Quantitative MRI changes in neck muscle size and composition following long-duration spaceflight.

Long-duration spaceflight poses several challenges to human health, including muscle loss and fat deposition. To mitigate these effects, exercise countermeasures are prescribed. This study aimed to measure neck muscle size and composition changes with long-duration spaceflight using quantitative magnetic resonance imaging (MRI) and examine associations with in-flight exercise modality and volume. Nine astronauts (mission duration: 7.1 ± 1.8 months) underwent pre- and post-flight 3T Dixon MRI of the cervical spine. Cross-sectional area (CSA) and muscle fat fraction (MFF) were measured bilaterally in 11 neck muscles. Mixed-effects models assessed monthly percentage changes and associations with resistance (ARED), cycle ergometer and treadmill training volumes. The CSA and MFF of most neck muscles did not change across long-duration missions. CSA increased for the sternocleidomastoid (+1.21%/month, P = 0.004) and the rhomboid minor (+1.71%/month, P = 0.057). Semispinalis capitis MFF increased (+1.10%/month, P = 0.051). More ARED repetitions of different exercises were associated with CSA increases in four muscles (P < 0.001-0.097) and MFF reductions in three muscles (P = 0.040-0.093). More cycle ergometer time was associated with a CSA increase in the sternocleidomastoid (P = 0.053) and an MFF increase in the scalenus medius (P = 0.019), while treadmill time was linked to CSA decreases in the trapezius (P = 0.095) and rhomboid minor (P = 0.047). Neck muscle size and composition were generally preserved during modern long-duration missions in which structured exercise countermeasures were performed. Because all crewmembers completed in-flight exercises, these findings should not be interpreted as evidence that exercise countermeasures prevented neck muscle changes.