Accuracy and reliability of an MR-compatible dorsiflexion ergometer for dynamic 31P-MRS: Comparison with a clinical dynamometer in individuals with and without obesity

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 (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 non-obese (BMI 18.5-30 kg/m2, 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. 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. Competing Interest Statement The authors have declared no competing interest. Funding Statement Yes Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Not Applicable The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Written informed consent was obtained from all subjects in accordance with the requirements of the Danish research ethics committee (1-10-72-385-21). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Not Applicable I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Not Applicable I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Not Applicable Data Availability A link to the 3D-printing files for essential MR ergometer components will be provided in the manuscript. The script used for analyzing muscle work and 31P-MRS data will be made publicly available. We intend to share de-identified data from human participants following acceptance.