Synopsis

The phosphocreatine (PCr) recovery rate of the tibialis anterior muscle after isometric exercise shows a strong gradient along the length of the muscle. In this study we demonstrate that this also holds for dynamic exercise of the TA. To further examine if mitochondrial metabolism is involved we determined acetylcarnitine (AC) levels along the length of the TA, but no differences were detected. Together with previous correlations found for blood oxygenation by NIRS experiments and perfusion by IVIM we conclude that muscle perfusion and oxygenation are main components determining the PCr recovery gradient in the TA.

Purpose

The phosphocreatine (PCr) recovery rate of muscles after exercise (k_PCr), reflecting oxidative capacity¹, varies between subjects^2,3 and muscle types^2–4. We recently demonstrated that k_PCr of the tibialis anterior (TA) muscle after isometric exercise also varies along the length of the muscle ⁵. In this study we examined if this also holds for dynamic exercise of the TA.

Acetylcarnitine (AC) is a compound buffering acetyl groups involved in mitochondrial metabolism. AC can be readily measured in human muscles by ¹H MRS at long echo times and in this way it was demonstrated that resting acetylcarnitine levels are positively associated with PCr recovery rates⁶. As k_PCr over the TA after exercise showed considerable variations we hypothesized that acetylcarnitine levels may also differ along the length of the TA.

Materials and Methods

Volunteers: Five volunteers participated in the MRS exams with dynamic exercise (age: 31 ± 4 years; BMI: 21.3 ± 1.6 kg/m²). Acetylcarnitine levels in the TA were obtained of 3 volunteers (age: 30±1y; BMI: 21.4±1.4kg/m²). In one volunteer these levels were determined at 2 different occasions.

MR acquisition:

All MR measurements were performed on a 3T MR system (Prisma Fit, Siemens).

³¹P MRS: A ¹H/³¹P birdcage coil (Rapid) for transmission and a home-built ladder-shaped ³¹P-phased- array coil containing five coil elements was positioned on the TA for reception (size=4x4.5cm each, signal localization through the coil element sensitivity profile). The in-bore exercise consisted of a dynamic ankle dorsiflexion exercise (frequency: 2Hz), starting at 10% MVC and incrementally increasing with 10% MVC every 30 sec. ³¹P MR spectra (TR: 2.06s, 2 averages per spectrum, 48° Ernst angle excitation, ¹H-³¹P NOE enhanced) were obtained for 20min36s during 1min rest, the dynamic exercise until exhaustion, and the recovery period.

¹H MRS: A 15-channel Tx-Rx knee coil was used covering the whole TA. An sLASER sequence was used to localise a voxel (17x17x40 mm). Spectra were measured from three voxels, one voxel placed in the middle of the TA and the second and third 5 cm more distally or proximally, respectively (Fig.1), avoiding contamination by subcutaneous fat and the extensor digitorum. TE was 350 ms, TR 6 sec and 64 to 128 scans were averaged.

Post-processing:

³¹P MRS: phosphor resonances were fitted with AMARES/jMRUI (Lorentzians). k_PCr was fitted with a mono-exponential model⁵.

¹H MRS: The signals of creatine (Cr) at 3.03 ppm and of AC at 2.13 ppm were fitted in jMRUI (Lorentzians) and the integrals corrected for T2 and for a short T2 component in the Cr signal. Tissue concentrations of AC were then determined assuming Cr at 30 mM/kg ww.⁶

Statistics: The k_PCr and PCr depletion at the different locations were compared with a repeated measures ANOVA.

Results

³¹P MRS: The maximum force reached during ³¹P MRS was 66±3% MVC and time to exhaustion was 187±4s. PCr decreased during dynamic exercise and recovered to baseline thereafter (Fig.2). The k_PCr differed significantly between the coil elements (p<0.001) with a higher k_PCr proximally than distally (Fig.3A). This is very similar to the k_PCr increase over the coil elements after isometric exercise (Fig. 3B). PCr depletion didnot correlate with the proximo-distal decrease in k_PCr (Fig. 3C&D).

¹H MRS: High quality ¹H MR spectra were obtained at a TE of 350 ms from 3 locations positioned along the TA with well-resolved resonances of AC and Cr without lipid signal contamination (Fig.4).

Conclusion and Discussion

Previously we reported a substantial increase in the PCr recovery rate from distal to proximal in the TA after isometric exercise⁵. Subsequently we demonstrated that also muscle oxygenation, measured by NIRS and perfusion, measured by IVIM, recovered faster proximally in this muscle⁷. As isometric exercise is partially ischemic we tested if the proximal-distal gradient in PCr recovery also occurs with dynamic exercise. The remarkable similar gradient indicates that intrinsic differences in oxygenation and perfusion are major factors in PCr recovery. The similarity between AC levels along the length of the TA indicates that no mitochondrial metabolic component is involved. The absence of a correlation between PCr depletion by the exercise with the proximal-distal k_PCr gradient also indicates that differences in energy metabolism are not involved and further supports that the PCr recovery rate is determined by oxygenation and perfusion increasing proximal along the TA.

Acknowledgements

References

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