Inactivity induced by cervical spinal cord injury does not affect diaphragm muscle fiber size

Inactivity of respiratory muscles following spinal cord injury (SCI) may result in life‐threatening deficits in the ability to sustain ventilation. In a commonly used model of SCI (the C2 hemisection model, C2HS), inactivity or grossly reduced activity persists over several weeks. Inactivity is commonly thought to result in significant muscle fiber atrophy. Since the diaphragm muscle (DIAm) is highly active, it may be particularly susceptible to inactivity imposed by C2HS. In adult rats, we examined DIAm fiber cross‐sectional area (CSA) and fiber type proportions for 6 weeks after C2HS. DIAm EMG recordings were used as a marker of activity level. Following C2HS, we found a progressive increase in the proportion of animals displaying spontaneous recovery of rhythmic DIAm EMG activity ipsilateral to C2HS over time. After 2 weeks, only 30% of animals displayed minimal DIAm EMG activity while after 6 weeks all animals displayed restored EMG activity albeit of reduced amplitude compared to control. Over the 6 week period, CSA of DIAm fibers increased especially at type IIx and/or IIb fibers. This growth was slightly blunted in C2HS animals in type IIx and/or IIb fibers but not in type I or IIa fibers. There were no differences in fiber type proportions in either group. These findings demonstrate that there is minimal DIAm remodeling following prolonged inactivity induced by cervical SCI. Supported by NIH grant AR096750.