ccuracy and Reliability of an MR-Compatible Dorsiflexion Ergometer for Dynamic 31P-MRS: Comparison With a Clinical Dynamometer in Individuals With and Without Obesity.

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 (31P-MRS) data remains unclear. This study aimed to validate an MR-compatible ergometer against a clinical dynamometer and to evaluate the applicability of 31P-MRS in individuals with severe obesity. Twenty adults (35-60 years) were recruited and divided into groups of nonobesity (BMI 18.5-30 kg/m2, n = 10) and severe obesity (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-min isotonic fatiguing exercise. 31P-MRS was continuously acquired during the in-scanner exercise. Agreement between devices was assessed using Bland-Altman plots and intraclass correlation coefficients (ICCs). 31P-MRS data quality was evaluated based on signal-to-noise ratio (SNR), uncertainty of fit (CRLB), and phosphocreatine (PCr) recovery fit (R2). Pearson's correlations examined relationships between muscle fatigue and metabolic parameters. 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. 31P-MRS met preset quality thresholds (SNR ≥ 5, CRLB < 20%, R2 ≥ 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). The MR ergometer demonstrated feasibility, acceptable reliability, and consistent 31P-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.