CrossTalk opposing view: Diffusion limitation of O2 from microvessels into muscle does not contribute to the limitation of

Oxygen is transferred from air to mitochondria in sequential steps by means of diffusion and convection. The theoretical ceiling for whole body aerobic uptake may be determined by any factor influencing the O2 transport and utilization chain. The dispute considered here is whether peripheral O2 diffusion from microvessels, particularly capillaries, into active skeletal muscle fibres limits/regulates maximal oxygen uptake () in healthy humans. is experimentally determined by the levelling off in O2 uptake observed with increasing workload during dynamic exercise involving more than half of total muscle mass (e.g. running, cycling) (Levine, 2008). This indicates that, empirically, is limited, i.e. primarily restricted prior to peak muscle activation. Moreover, such limitation is attributed to a finite oxygen supply to muscle given that mitochondrial oxidative capacity exceeds that of oxygen delivery at (Boushel et al. 2011). Thus, any factor related to the transport of O2 into the mitochondria might limit . In this regard, there is sound evidence that is proportionally modified in accordance with acute changes in blood O2-carrying capacity and content (Calbet et al. 2006a). In contrast, a decrease in arterial O2 partial pressure, and thereby reducing the driving force for O2 diffusion, does not affect maximal O2 uptake if O2 delivery to the exercising limbs at the same time remains preserved (Calbet et al. 2003, 2009). Importantly, these observations were not associated with any particular individual's fitness status. Accordingly, at in healthy individuals there must be a physiologically relevant reserve in muscle O2 diffusing capacity, which also precludes that is limited by peripheral O2 diffusion from capillary into muscle. While peripheral O2 diffusion does not limit , the question arises as to whether could be influenced by muscle O2 diffusing capacity in healthy individuals. This would be entirely refuted if O2 were fully extracted from capillaries supplying active muscle fibres. However, such level of resolution for O2 extraction within human muscle fibres is beyond reach with current methods (Koga et al. 2014). Instead, the large body of empirical evidence derives from O2 measures in venous blood exiting the active limb (Rud et al. 2012), in which a 100% O2 extraction seems unachievable considering temporal and spatial characteristics (Heinonen et al. 2015). For instance, the high intramuscular pressure generated during the contraction phase partly diverts blood flow toward less metabolically active tissue (Clark et al. 2000). Also, the perfusion of active muscle fibres is inherently inefficient as regards O2 delivery, since microvascular units (i.e. terminal arteriole and downstream capillaries) are not spatially coordinated with individual motor units and this may result in the overperfusion of inactive fibres (Emerson Calbet et al. 2006b). Of note, blood flow distribution is determined by the complex interplay of factors including the sympathetic drive, concentration of vasoactive substances, arterial dilator/constrictor function and microvascular structure. Therefore, a perfect matching between leg O2 delivery and metabolic demand during exercise is not expected even if neglecting the previously mentioned temporal and spatial intrinsic constraints; here, blood flow distribution is improved and O2 extraction enhanced within exercising muscles in long-term trained individuals (Kalliokoski et al. 2001). Ignoring or trivializing this fact may have distorted the contribution of muscle O2 diffusion to the limitation (Piiper, 2000; Koga et al. 2014). Yet O2 extraction across the leg commonly rises to 85% or more at in untrained and trained individuals (Lundby et al. 2006; Rud et al. 2012), attaining an astonishing 97% in some elite athletes (Calbet et al. 2005). It follows that for muscle O2 diffusing capacity to contribute to the limitation of , an average of approximately 85% or more of leg blood flow should be continuously perfusing active muscle fibres. If leg O2 extraction, and thus , were regulated by leg muscle O2 diffusing capacity, any increase in diffusion capacity would be reflected, at least to a degree, in an augmented leg O2 extraction. In this respect, one-leg training studies have provided compelling evidence because the impact of peripheral adaptations on may be isolated from central haemodynamic adaptations (Gleser, 1973; Saltin et al. 1976; Klausen et al. 1982; Rud et al. 2012). All these studies have shown marked increases in one-legged cycling peak blood flow, O2 diffusing capacity, and uptake in individuals who, before training, had in the normal range (Gleser, 1973; Saltin et al. 1976; Klausen et al. 1982; Rud et al. 2012). However, peak O2 uptake during two-legged cycling (i.e. ) remained unaltered in the presence of unchanged maximal cardiac output () following one-legged training (Gleser, 1973). In line with this, the increase in following two-legged training was reverted to the baseline level after negating the training-induced gain in by means of phlebotomy (Bonne et al. 2014). Likewise, was identical prior to and after one-legged training despite the fact that blood flow, O2 extraction and uptake were enhanced during two-legged cycling in the trained versus control leg (Rud et al. 2012). This strongly suggests that two-legged O2 extraction (∼85%) was maximized relative to blood flow at , irrespective of any training adaptation in muscle O2 diffusing capacity. Overall, these data indicate that is not modulated by muscle O2 diffusing capacity. Rather, seems to be governed, in a tyrannical manner, by the amount of blood flowing into the exercising limbs. Several canine studies have been aimed aet examining the contribution of muscle O2 diffusing capacity to O2 extraction (Schumacker et al. 1985; Barclay, 1986; Hogan et al. 1989, 1991; Richardson et al. 1998). In these, blood flow or haemoglobin O2 affinity (P50) have been manipulated while maintaining O2 delivery constant to contracting muscle. Hence a regulatory role for O2 diffusion from capillary into muscle in O2 extraction could be pinpointed, provided that O2 delivery to active muscle fibres is not influenced by blood flow or P50 (Schumacker et al. 1987). Regardless, the majority of the evidence comparing experimental versus control conditions indicates that muscle O2 uptake is uniquely dependent on O2 delivery (Schumacker et al. 1985, 1987; Barclay, 1986; Richardson et al. 1998). Furthermore, the presence of statistical procedures raising the likelihood for type I errors to occur (i.e. the probability of making false discoveries) in divergent findings is noteworthy (Hogan et al. 1989). Taken together, the proposed regulatory role for O2 diffusion from capillary into muscle during exercise (Wagner, 1992) cannot be induced from the above animal experiments, let alone extrapolating it to humans exercising at , in which, as a matter of fact, such a role is empirically absent (Calbet et al. 2003; Lundby et al. 2006). Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief (250 word) comment. Comments may be submitted up to 6 weeks after publication of the article, at which point the discussion will close and the CrossTalk authors will be invited to submit a 'Last Word'. Please email your comment, including a title and a declaration of interest to jphysiol@physoc.org. Comments will be moderated and accepted comments will be published online only as 'supporting information' to the original debate articles once discussion has closed. Carsten Lundby (left) and David Montero (right) both work at the University of Zürich in a small team focusing on Oxygen Transport and Utilization in its broadest sense. The strength of the group is its integrative approach which allows them to study O2 as it is transported all the way from inspired air and until it is oxidized in the mitochondrion. Their most recent work has focused on the relative importance of skeletal muscle and circulatory adaptations to exercise training for improvements in and hence is in direct line with the current cross talk. Disclaimer: Supplementary materials have been peer-reviewed but not copyedited. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. None declared.