Contractile properties and myosin phenotype of single motor units from neonatal rats.

The unloaded shortening velocity (Vus), twitch time to peak force (TTP), and myosin heavy chain (MHC) composition of motor units from soleus muscles of neonatal rats (7-8 and 16-18 days) were compared. The Vus range was from 2.2 to 7.1 fiber lengths (fl).s-1 and was unchanged from 7-8 to 16-18 days. TTP shifted from 7-8 (range, 66-120) to 16-18 days (range, 41-95). MHC-specific antibody staining of motor units revealed a correlation between MHC and Vus of individual motor units. Slowest motor units contained type I MHC. Intermediate Vus motor units contained both embryonic, neonatal, and/or type IIa MHCs. Fastest motor units contained neonatal and/or type IIa MHCs. These findings demonstrate that an individual motor unit of a neonatal muscle contains a nonrandom distribution of fiber types. The range of myosins present within the motor units of the soleus translates into a range of possible Vus.