Phenotypic changes in the composition of muscular fibres in rat soleus motor units after 14 days of hindlimb unloading.

The main goal of this study was to identify the different fibre types of the motor units (MUs) contained in the soleus muscles from control (CONT) rats and from rats submitted to 14 days of hindlimb unloading (HU). The MU types were classified according to their contractile properties and also using glycogen depletion followed by adenosine triphosphatase (ATPase) staining. In CONT rats, the soleus muscle contained two MU types identified as slow and fast types. After HU, the MU distribution showed three populations: slow, intermediate and fast. All the MUs from HU soleus were heterogeneous in terms of fibre type composition, indicating a complex remodelling of the muscle.