RESUMEN
The extensor digitorum longus muscle (EDL) or soleus muscle (SOL) in rats was mechanically overloaded on one side. The muscles were (i) untreated (normal) or (ii) self- or foreign-reinnervated (leading to persisting muscle fibres) or transplanted (leading to regenerating muscle fibres). The effects of the different procedures were studied in the treated and untreated muscles on the operated side and in the untreated muscles on the contralateral side. Overloading led to an absolute increase in mass (versus control values) in the normal muscles and to a relative increase in mass (versus the lower mass after reinnervation) in the treated muscles. The mechanism underlying this gain in mass was usually a compensatory hypertrophy. Overloading was followed by transformation of fibres from fast to slow in normal muscles. In the reinnervated muscles, the fibre distribution changed in response to the new nervous input and then remained constant. The majority of the experimental procedures elicited significant muscular changes in the contralateral muscles, including hyperplasia, fibre transformation and fibre hypertrophy or atrophy. The changes are interpreted as the consequence of a general compensatory neuromuscular activity designed to maintain a symmetrical posture during walking and running. These frequent and substantial muscular changes in the unoperated muscles clearly show that the muscles of the contralateral side cannot be used as normal controls.