Muscle stiffness and continuous electromyographic activity in old rats; an animal model for spasticity?

A mechanomyographic response of the hind foot to passive straightening and bending, as well as an electromyographic activity of the gastrocnemius and tibialis anterior muscles were recorded in old (35-44-month-old) and young female rats. In old rats, spontaneous, tonic electromyographic activity patterns were concurrently observed in both antagonistic muscles; they were low-amplitude, dense tonic activity and continuous, high-amplitude, sparse electromyographic activity. The tonic electromyographic activity was correlated with a decline in the strength and mass of muscles, as well as with motor disturbances, including paresis of the rigidly straightened backward hind legs, dragged behind by an animal. In muscles of old rats, morphological features of a chronic denervation atrophy were found. Baclofen (10 and 15 mg/kg, i.p.) diminished the spontaneous tonic electromyographic activity and potently decreased the whole body muscle tone, whereas Madopar (50 mg/kg of L-DOPA+12.5 mg/kg of benerazide) was ineffective. It is suggested that old rats in which the above-described pathologic alterations are observed might be a useful animal model in the search for basic etiopathological mechanisms of spasticity and similar disturbances found in humans.