Since a period of rapid technical discovery and development in the 1980s, nuclear MRS has provided an unrivalled tool to study the biochemistry (and biophysics) of living cells in real time. Now that the reconciliation of cell and tissue data with whole organism physiology is becoming a priority, MR will continue to offer unique advantages over more destructive methods (mass spectrometry) or those that can only be used under very specific circumstances (live cell imaging). In this paper, we report our results from an unusual data set – combined 31P MRS and applied physiology measurements made on a particularly large cohort of healthy humans of mixed ethnicity and gender. As a result, we were able to test a number of hypotheses relating muscle biochemistry to whole body physiology in normal humans with unusually high statistical power.
In general we found that muscle biochemistry was reflected in whole body physiology in a manner that was consistent with existing theory and data, and with our a priori
hypotheses. For example, based on earlier findings in smaller, more specialized cohorts21,22,23
we predicted that, in a large cohort of mixed gender and ethnicity, whole-body VO2
max would correlate with muscle respiratory capacity, measured using 31
P-MRS. Our hypothesis was supported: PCr1/2t
(negatively) and Qmax and ATPmax (positively) correlated with both absolute and mass-corrected VO2
max (). Although the correlation coefficients appear low this supports the notion that VO2
max is a complex measure of integrative physiology that is only partly determined by mitochondrial function. Indeed, the magnitude of these relationships suggested that approximately 20–25% of the variation in mass-corrected VO2
max could be explained by variations in muscle respiratory capacity, a finding that was also consistent with previous work. The remainder of the variation in VO2
max was presumably due to extra-muscular physiological factors such as diffusive and/or convective oxygen delivery (and possibly the neuropsychological factor of motivation). The halftime of PCr recovery appeared to be a more robust indicator of VO2
max than either of the extrapolated/calculated values (Qmax or ATPmax), while ATPmax was a better predictor of both absolute and mass-adjusted VO2
max than was Qmax. This may however simply reflect the greater mathematical/theoretical simplicity of ATPmax, resulting in reduced noise (but not necessarily better reflecting underling physiology).
Delta efficiency during cycling is related to the percentage of oxidative fibres in the locomotor muscles2
. Given that PCr concentration in oxidative fibres is lower than in glycolytic fibres24
and therefore mean PCr concentration in mixed muscle reflects fibre type distribution15,25,26
, we tested the hypothesis that muscle PCr content, measured using 31
P-MRS, would be positively correlated with DE (the slope of VO2
against work rate at work rates below VT27
) during incremental exercise testing. In this case our hypothesis was supported by the data (r
= .22, p
= .046), strengthening not only the earlier (but disputed28
) findings of Coyle et al.14
, but also the utility of [PCr] as an index of muscle fibre type distribution between individuals.
There is an ongoing debate as to whether muscle mitochondrial function and delta efficiency are related (cf the introduction in29
). For example, Lucia et al.
reported an inverse relationship between VO2
max and efficiency (during a steady-state exercise task)30
suggesting an inverse relationship between muscle oxidative capacity and delta efficiency. However, this study was carried out in a highly-selective group (world-class endurance cyclists), so the results may have reflected not ‘normal’ physiology but the result of a selective process within tight constraints - those with marginal aerobic capacity may have been nevertheless able to succeed as professional road cyclists precisely because they were highly efficient. Thus we decided to test for a relationship between delta efficiency (as defined above) and muscle respiratory capacity. We found a significant inverse correlation between PCr1/2t
and DE (r
= −.29, p
= .006), such that better mitochondrial function was associated with higher delta efficiency. Our findings are at odds with those of Hunter et al.
), who reported an inverse correlation between exercise economy (defined as the oxygen cost of a given walking velocity during steady-state walking) and muscle respiratory capacity (measured, as here, using 31
P-MRS). However, these disparate results might be reconciled by differences in both methodology (DE compared with VO2
during steady state walking) and cohort (pre-menopausal women only, compared with a mixed gender and ethnicity cohort). Given that we also found a positive correlation between PCr concentration and delta efficiency, it is tempting to suggest that both correlations ([PCr] vs. DE and PCr1/2t
vs. DE) are the result of underlying differences in fibre type distribution. Unfortunately it is a limitation of the present study that we did not measure fibre type distribution directly.
Given an earlier finding that resting metabolic rate was positively related to muscle ATP-synthase content31
, we decided to test our hypothesis that resting metabolic rate was related to muscle respiratory capacity. The data supported our hypothesis: PCr1/2t
was significantly related to total daily energy expenditure (TDEE) (r
= −.23, p
= .030) (). Therefore those with higher muscle respiratory capacity had increased daily energy expenditure. The nature of causality between these variables is, as yet, unclear. One might reasonably hypothesize that increased daily activity led to improved muscle respiratory capacity and this view is somewhat supported by the absence of a positive relationship between resting VO2
and muscle oxidative capacity.
Intriguingly, we found good evidence linking body composition to muscle mitochondrial function. Again, the direction of causality here is unclear. One would expect those who are more active to use more energy and therefore to have higher muscle (and whole-body) respiratory capacities and less body fat. By Occam's razor, this explanation would seem to be more likely than the concept that increased fat mass was somehow inhibiting muscle mitochondrial function However, there was a very significant relationship between resting oxygen uptake and TDEE (r = .43, n = 86, p = 0.00003), such that one might also reasonably argue that some subjects were genetically predisposed to having a higher resting energy cost, better muscle mitochondrial function and therefore had a higher TDEE and lower fat mass.
There were no consistent differences in muscle biochemistry between genders or by ethnicity, with one exception. Muscle pH was significantly lower in females compared with males (7.06 ±.003 vs. 7.09 ±.004, p
). Although this appears at first to be only a minor difference it amounts to a 7% higher proton concentration in males. The determinants of intracellular pH setpoint in themselves remain far less well understood in skeletal muscle than in cardiac muscle32
, although the principle is that basal pH is determined by the steady state balance between a number of sarcolemmal proton exchangers and uniporters. Furthermore the significance of this pH difference by gender is difficult to determine, although it must be considered if comparing any pH-dependent variable between groups that are unbalanced for gender. Beyond this, our unsupervised multivariate analysis of the resting spectra revealed no unexpected grouping.
This dataset represented a unique opportunity to study relationships between non-invasive measures of muscle biochemistry with whole-body exercise-physiology, with particular reference to oxidative metabolism, in a group of normal subjects large enough to establish these with high statistical power. Unlike earlier studies in this area we did not study the effects of formal exercise training, and especially not adaptions in professional athletes where, as noted in the Discussion, selection effects can give apparently paradoxical results. The results broadly confirmed our pre-specified hypotheses that a substantial fraction of variability in whole body aerobic fitness could be explained by variations in muscle mitochondrial function, and that [PCr] correlated with delta efficiency in a way consistent with its being a surrogate for average muscle fibre type composition. We also found that muscle mitochondrial function was a positive predictor of both daily energy expenditure and leanness. Finally, we found that there was a previously unreported gender difference in resting muscle pH. These findings are important for those with an interest in the genetic and physiological determinants of whole-body aerobic capacity, as well as those with broader interests in human physiology and health (given that VO2
max is a strong predictor of all-cause mortality33