensity interval training in healthy and patient populations (Gibala et al. 2012). The double‐edged sword here is that such exercise training programmes must be designed judiciously for the intended outcome and that signals driving muscle adaptations (bioenergetics fluctuations, for example) may be very different from those for systemic O₂ transport improvements. It is to be hoped that the novel data in Davies et al. (2017) will provide added impetus and allow development and refinement of training protocols tailored to best drive the most beneficial adaptations in the candidate individual or population. The importance of this undertaking should not be underestimated. In the developed world afflicted individuals number in the tens of millions and they, together with their healthy ageing compatriots, rely upon the capacity for physical activity to prolong their functional lifespan and offset their reliance on dependent care. For enhancing patient rehabilitation, employment of an intermittent exercise protocol that evokes a relatively modest rate of perceived exertion (so‐called RPE) while promoting substantial muscle and/or O₂ transport adaptations would likely improve patient participation, sustainability and outcomes.

Over two thousand years after Virgil recognized the importance of separating the task from the toil, Davies and colleagues (2017) have provided fundamental clues regarding the control of muscle energetics as to when and how this may be accomplished. Perhaps this will offer affected populations the real hope of escaping their underworld of exercise dysfunction that predicates premature morbidity and mortality.

Additional information

Competing interests

None declared.