Although muscle is not homogeneous, the rate of phosphocreatine (PCr) recovery after exercise as a correlate of the muscle’s oxidative capacity is traditionally determined in a non-localized way. Fiber type distribution varies within muscles, including the tibialis anterior (TA), in which the proportion of glycolytic type II fibers increases towards the distal portion of the muscle¹. Therefore, we hypothesized the rate of PCr recovery would vary along the length of this muscle in healthy volunteers. In addition, muscle functional MRI (mfMRI) was applied using T2*-weighted GE-EPI imaging to examine muscle recruitment and to assess the kinetics of the post-exercise signal intensity response².

Subjects: 10 healthy male young volunteers (29±3y, 79±9kg, BMI: 23.1±2.6kgm^-2).

Hardware: A home-built ladder-shaped ³¹P-phased array receive coil consisting of 5 individual coils (size:4x4.5cm each, total size: 4x20cm, signal-localization through the limited sensitivity profile of the elements) was positioned on the right TA. For ³¹P-transmission and ¹H-imaging, a ¹H/³¹P birdcage coil (Rapid) was used. A home-built ergometer with visual feedback of applied force was used during isometric dorsiflexion of the foot.

Study-design: Each volunteer underwent 2 experimental sessions performed inside the 3T MR-scanner (Siemens TIM Trio). The first session served to determine the maximum voluntary contraction (MVC) and for habituation to set-up and exercise. During the second session, ³¹P- and ¹H-imaging was first performed at rest. Thereafter, T2*-weighted GE-EPI images (TE=29ms, TR=1s) from 5 transversal slices (3mm), corresponding to the centre of each ³¹P-coil were obtained during 1min of rest, 40s of isometric contractions (60%MVC), and 15min recovery. Afterwards, ³¹P-MR spectra (pulse-and-acquire, 500ms hard pulse, FA=48°, TR=2s, 2 averages/spectrum, 1 spectrum/coil-element) were obtained during 30s rest, 40s isometric contractions (60%MVC, SUBMAX), and 5min recovery. Finally, ³¹P-MRS was obtained for 16min during an incremental exercise to exhaustion (EXH) starting at 10%MVC, increment:+10%MVC/30s.

Post-processing: mfMRI: For the post-exercise period, two ROIs surrounding the dorsiflexors, i.e. TA and extensor digitorum longus (EDL), and a third ROI surrounding the calf and peronei (C) were drawn. Average signal intensities for the 3 ROIs and time-to-peak (ttp) for the dorsiflexors were computed. ³¹P-MRS: AMARES/jMRUI was used for spectral fitting (Lorentzians). PCr recovery was fitted to a monoexponential model using matlab:$$PCr(t)=PCr_{0}+\triangle PCr( e^{-k_{PCr}\times t})$$ where kPCr: the recovery rate. pH at the end of exercise (pH_endex) and minimum pH (pH_min) were determined from the chemical shift-difference between inorganic phosphate (Pi) and PCr.

Isometric exercise: Average maximal force was 200±24N (SEM). The force during mfMRI and SUBMAX was 59±0.5% and 59±0.4%, respectively. Average time to exhaustion during EXH was 165±7s.

An example of the ³¹P-results from one volunteer is shown in Fig.1. While the depletion of PCr and pH_endex during SUBMAX was similar, these parameters varied slightly, but significantly between elements in EXH (Fig.2). The k_PCr varied significantly along the length of TA, being lower in distal compared to proximal regions during both, SUBMAX and EXH (Fig.3). Average post-exercise mfMRI indicate that both TA and EDL (but not C) were recruited (Fig.4). The ttp for TA varied significantly with slice and correlated with ttp for EDL (Fig.5A/B). Moreover, ttp TA significantly correlated with the half-time of PCr recovery (t_1/2PCr, Fig.5C).

By using two separate MR-techniques, ³¹P-MRS and ¹H-mfMRI, we observed remarkable metabolic differences along the length of the TA in healthy volunteers. The k_PCr was approximately twice as high in proximal as compared to distal parts in the muscle. This within-subject variation is higher than what was previously observed for TA when comparing untrained with endurance-trained subjects³, or sprinters with distance-runners⁴ using traditional surface coils. Moreover, ttp in the mfMRI was significantly slower in distal parts compared to proximal parts and it was significantly correlated with t_1/2PCr. Such a correlation, however for single-slice EPI-acquisitions, has previously been reported². Osmotically driven water shifts from the intracellular (short T2*) to the interstitial compartment (long T2*) as a result of a rapidly decreasing metabolite concentration (overall reaction: Pi+Cr→PCr), may be the main reason for the initial post-exercise signal increase observed in mfMRI^2,5, which also explains the tight correlation of ttp and PCr-recovery time. pH_endex and pH_min were similar between coil elements during SUBMAX (Fig2A), indicating that pH is not the main reason for the pronounced variation in k_PCr and ttp along the length of TA. Differential intra-muscular fiber-type distribution¹, or varying O₂-supply due to heterogeneous capillary density could, however, potentially explain the functional gradients presently observed.

CONCLUSION: A strong gradient in functional oxidative capacity was found in healthy tibialis anterior. This may reflect regional variation in the metabolic/biomechanical demands of everyday activities on this muscle.