SU‐F‐I‐25: Reproducibility of Phosphorus MRS and Work Measurements for Evaluating Skeletal Muscle Metabolism

Abstract

Purpose:Skeletal muscle energetics were studied by phosphorus‐31 MRS (31P‐MRS) during exercise. The reproducibility of work performed and the MRS parameters were investigated.Methods:Reproducibility of the 31P‐MRS exercise parameters was validated in five healthy volunteers (2 M/3 F, age 39.2 ± 21.9 yr, BMI 25.1 ± 4.8 kg/m2). Five minute exercise of the vastus lateralis (VL) was performed on two separate occasions during MRS in a 3 Tesla MRI system. The exercise (up to 13.5 kg resistance) was measured on a strain gage. The summation of force applied (strain level) over time (Δt = 20 ms) was taken as the work output. The phosphocreatine (PCr) recovery time constant (k) following exercise was determined from nonlinear least squares fitting of MRS spectra acquired every 3 s to the Levenberg‐Marquardt equation. Maximum oxidative capacity was calculated from the baseline [PCr] and k, as previously described. The protocol was approved by the IRB.Results:The percent drop of PCr = 40.7 ± 13.4% and the rate of PCr recovery was 0.16 ± 0.03 mM•s−1 (k = 0.018 ± 0.003 s−1). The mean recovery time, τ, of PCr was 40.0 ± 8.1 s, which corresponds well with current literature values in healthy subjects (32 ± 3 s to 53 ± 25 s). Bland‐Altman analysis showed poor reproducibility (LOA ratio = 55.7) in work performed h, the k‐value for PCr was very reproducible (bias = −1.28*10−4, random error = 3.32*10−3). The % drop in PCr was negatively but significantly correlated with the oxidative phosphorylation capacity (r= −0.84, p=0.003).Conclusion:[PCr] changes during VL exercise and recovery were reproducible and not affected by inconsistent work being performed during exercise bouts. The results suggest that this system should be useful for relating work vs. metabolism differences between subjects with differing metabolic profiles.