The time-course of extrinsic foot muscle volumes during prolonged running: an mri study

The volume of intra-muscular fluid might play an important role in the contribution of intramuscular connective tissue to muscle force development at longer muscle lengths. Additionally, prolonged running may affect intra-muscular fluid volumes by amplified blood flow or dehydration. Therefore, we analyzed the changes in volume of the extrinsic foot muscles during a prolonged run using intermittent magnetic resonance imaging scans. We found rapid increases in individual muscle volumes immediately after the onset of activity that was followed by a decline in muscle volume after approximately 15 minutes of running. At the end of the run, muscle volume fell below the resting values observed before the run. Under the assumption that these changes are attributable to muscle-fluid volume changes, it is likely that passive contributions from intramuscular connective tissues change throughout a prolonged run.