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

KEY POINTS: During exercise skeletal muscles use the energy buffer phosphocreatine. The post‐exercise recovery of phosphocreatine is a measure of the oxidative capacity of muscles and is traditionally assessed by (31)P magnetic resonance spectroscopy of a large tissue region, assuming homogeneous energy metabolism. To test this assumption, we collected spatially resolved spectra along the length of human tibialis anterior using a home‐built array of (31)P detection coils, and observed a striking gradient in the recovery rate of phosphocreatine, decreasing along the proximo‐distal axis of the muscle. A similar gradient along this muscle was observed in signal changes recorded by (1)H muscle functional MRI. These findings identify intra‐muscular variation in the physiology of muscles in action and highlight the importance of localized sampling for any methodology investigating oxidative metabolism of this, and potentially other muscles. ABSTRACT: The rate of phosphocreatine (PCr) recovery (k (PCr)) after exercise, characterizing muscle oxidative capacity, is traditionally assessed with unlocalized (31)P magnetic resonance spectroscopy (MRS) using a single surface coil. However, because of intramuscular variation in fibre type and oxygen supply, k (PCr) 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 (31)P/(1)H volume transmit coil combined with a home‐built (31)P phased‐array receive probe, consisting of five coil elements covering the TA muscle length. Mono‐exponential k (PCr) 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 ((1)H mfMRI) was performed using the volume coil after another 40 s of SUBMAX. A strong gradient in k (PCr) was observed along the TA (P