Skeletal Motor Unit Recruitment During Periodic Auditory Cueing: A Simultaneous Behavioral and Motor Unit Magnetic Resonance Imaging (MUMRI) Study.

Rhythmic motor paradigms are widely used to study sensorimotor timing, yet magnetic resonance imaging (MRI) research has largely focused on central processes, with limited insight into peripheral neuromuscular mechanisms. Motor unit MRI (MUMRI), a motion-sensitive technique in which muscle contraction induces intravoxel water redistribution and transient signal attenuation, enables in vivo visualization of muscle activity. In this study, we developed and validated a combined behavioral-MUMRI paradigm to characterize muscle recruitment during rhythmic foot tapping. Healthy participants performed an auditory-paced tapping task inside an MRI scanner while timing was recorded via an MRI-compatible force transducer and muscle activity was measured using single-slice MUMRI. A variable-latency cueing design systematically sampled the temporal relationship between auditory cues, motor execution, and image acquisition, allowing identification of the optimal latency window for detecting contraction-related signal changes. Fixed-latency acquisitions were then used to assess reproducibility. Behavioral results showed stable performance across conditions, with low variability in tapping accuracy (mean coefficient of variation [CoV] ≈0.078). Transient, localized signal reductions consistent with muscle contraction were observed in anterior lower leg muscles during dorsiflexion. Voxel-wise analyses demonstrated high within-condition reproducibility and latency-dependent spatial patterns, with the greatest average consistency when tapping aligned with scanner rhythm (r ≈0.68). These findings establish a robust framework for integrating rhythmic motor tasks with MUMRI, highlighting the importance of precise temporal alignment for reliable measurement of muscle activity. This approach provides a reproducible method for linking motor behavior to peripheral neuromuscular dynamics and offers potential for advancing both basic and clinical MRI research.