Heterogeneity of energy metabolism within the tibialis anterior: is it just an outcome of a specific muscle activation, architecture and composition?

The non‐invasive characterization of skeletal muscle energy metabolism in vivo remains essential for the prognosis, diagnosis and follow‐up assessment of several human diseases. Phosphorus‐31 nuclear magnetic resonance spectroscopy (³¹P‐MRS) is now widely accepted as the “gold standard” method for this kind of non‐invasive measurement, which allows for the quantification of phosphorylated metabolite concentrations, including phosphocreatine (PCr), on a large spatial coverage within skeletal muscle before, during and post‐exercise (Kemp et al. 2015). From these data, skeletal muscle oxidative capacity can be characterized through the post‐exercise rate constant of PCr concentration recovery (k _PCr), which is a parameter commonly used as an indicator of skeletal muscle mitochondrial function (Kemp et al. 2015). Therefore, ³¹P‐MRS appears as a powerful tool to detect mitochondrial dysfunction associated with pathologies and injuries of healthy skeletal muscle. Most previous studies have assessed non‐localized muscle energy metabolism. Only a few studies have investigated the discrepancies in oxidative capacities among muscles using localized evaluation to overcome potential inter‐muscular inhomogeneity, and none have assessed the potential intramuscular heterogeneity in energy metabolism.

The recent study published in The Journal of Physiology by Boss et al. provided new information about the spatial variability of energy metabolism within a single lower leg skeletal muscle (Boss et al. 2018). The authors investigated the oxidative capacities along the tibialis anterior (TA) muscle using a dedicated phased array probe with five coil elements allowing ³¹P‐MRS measurements both during and after submaximal and exhaustive voluntary ankle dorsiflexion contractions in 10 healthy young men. Changes in PCr concentration and pH were quantified during the exercise, as well as the k _PCr to estimate the muscle's oxidative capacity. A non‐uniform oxidative capacity was measured within the TA with a lower k _PCr in the distal compared with the proximal portion of the muscle. However, no significant difference was found among measurements performed with each coil element in relative PCr depletion rate or in minimal and end‐exercise pH during the exhaustive and submaximal voluntary exercises. Therefore, the spatial discrepancy in oxidative capacities along the TA was unrelated to the relative changes in PCr and pH during the exercise. The authors ensured that the major part of the ³¹P MR signal was associated with the activation of the TA and ruled out the potential co‐activation of other agonist or antagonistic muscles using functional proton magnetic resonance imaging (¹H‐MRI). The relative signal from the ³¹P‐signal attributed to the TA detected by the multi‐array coil was evaluated to be between 64 and 84% depending on the coil element, with the remaining signal mainly attributed to activation of the extensor digitorum longus (i.e. dorsiflexion agonist muscle of the anterior leg compartment). It should be noted that the maximal normalized ¹H‐MRI signal and slope determined during the recovery after the exercise was higher in the most proximal coil element as compared to the distal ones. On that basis, muscle activation can potentially be lower in the TA distal part during the exercise which could have a major effect on the related metabolic activity in this muscle area, i.e. lower metabolic demands distally. This would be in accordance with the spatial inhomogeneity in TA activation detected through the lower surface integrated electromyographic signal in the distal vs. proximal portions of the TA during exercise (Yamada et al. 1987).

Boss et al. (2018) assumed that the lower relative ¹H‐MRI signal in the TA distal part was at least in part related to a lower oxygen supply and/or a lower local perfusion due to a potential difference in muscle fibre‐type composition along the length of the TA. Nevertheless, the authors noted that muscle tension production and energy requirement during the submaximal voluntary contractions could have been non‐uniform along the TA. Firstly, the potential difference in muscle fibre typology in distal and proximal parts of the TA may lead to discrepancies in the effective muscle fibre tension being produced and transmitted. Animal studies suggest a decreasing proportion of type I fibre area from proximal to distal parts of the TA, which could explain the non‐uniform oxidative metabolism activity along the TA during submaximal voluntary contractions. Secondly, a complex architecture was reported for the TA based on observations from human cadavers (Wolf & Kim, 1997). In the proximal part of the TA, a central aponeurosis splits the muscle into a deep and a superficial area. In the distal part, the muscle fascicle arrangement led to a more fusiform architecture. The specific architecture in the distal and proximal parts of the TA can result in non‐homogeneous stress and/or strain during submaximal voluntary contractions among fibres within the muscle. In the distal part of the TA, a higher proportion of muscle fibres oriented along the whole muscle axis would generate similar muscle tension with less strain and lower energy requirement as compared to pennate muscle fibres in the proximal part.

Therefore, further investigations of the TA muscle during a constant submaximal exercise performed at a higher relative force level, e.g. ∼80–90% maximal voluntary contraction would be of interest to explore the potential cofounding influence of fibre‐type relative contribution in the original results obtained by Boss et al. (i.e. the spatial variability of energy metabolism along the TA). Indeed, on the basis of the Henneman's size principle, a higher relative voluntary force level should generate a high proportion of type II fibre involvement within the TA, especially in the proximal part, during the contraction providing additional relevant information about the influence of muscle fibre type in the different metabolic demands observed along the TA. Additionally, an investigation of other muscles with homogeneous architecture would have been of interest to assess the potential confounding effect of muscle structural organization on the spatial variability of metabolism within the TA muscle reported by Boss et al. (2018). Moreover, an assessment of the reproducibility of ³¹P‐MRS measurements performed with the multi‐array coil used by Boss et al. (2018) would have been an additional argument regarding the importance of the coil positioning to avoid potential misinterpretation of results in longitudinal assessment from non‐localized ³¹P‐MRS measurements including repetitive explorations over time. To reinforce the importance of coil positioning in human experiments, recent studies have also reported variabilities in the diffusion properties and architecture of lower leg muscles in healthy subjects (Fouré et al. 2018) or fat infiltration in dystrophic patients from multi‐slice magnetic resonance imaging assessments.

Overall, Boss et al. (2018) shed light on the potential of localized metabolic measurements to provide relevant additional information and to accurately examine intramuscular changes associated with injuries, pathologies and the effects of chronic interventions (e.g. exercise training and therapeutic treatments).

Additional information

Competing interests

None declared.

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