RESUMO
The changes in contraction time (CT) and histochemical muscle fibre pattern are compared with respect to ATPase activity of the slow soleus muscle of the guniea-pig during regeneration in free grafts and reinnervation after nerve interruption. Interruption of the nerve by crushing at birth results first in prolongation of CT which later returns to normal (control) values. A homogeneous fibre pattern is established related to the homogeneity of the motor units constituting the muscle. In the regenerating muscle CT and muscle fibre pattern of the graft repeat the changes during postnatal development, i.e. CT shows progressive prolongation accompanied by transformation of a heterogeneous (fibres of high and low ATPase activity) to a homogeneous (fibres of low activity only) fibre pattern. However, the regenerating muscle also shows an initial phase with slow CT. Thus the changes in reinnervation and regeneration of the muscle after birth differ, the reinnervated muscle revealing only prolongation, the regenerated muscle a temporary shortening followed by prolongation, i.e. a biphasic development of CT.