Accuracy and reliability of an MR-compatible dorsiflexion ergometer for dynamic 31P-MRS: Comparison with a clinical dynamometer in individuals with and without obesity
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Background
The clinical application of MR-compatible ergometers for muscle contractile assessment is limited by a lack of validation against standard clinical dynamometers. Moreover, the impact of obesity on the reliability of MR ergometer-based muscle contractile assessments and the quality of phosphorus-31 magnetic resonance spectroscopy ( 31 P-MRS) data remains unclear. This study aimed to validate an MR-compatible ergometer against a clinical dynamometer and to evaluate the applicability of 31 P-MRS in individuals with severe obesity.
Methods
Twenty adults (35–60 years) were recruited and divided into groups of non-obese (BMI 18.5-30 kg/m 2 , n=10) and severely obese (BMI ≥35 kg/m², n=10), matched for age, sex, and height. Ankle dorsiflexion was assessed using both a clinical dynamometer and an MR ergometer, measuring maximal voluntary isometric contraction (MVIC) and a 4-minute isotonic fatiguing exercise. 31 P-MRS was continuously acquired during the in-scanner exercise. Agreement between devices was assessed using Bland-Altman plots and intraclass correlation coefficients (ICCs). 31 P-MRS data quality was evaluated based on signal-to-noise ratio (SNR), uncertainty of fit (CRLB), and phosphocreatine (PCr) recovery fit (R 2 ). Pearson’s correlations examined relationships between muscle fatigue and metabolic parameters.
Results
All subjects successfully completed the protocol on both devices. The MR ergometer demonstrated moderate-to-excellent reliability (ICC ≥0.50) for most contractile parameters. While maximal torque, power, and work were underestimated on the MR ergometer (16-28%), this bias was consistent across BMI groups. 31 P-MRS met preset quality thresholds (SNR≥5, CRLB <20%, R 2 ≥0.70) in both groups. Dorsiflexion fatigue (reduction in power) correlated strongly (r ≥0.77) with metabolic changes, including PCr depletion (R²=0.68), pH drop (R²=0.59), PCr recovery time constant (R²=0.62), and inorganic phosphate accumulation (Pi/PCr) (R²=0.67).
Conclusion
The MR ergometer demonstrated feasibility, acceptable reliability, and consistent 31 P-MRS data quality across BMI groups. These findings support the use of the MR ergometer for in-scanner dorsiflexor assessments, even in individuals with severe obesity.
