Mechanographic analysis of muscle rigidity after morphine and haloperidol: a new methodological approach

The new method described in this study was based on consecutive repeated measurements of the resistance of flexor and extensor muscles of the hind foot of the rat to forced flexions and extensions of the foot. Locomotor movements of the rat were restrained with a metaplex box which had a slot for the hind limb. The control muscle tone measured by this method was constant for more than 2 h, and amounted to approx. 25 g for flexor muscles, and approx. 45 g for extensors. Morphine (2.5, 5, 10, 20 mg/kg) enhanced dose-dependently the resistance of flexor muscles up to approx. 45 g, 70 g, 100 g and 140 g, respectively, and the resistance of extensors of the paw up to approx. 100 g, 140 g, 180 g and 240 g, respectively. Haloperidol (5 and 10 mg/kg) enhanced dose-dependently the resistance of flexor muscles up to approx. 45 g and 70 g, respectively, and that of extensors of the foot up to approx. 75 g and 120 g, respectively. Morphine rigidity, measured as resistance of respective muscles to forced movements, was almost completely inhibited by a consecutive injection of 0.2 mg/kg of naloxone. The new method seems to have considerable advantages in comparison with electromyographical (EMG) or other kinds of mechanographical measurements of the muscle tone.