Multiparametric MRI Assessment of Morpho-Functional Muscle Changes Following a 6-Month FES-Cycling Training Program: Pilot Study in People With a Complete Spinal Cord Injury

Abstract

Background: Spinal cord injuries (SCIs) cause debilitating secondary conditions such as severe muscle deterioration, cardiovascular, and metabolic dysfunctions, significantly impacting patients' quality of life. Functional electrical stimulation (FES) combined with cycling exercise (FES-cycling) has shown promise in improving muscle function and health in individuals with SCI.

Objective: This pilot study aimed to investigate the potential role of multiparametric magnetic resonance imaging (MRI) to assess muscle health during and after an FES-cycling rehabilitation program.

Methods: Four male participants with chronic SCI underwent a 6-month FES-cycling training program, consisting of two 30-minute sessions per week. MRI scans were performed at baseline (T0), after 3 months (T1), at the end of the training (T2), and 1-month posttraining (T3). The MRI protocol included T1-weighted imaging for volume quantification, Dixon imaging for fat fraction, multi-echo spin echo for T2 relaxation times, and diffusion tensor imaging to assess diffusion parameters.

Results: Muscle hypertrophy was observed, with an average increase in muscle volume of 22.3% at T1 and 36.7% at T2 compared with baseline. One month posttraining, muscle volume remained 23.2% higher than baseline. Fat fraction decreased from 11.1% at T0 to 9.1% at T2, with a rebound to 10.9% at T3. T2 relaxation times showed a reduction even though this was not consistent among participants. Diffusion tensor imaging parameters revealed subtle changes in muscle tissue microstructure, with a decrease in fractional anisotropy mainly associated to an increase of radial diffusivity.

Conclusions: Although preliminary, this study provides evidence that 6 months of low-intensity FES-bike training can increase muscle volume and decrease fat infiltration in individuals with SCI. The study demonstrates that the use of a multiparametric MRI provides comprehensive insights into both macroscopic and microscopic changes within muscle tissues, supporting its integration into clinical practice for assessing the efficacy of rehabilitation interventions.

Keywords: FES; MRI; cycling; exercise; functional electrical stimulation; image; imaging; magnetic resonance imaging; mpMRI; multiparametric MRI; muscle; muscular; musculoskeletal; physical activity; rehabilitation; skeletal muscle; spinal cord injury; spine.

Figure 1.. Multiparametric magnetic resonance imaging protocol used for assessing muscle volume, fat fraction, T₂ relaxation times, and diffusion parameters. All sequences were acquired according to the axial plane placed perpendicular to the femur and with the upper part of the field of view placed in the middle of the head of the femur. All sequences share the same field of view (size 256 × 256 × 300 mm³), regardless of their acquisition matrix and reconstructed voxel size.

Figure 2.. Representative axial magnetic resonance imaging cross-sections of the thigh for 4 participants (S1 to S4), with segmented anatomical regions representing individual muscles. Each muscle is outlined in a specific color corresponding to the legend on the right. Muscles include the vastus lateralis, vastus medialis, vastus intermedius, rectus femoris, sartorius, gracilis, adductor magnus, semimembranosus, semitendinosus, biceps femoris (long and short heads), and adductor longus.

Figure 3.. Extension of thigh muscle regions of interest across 4 participants (S1‐S4) in a representative sagittal plane for each time point (T₀-T₃). Each muscle is outlined with a distinct color. The regions of interest boundaries were consistently defined from the beginning of the semimembranosus to the last available slice of the rectus femoris, ensuring reproducibility across participants and time points. Magnetic resonance imaging scans covered a 30 cm range along the head-to-feet axis from the midpoint of the femoral head.

Figure 4.. Changes in normalized muscle volume over the 6-month functional electrical stimulation–cycling training and 1-month posttraining. The left panel shows 3D-reconstructed volume renderings for an example volunteer at each time point (T₀, T₁, T₂, and T₃), illustrating the changes in muscle size. The right panel presents box plots of normalized muscle volume measurements for all participants at each time point.

Figure 5.. Changes in a muscle’s largest cross-sectional area (CSA) over the 6-month functional electrical stimulation–cycling training program and 1-month posttraining. The right panel shows box plots of cross-sectional area measurements for all participants at each time point (T₀, T₁, T₂, and T₃). The left panel illustrates the magnetic resonance imaging cross-sectional images of the thigh muscles for an example volunteer at each time point, highlighting the visual changes in muscle size.

Figure 6.. Changes in muscle fat fraction over the 6-month functional electrical stimulation–cycling training and 1-month posttraining. The right panel shows box plots of fat fraction measurements for all participants at each time point (T₀, T₁, T₂, and T₃). The left panel displays magnetic resonance imaging cross-sectional images of the thigh muscles for an example volunteer at each time point, illustrating the reduction in fat infiltration.

Figure 7.. Changes in T₂ relaxation times over the 6-month functional electrical stimulation–cycling training and 1-month posttraining. The right panel shows box plots of T₂ relaxation times for all participants at each time point (T₀, T₁, T₂, and T₃). The left panel displays T₂ relaxation maps of the thigh muscles for an example volunteer at each time point.

Figure 8.. Changes in diffusion tensor imaging parameters over the 6-month functional electrical stimulation–cycling training and 1-month posttraining. The top panel displays axial magnetic resonance imaging images for fractional anisotropy, mean diffusivity, radial diffusivity, and axial diffusivity of 1 example volunteers at each time point (T₀, T₁, T₂, and T₃). The bottom panel shows box plots of these diffusion tensor imaging parameters for all participants over time.