Nelson_1994_Am.J.Physiol_266_C919

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.