EMG stability as a biofeedback control

Biofeedback and Self-Regulation, VoL 17, No. 2, 1992

EMG

S t a b i l i t y as a B i o f e e d b a c k C o n t r o l 1

Andrew Harver University of North Carolina, Charlotte

Joyce Segreto Youngstown State University

Harry Kotses 2 Ohio University

Factors that may confound comparisons between electromyographic (EMG) biofeedback training and its control conditions include feedback quality and experience of success. We investigated the usefulness of a control procedure designed to overcome these potential sources of confounding. The procedure consisted of training muscle tension stability. We used it as a control for frontal EMG relaxation training in children with asthma. To equate the groups for feedback quality and experience of success, we gave each child in the control condition audio feedback decreasing in pitch when muscle tension was at or near baseline levels, and feedback increasing in pitch when muscle tension was either substantially above or below baseline levels. Children in both groups were instructed to decrease the pitch of the tone. In comparison to children in the relaxation condition, the children in the control condition exhibited stable levels of muscle tension throughout eight training sessions. We concluded that feedback for stable muscle tension may be a useful control procedure for EMG biofeedback training whenever experimental and control procedures differ in either feedback quality of degree to which they permit subjects to experience success. Descriptor Key Words: electromyographic ( E M G ) control; E M G stability training; biofeedback control. 1This research was supported by NIH-Grant HL 27402. We are grateful to Paul Schnitter who constructed the EMG stability feedback device. 2Address all correspondence to Harry Kotses, Psychology Department, Ohio University, Athens, Ohio 45701. 159 0363-3586/92/0600-0159506.50/0© 1992PlenumPublishingCorporation

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Few aspects of electromyographic (EMG) biofeedback research have generated more controversy than the procedures used to control biofeedback experiments. The controversy stems from efforts to resolve problems in isolating the effects of EMG biofeedback training. The biofeedback procedure usually is associated with a quality of feedback and with an experience of success or mastery that is' difficult or awkward to duplicate in a control condition. In addition, subjects can sometimes recognize control procedures, thereby compromising the function of the control procedures. Further complicating the control problem is the practice of instructing subjects to alter tension, either generally or in muscles directly relevant to biofeedback training. Instructions have their own unique effects on the musculature (Fridlund & Cacioppo, 1986) and may interact with concurrent biofeedback training. Potentially confounding instructional effects in studies of EMG biofeedback training may be negated by giving as few instructions as possible. Unfortunately, doing so is not always convenient. In research dealing with the effects of EMG biofeedback on the treatment of a disorder, for example, it may be important to use procedures similar to those used in clinical practice wherein clients typically receive information about the objectives of training. When it is necessary to tell subjects about muscle tension or about biofeedback training, feedback designed to stabilize muscle tension may be a useful control procedure. EMG stability training permits administration of the same instructions to experimental and control subjects, and thereby avoids confounding (Hatch, 1982). In addition, E M G stability training greatly reduces the likelihood that subjects will recognize control procedures as it controls both for quality of feedback and for experience of Success.

Audio feedback for EMG stability has been used in the past but not always for the purpose of control (Cram, 1980). As a control, it has proved ineffective (Hodes & Howland, 1986); it led to reliable changes that were similar, though not as large, as those brought about by feedback given to decrease EMG. Although discouraging, the failure may have had less to do with the procedure than with the difficulty of the task that subjects were asked to perform. Hodes and Howland required each control subject to balance muscle tension around a specific value, median baseline EMG. They signaled either positive or negative changes in tension from the median baseline value by feedback increasing in pitch. These conditions made for a difficult task: In the vicinity of the baseline value, the subjects heard changes in the direction of the feedback when muscle tension changed very

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little. To make matters worse, the difficulty associated with learning to stabilize E M G was augmented by allowing subjects little time, only one session, to learn the task. We were more successful in using feedback for stabilizing EMG. Our procedure differed from that used by Hodes and Howland in two ways: We employed the baseline range rather than the baseline median as the target and we trained subjects for a number of sessions. As control for an EMG decrease task, we provided decreasing feedback to subjects during training so long as they maintained tension levels within their baseline EMG range. We signaled tension levels above or below that range by feedback increasing in pitch. The use of the baseline range as the target provided subjects with a larger margin of error than use of the baseline median and thereby simplified the biofeedback task. In addition, the use of several training sessions increased the likelihood that individuals would learn the task.

