AGE-RELATED MUSCLE-STIFFNESS - PREDOMINANCE OF NON-REFLEX FACTORS

This study was aimed at assessing the contribution of reflex and non-r eflex factors to the muscle tone of old female Wistar rats. The hind f oot of a rat was flexed or extended at the ankle joint by 25 degrees o ver 250 ms. The resistance of the foot to passive movements (torque, m echanomyogram), as well as the reflex electromyographic activity in th e gastrocnemius and tibialis anterior muscles, were recorded simultane ously. Moreover, the impact of the blockade of the reflex activity cau sed by the local anesthetic lignocaine (1-2 ml of a 2% solution, injec ted in the vicinity of the sciatic nerve) on the muscle tone was inves tigated. Additionally, old rats' hind leg muscle samples were analysed using fluorescent microscopy for the expression of fibronectin, which is an early marker of connective tissue formation. It has been shown that old rats are characterized by (i) a substantially increased resis tance of flexor muscle stiffness (measured during extension) and uncha nged resistance of extensors (measured during flexion), (ii) the loss of a major part of the reflex electromyographic activity and (iii) the increased content of fibronectin in muscles. Moreover, it has been sh own that lignocaine, which completely blocked the electromyographic re flex activity in the gastrocnemius and tibialis anterior muscles in yo ung animals, was unable to counteract the resistance of these muscles to passive movements in old rats. The present results suggest that the muscle stiffness seen in old rats is not due to a reflex response, bu t depends mainly on non-reflex factors-chiefly on a large overgrowth o f non-elastic connective tissue replacing degenerated active muscle fi bers. (C) 1997 IBRO.