Structural Changes in Skeletal Muscle Fibers after Icing or Heating on Downhill Running in Mice

An experimental animal model that causes mild structural disorders of skeletal muscles is essential to understand general exercise-induced muscle damage. Thermal stimulations such as icing and heating are commonly used as treatments for muscle injuries in sports. We established a downhill running (DR) protocol that leads to structural muscle disorders without sarcolemmal disruption and directly compared the structural changes produced by icing and heating after DR. Male ddY mice were divided into the DR, DR plus icing (Ice), and DR plus heating (Heat) groups. All mice ran at 20 m/min, -20% grade on a treadmill for a total of 90 min (three rounds of 30 min). In the Ice and Heat groups, an ice pack and a hot pack were, respectively, applied to the exercised triceps brachii muscles for 20 min just after DR. The proportion of myofibers with structural disorders was higher in the Ice group than in the DR and Heat groups at days 1 and 7 after DR. Moreover, the structural disorder of myofibers was slightly improved in the Heat group at day 1 after DR compared with the DR group. These findings suggest that icing treatment might aggravate the structural changes after DR.