THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES Dependence of EMG Responses Evoked by Imposed Wrist Displacements on Pre-existing Activity in the Stretched Muscles

The relationship between the segmented EMG activity in flexor carpi radialis evoked by imposed angular wrist displacement was studied with respect to the level of pre-existing background activity in 30 normal human subjects. Input-output response planes demonstrate that the magnitude of the Ml & M2-3 segments is dependent on both the displacement parameters and the level of pre-existing EMG activity in the stretched muscle. If the level of background activity exceeded 4-5% of the maximum voluntary contraction, the onset latency of the M1 segment and duration of the Ml and the M2-3 segments remained constant (within ± 2 msec) for different magnitudes of step load displacements, despite marked variation in the range of the displacement's amplitude, duration, velocity, and acceleration. We propose that the dependency of the relationship between reflex magnitude and imposed movement parameters on tonic motoneuron activity, as represented by pre-existing EMG levels, may reflect an automatic adjustment mechanism that could be utilized in servo compensation of movements requiring markedly different force levels. RESUME: Nous avons compare chez 30 sujets normaux l'activite^ EMG d'arriere-plan pre-existante et celle reTlexe provoquee par un defacement passif du poignet. Les plans de reponse "input-output" demontrent que l'amplitude des reflexes Ml et M2-3 est ddpendante des parametres de deplacement et du niveau d'activite' EMG pre-existante. Se le niveau d'activit6 d'arriere-plan exc6de de 4 a 5% du'maximum de la contraction volontaire, la latence de d6but de Ml et la duree de l'activite^ Ml et M2-3, par contre, semblent demeurer constant (± 2 msec) malgr6 la variation des poids des deplacements et malgre une grande variety d'amplitude, de durde, de velocity, et d'acceleration. Nous proponons que la dependance a l'egard de l'activite' tonique du motoneurone, de la relation entre I'ampleur du reflexe et les parametres de mouvement representee par les niveaux EMG pr6-existants, ceci reflete l'ajustment automatique qui serait utilise comme retro-compensation pour les mouvements exigeant des niveaux de force difterents. Can. J. Neurol. Sci. 1984; 11:272-280 Over the last thirty years, electromyographic (EMG) activity evoked by imposed angular joint displacements has been used to study reflex systems responding to sensory input from limb mechanoreceptors. Both the analysis techniques employed and perspectives as to the peripheral and central neural mechanisms underlying the responses have evolved markedly since Hammond (1954) first described the segmentation of the biceps EMG activity evoked by imposed extension of the human elbow (see Wiesendanger and Miles, 1982 for a recent review). Initially, most of the interest focused on the latencies of the different EMG segments (Hammond, 1954; Melvill Jones and Watt, 1971; Marsdenetal., 1972; Tatton etal., 1975) as providing a possible means of elucidating the central connections involved in the generation of the responses. The latency considerations were based on the assumption that each of the EMG segments (now frequently labelled Ml, M2 and M3) were mediated by separate central pathways with different "loop times". More recently, the latency assumption has been questioned and a number of alternate central and/or peripheral mechanisms that may contribute to the generation of the segmented EMG responses have been proposed (see Eklund et al., 1982 and Lee and Tatton, 1982 for references and considerations of possible mechanisms). Although research has not yet been able to unambiguously define the relative roles played by the various proposed mechanisms, it has been established that the involvement of specific CNS structures in the generation of the segmented responses can differ depending on the species and/or jointmuscle system examined. For example, the M2 segment of distal upper limb flexor muscles in primates is dependent on the integrity of the motor cortex for its elaboration (Lenz et al., 1983b), and spike triggered averaging has shown motor cortical From the Playfair Neuroscience Unit and Department of Physiology, University of Toronto and West Park Hospital, Toronto, Ontario Received November 10, 1983. Accepted for publication March 5, 1984 Reprint requests to: Dr. W.G. Tatton, Playfair Neuroscience Unit, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 2S8 272 EMG Effect in Stretched Muscle — Bedingham and Tatton LE JOURNAL CANADIEN DES SCIENCES NEUROLOGIQUES neurons must contribute to the generation of the M2 segment in monkey wrist muscles (Cheney and Fetz, 1983). A similar dependency is not evident for the M2 segment evoked in the proximal limb flexors in the same animals (Miller and Brooks, 1981;Lenzetal., 1983a, 1983b) or for the second segment in the cat triceps (Ghez and Shinoda, 1978; Tatton et al., 1983). Hence, the evidence related to peripheral and central mechanisms and the resultant response properties determined for one jointmuscle system do not necessarily hold for another. Other research has been directed toward determining the functional role of the segmented responses in the control of movements. Initially, research using imposed movements of the human thumb (Marsden et al., 1972) and ankle (Melvill Jones and Watt, 1971) was interpreted as suggesting that the responses were appropriate to "automatically" compensate for unexpected loads encountered during a voluntary movement. Subsequently, a number of other proposals for the functional role of the responses have been offered. For example, it has been proposed that the segmented responses may serve as "test pulses" to measure unexpected loads (Allum, 1976) or alternately, to maintain constant muscle stiffness despite changing loads (Kwan et al., 1979). As one step in exploring the validity of the different proposals, the input-output relations for the segmented responses have been examined. Averages of the rectified EMG responses have shown them to be monotonically graded with a number of related mechanical parameters for movements imposed on the cat elbow (Ghez and Shinoda, 1978; Tatton et al., 1983), the primate wrist (Tatton and Bawa, 1979; Jaeger et al., 1982a; Lenz et al., 1983b) and the human ankle (Gottlieb and Agarwal, 1979). Several of the input-output studies attempted to control for variations in the excitability of the motoneuron pool innervating the stretched muscles (for an example see Marsden et al., 1976 as well as Tatton and Bawa, 1979 for details concerning the necessity of controlling for changing levels of motoneuron excitability). Yet, despite recognition of the likely importance of the level of pre-existing motoneuron excitability, its effect on the magnitude and timing of the segmented responses relative to that of changes in the mechanical parameters of the imposed movements has not been systematically determined for any joint-muscle system. Accordingly, we have examined the time course and grading of the segmented responses in one jointmuscle system, the human wrist flexor muscles, to step load imposed angulardisplacements relative to changing background levels of motoneuron activity. In response to step loads, the segmented EMG responses are shown to constitute constant duration pulses of activity whose input-output relations can be described by response planes relating their magnitude simultaneously to both the level of pre-existing EMG activity in the stretched muscle and the initial velocity of the imposed displacements. It is shown that determinations of the volumes containing the response planes for populations of normals should be used if alterations in the segmented responses induced by different movement paradigms or in patients with various neurological conditions are to be meaningfully assessed.