Oxidative capacity varies along the length of healthy human tibialis anterior

Abstract

The rate of phosphocreatine (PCr) recovery (k ) after exercise, characterizing muscle oxidative capacity, is traditionally assessed with unlocalized P magnetic resonance spectroscopy (MRS) using a single surface coil. However, because of intramuscular variation in fibre type and oxygen supply, k may be non-uniform within muscles. We tested this along the length of the tibialis anterior (TA) muscle in 10 male volunteers. For this purpose, we employed a 3T MR system with a P/ H volume transmit coil combined with a home-built P phased-array receive probe, consisting of five coil elements covering the TA muscle length. Mono-exponential k was determined for all coil elements after 40 s of submaximal isometric dorsiflexion (SUBMAX) and incremental exercise to exhaustion (EXH). In addition, muscle functional MRI ( H mfMRI) was performed using the volume coil after another 40 s of SUBMAX. A strong gradient in k was observed along the TA (P < 0.001), being two times higher proximally vs. distally during SUBMAX and EXH. Statistical analysis showed that this gradient cannot be explained by pH variations. A similar gradient was seen in the slope of the initial post-exercise H mfMRI signal change, which was higher proximally than distally in both the TA and the extensor digitorum longus (P< 0.001) and strongly correlated with k . The pronounced differences along the TA in functional oxidative capacity identify regional variation in the physiological demand of this muscle during everyday activities and have implications for the bio-energetic assessment of interventions to modify its performance and of neuromuscular disorders involving the TA.

Interleaved 31P MRS/1H ASL for analysis of metabolic and functional heterogeneity along human lower leg muscles at 7T

“…The significantly higher end‐exercise PCr depletion and higher postexercise perfusion found proximally are in good agreement with results of Boss et al, who used an exhaustive exercise paradigm in tibialis anterior. Although submaximal exercise was performed in our study, significant differences of end‐exercise pH and PCr recovery times were found, in contrast to . This could be explained by the fact that the fraction of oxidative Type I muscle fibers is higher in the tibialis anterior which is considered to work in a more oxidative regime compared to the GM .…”

“…Functional and metabolic heterogeneity along a single muscle group has been found recently in a study using 31 P MRS and 1 H MRI during multiple successive exercise recovery experiments in tibialis anterior at 3 T. The simultaneous acquisition of complementary data, in combination with the the high specificity of a multi‐slice and multi‐voxel protocol and increased sensitivity of measurements at 7 T, allow for an advanced investigation of intramuscular heterogeneity in functional and metabolic exercise response along human calf muscles.…”

“…In most studies published so far, the recruitment of tissue along a specific muscle was implicitly assumed to be homogeneous, largely ignoring potential spatial inhomogeneities within, and often even between adjacent muscles. Using 31 P MRS and 1 H MRI in separate experiments Boss et al recently discovered variations of oxidative capacity not only between muscle groups, but also along the studied muscle, (tibialis anterior). This heterogeneity was confirmed in a follow‐up study, reporting a gradient of perfusion and supply measured with NIRS, intra‐voxel incoherent motion imaging (IVIM) and 31 P MRS in consecutive experiments…”

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

“…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 31 P‐MRS measurements both during and after submaximal and exhaustive voluntary ankle dorsiflexion contractions in 10 healthy young men.…”

“…Boss et al . (2018) assumed that the lower relative 1 H‐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.…”