Oxidative capacity varies along the length of healthy human tibialis anterior

Abstract

The rate of phosphocreatine (PCr) recovery (k_PCr) after exercise, characterizing muscle oxidative capacity, is traditionally assessed with unlocalized ³¹P 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 ³¹P/¹H volume transmit coil combined with a home-built ³¹P phased-array receive probe, consisting of 5 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 (¹H 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 < 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 ¹H mfMRI signal change, which was higher proximally than distally in both the TA and the extensor digitorum longus (P < 0.001) and it strongly correlated with k_PCr. 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.

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