Since the rate of phosphocreatine recovery after exercise varied substantially within the tibialis anterior, we wondered if O2 supply also varied along this muscle. Therefore, we applied near infra-red spectroscopy to study the dynamic equilibrium of O2 supply and O2 utilization, intravoxel incoherent motion imaging for muscle perfusion and 31P-MRS during and after isometric exercise of the tibialis anterior. We observed higher post-exercise perfusion and faster phosphocreatine and oxyhemoglobin recovery proximally than distally in the tibialis anterior, indicating a proximo-distal gradient in O2 supply. This may be an adaptation to a higher proximal energy requirement.
Study design
In two sessions twelve healthy male volunteers (age: 26±3y; BMI: 22.9±1.6kg/m2) performed isometric dorsiflexion of the right ankle until exhaustion at 30% of their maximum voluntary contraction (MVC; determined at the start of both sessions).
Session 1: Oxy- and deoxyhemoglobin changes were measured simultaneously at seven positions along the TA (Fig1) using continuous-wave NIRS (OxyMon, Artinis) for five minutes rest, exercise, and fifteen minutes recovery (N=12).
Session 2: The subjects performed the exercise twice inside a 3T MR system (Prisma Fit, Siemens). First, nine transverse diffusion-weighted (DW) slices (Fig1) were acquired (SE-EPI; SPAIR; TR/TE=2000/40ms; voxel size=2.75x2.75x10mm3; gap=11.3mm; acquisition time=1min18s; b=0,5,10,15,20,40,60,80,100,150,200,400,600s/mm2) using a knee coil; four acquisitions during rest and twelve during recovery (N=10). Second, 31P-MR spectra were collected (pulse-acquire; FA=48°; TR=2060ms; NA=2; 1 spectrum/coil element; NOE) during one minute rest, exercise and recovery (total:±20min; N=8). We used a 1H/31P birdcage coil (Rapid) for transmission and home-built ladder-shaped 31P-phased array coil containing five coil elements positioned on the TA for reception (Fig1; size=4x4.5cm each, signal localization through the coil element sensitivity profiles).
Post-processing
NIRS: We applied a moving average filter (span=10s) and determined the time to reach 50% of the maximum oxyhemoglobin value during recovery (TTP_50perc).
IVIM: We determined the average signal magnitude for the TA per slice, b-value and time-series by delineating the TA on all b0 images. The signal decay was fitted per slice and time-series with a bi-exponential model6 (Eq1) in two steps7.
$$$S_b=S_0'((1-F_p)e^{-b\times{D}}+F_pe^{-b\times{D^*}})$$$ (Eq1)
First, diffusion coefficient (D) and S0’’ were computed by a linear least-squared fit to the log-transformed signal for b-values>200s/mm2 (Eq2).
$$$log(S_b)=-b\times{D}+log(S_0'')$$$ (Eq2)
Second, perfusion coefficient (D*), perfusion fraction (Fp) and S0’ were fitted with a non-linear least-squared fit (Eq1) to b-values=5-600s/mm2 with D fixed. D, D*, Fp, and blood flow related parameter Fp×D* during rest and recovery were defined as the average over the first four and last eleven acquisitions, respectively.
31P-MRS: PCr, inorganic phosphate (Pi) and ATP resonances were fitted with AMARES/jMRUI (Lorentzians). kPCr was fitted with a mono-exponential model (Eq3).
$$$PCr(t)=PCr_0+\triangle{PCr}(1-e^{-k_{PCr}\times{t}})$$$ (Eq3)
PCr0=end-exercise PCr; ΔPCr=end-recovery PCr (PCrrecovery) minus PCr0
PCr depletion was defined as ΔPCr/PCrrecovery and end-exercise pH (pHendex) was determined from the chemical shift difference between Pi and PCr.
Statistics
Linear mixed models were used to assess the linear dependence of kPCr, TTP_50perc and IVIM outcomes to position. kPCr was correlated with NIRS and IVIM using Pearson’s correlation on the pooled data of all volunteers per position.
Exercise: MVC was 209±36N and 207±25N during session 1 and 2, respectively. Force during NIRS, IVIM and 31P-MRS was similar (all: 29±1% MVC) and time to exhaustion was 276±98s, 271±102s, and 158±39s, respectively.
NIRS (Fig2): Oxyhemoglobin decreased and deoxyhemoglobin increased during exercise, both recovering to baseline. TTP_50perc showed significant linear relationships with measurement position (p=0.01), being lower proximally.
IVIM (Fig3): Fig3A-C shows example DW images for b5 and corresponding signal decay. D, Fp, D*, and Fp×D* significantly increased after exercise (D:+4±3%, Fp:+62±48%, D*:+131±53%, Fp×D*:+283±135%, all p<0.001). Fp and Fp×D* showed significant linear relationships with slice position during recovery (Fp: p=0.011; Fp×D*: p=0.031), being higher proximally. D* showed a similar trend (p=0.062).
31P-MRS (Fig4): PCr decreased during exercise and recovered to baseline after exercise. kPCr showed significant linear relationships with coil elements, being higher proximally (p<0.01), while pHendex (p=0.060) and PCr depletion did not (p=0.806).
31P-MRS vs. NIRS and IVIM (Fig5): PCr recovery correlated significantly with TTP_50perc (r=0.932,p=0.021), D* (r=0.986,p=0.014) and tended to correlate with Fp×D* (r=0.910,p=0.090).
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