202 BRES 16835 Brain Research, 555 (1991) 202-214 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.50 A DONIS 0006899391168355 Coordination of motor pools controlling the ankle musculature in adult spinal cats during treadmill walking Camille P. de Guzman 2, Roland R. Roy 1, John A. Hodgson I and V. Reggie Edgerton 1'2 1Brain Research Institute and 2Neuromuscular Research Laboratory, Department of Kinesiology, Los Angeles, CA 90024 (U.S.A.) (Accepted 5 March 1991) Key words: Adult spinal cat; Treadmill locomotion; Electromyography; Agonist; Antagonist; Motor control The coordination of the motor pools of two ankle plantarflexors, i.e. the soleus (Sol) and medial gastrocnemius (MG), and an ankle dorsiflexor, i.e. the tibialis anterior (TA) was quantified by comparing the EMG amplitude relationships in muscle pairs in normal and trained adult spinalized cats during treadmill walking across a range of relatively slow speeds (0.1 to 1.0 m/s). The effects of increased tactile stimulation or loading on locomotor performance were also studied in the spinal cats. Joint probability density distributions in the spinalized cats showed a low level of MG activation relative to Sol which did not change as speed increased. In general, the coordination between Sol and MG was similar in normal and spinal cats. However, towards the final phase of the extensor burst, the MG EMG amplitude decayed prematurely in spinal cats, particularly at higher speeds. Preferential elevation of MG relative to Sol activity was seen as a result of tactile stimulation. An elevated load resulted in a higher level of MG activation relative to Sol, prolonged MG activity at the end of the extensor burst, and the reduction in the clonic pattern of EMG typical of spinal cats. Spinalized cats showed an increased incidence of SoI-TA coactivation, especially at the higher speeds, due in part to the tonic activity in the TA. However, the overall reciprocal relationship between these antagonists was maintained. This reciprocity was preserved, but the high level of coactivation was unaffected by tactile stimulation. An elevated load, however, resulted in less SoI-TA coactivation. These results suggest that the coordination between synergists (SoI-MG) and between antagonists (SoI-TA and MG-TA), as well as the level of activation are modulated in the adult spinal cat similar to that observed in the normal cat. Further, there are specific types of proprioceptive-cutaneous information that can affect selected phases of the step cycle such that full weight-supporting stepping is significantly improved. INTRODUCTION It is generally believed that the recruitment of motor units within a motor pool occurs in a relatively constant order related to motoneuron size or closely associated parameters 9'1°'1s. However, the recruitment pattern among agonist and antagonist motor pools has been characterized only qualitatively. For example, it is known that the onset and termination of activation of the soleus (Sol) and medial gastrocnemius (MG) occur at about the same time and that the relative level of activation of the Sol motor pool exceeds the MG during slow loco- motion 24'3°. The MG becomes more highly activated only in movements that have high tension or power re- quirements 19'2°'24'3°'32. Also it is well recognized that the activation sequence of antagonists such as the tibialis anterior (TA) and the MG are generally out of phase, although some overlap in the initiation of TA activity and the termination of MG activity (i.e. co-contraction) usually occurs 14'15'24. The regulation of the relative activation of agonist and antagonist motor pools has been described more quantitatively. For example, the proba- bility of occurrence of any given combination of EMG amplitudes in normal rats during locomotion at varying speeds and grades and during swimming, when the demands of weight support are eliminated, have been determined 2°. The relationship between the Sol and gastrocnemius in normal cats during various motor tasks has been described quantitatively in a similar manner 19. Previous studies have detailed the kinematics and EMG characteristics of hindlimb stepping on a treadmill of cats spinalized at a low thoracic level at 1-2 weeks of age 12'13, at 2-12 weeks of age 8'12'28'29 or as an adult 2' 7,11,22,23,25,26 From these data, it is clear that the hindlimbs of spinalized cats trained daily to step on a treadmill can recover a level of coordination that in many respects resembles normal locomotion. In the present study, the coordination of motor pools during locomotion was quantified by comparing the rectified and smoothed EMG amplitudes in muscle pairs in normal cats and in cats spinalized as adults. The probability and sequence of occurrence of pairs of amplitudes during the activation of Correspondence: R.R. Roy, Brain Research Institute, Center for the Health Sciences, University of California, Los Angeles, 10833 Le Conte Avenue, Los Angeles, CA 90024-1761, U.S.A. Fax: (1) (213) 825-6616.