METHOD

We outlined the details of both the apparatus and the software for providing feedback for stable muscle tension elsewhere (Schnitter, Harver, & Kotses, 1984). In this article, we describe our effort to train individuals to maintain a stable EMG level using audio feedback for muscle tension stability. The experiment in which these observations were made was reported previously (Kotses, Harver, Segreto, Glaus, Creer, & Young, 1991). Briefly, 29 children with asthma whose ages ranged from 7 to 16 were assigned randomly to one of two conditions: a Frontal Stable group (N -- 14) and a Frontal Decrease group (N = 15). The children assigned to the Frontal Stable Group were trained to maintain a stable level of frontal tension in each of eight sessions. Each session consisted of a 4minute basal recording period followed by a 16-minute training period. During the basal period of each session, a voltage window was established based on the upper and lower limits of the continuous EMG signal conditioned by a contour-following integrator. During the training period, feedback decreasing in pitch was given as long as EMG levels were within the window, and feedback increasing in pitch was given when EMG levels were either above or below the window. Except for variation in pitch direction, which depended on the relationship of the child's EMG to the voltage window, the pitch of the feedback was independent of the EMG. Prior to each recording session, we instructed each child to reduce the pitch of the tone.

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Children assigned to the Frontal Decrease Group were treated in the same way as children in the Frontal Stable Group except that the feedback during the training portion of each session was proportional to EMG. The pitch of the tone decreased when muscle tension decreased and increased when muscle tension increased. Thus, children in the two groups were equated for instructions and for direction of feedback signalling successful performance. In the assessing of our control procedure, we were concerned with evaluation of two quantities: the boundary levels of the voltage window, and the EMG scores of children in the two groups. Of special concern to us were the EMG scores of children in the Frontal Stable group. RESULTS AND DISCUSSION Both the average minimum and maximum values of the voltage window computed for children in the Frontal Stable group were fairly constant for each of the eight sessions. This resulted in a constant window width as shown in Figure 1. A repeated measures analysis of variance which tested for differences between minimum and maximum voltage levels and for differences between sessions yielded a significant term only for differences between minimum and maximum levels, F(1,13) = 81.77, p < .001. We reported statistical comparisons between the two groups of children earlier (Kotses et at., 1991). As shown in Figure 2, children in the

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Frontal Stable group exhibited a relatively stable E M G record between sessions, whereas children in the Frontal Decrease group showed a progressive E M G decline. 3 The interaction of sessions with group assignment was reliable beyond the .05 level [F(4,189) = 3.18]. Children in the stable group evidenced more within-session variability than children in the Frontal Decrease condition; the Groups x Minutes interaction was significant beyond the .05 level [F(6,405) = 2.24]. Tests of simple effects conducted on the scores only of children in the Frontal Stable group failed to yield a reliable sessions term. The children in the Frontal Stable group, however, exhibited within-session variability in E M G levels which resulted in both a reliable minutes term, F(15,195) = 2.47, p < .05, and a reliable Minutes x Sessions interaction, F(105,1365) = 1.87, p < .05. As training proceeded, within-session variability decreased. These observations showed that muscle tension stability was achieved midway through training of the Frontal Stable children and persisted throughout the balance of training. 3We recorded EMG activity as a continuous, integrated signal, and expressed EMG activity as the integral of accumulated voltage and time (microvolt-seconds).

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W e did not systematically evaluate subjective variables in children assigned to the Frontal Stable group because, in our judgment, the operations needed to evaluate these variables were disruptive to the primary purpose of the study. We did, however, obtain some information concerning subjective experiences. Occasionally, a child in the Frontal Stable g r o u p volunteered information that suggested he or she experienced success in performing the biofeedback task. Specifically, 7 of the 14 children in the Frontal Stable group reported experiences of success at some point in their training (Kotses, Harver, & Schnitter, 1983). Because we made no effort to d e t e r m i n e such i n f o r m a t i o n systematically, o u r o b s e r v a t i o n m a y represent a low estimate of the success experienced by research participants. A dependable evaluation of subjective experiences would follow from an assessment more orderly than ours. O u r observations indicate that feedback for stable muscle tension resuits in E M G stability and m a y be useful as a control p r o c e d u r e for biofeedback training, especially when information is given to subjects regarding the target response or the purpose of the training. Our findings also suggest that use of the baseline range as the target is superior to use of the baseline median. Definitive information as to the latter point, however, must await direct comparison of the two procedures. REFERENCES Cram, J. (1980). EMG biofeedback and the treatment of tension headaches: A systematic analysis of treatment components. Behavior Therapy, 11, 699-710. Fridlund, A. J., & Cacioppo, J. T. (1986). Guidelines for human electromyographic research. Psychophysiology, 23, 567-589. Hatch, J. P. (1982). Control group designs in biofeedback research: Ask, "What does the control group control for?" Biofeedback and Self-Regulation, 7, 377-401. Hodes, R. L., & Howland, E. W. (1986). Ocular and stabilization feedback: An evaluation of two EMG biofeedback control procedures. Biofeedback and Self-Regulation, 11, 207-220. Kotses, H., Harver, A., Segreto, J., Glaus, K. D., Creer, T. L., & Young, G. A. (1991). Long-term effects of biofeedback-inducedfacial relaxation on measures of asthma severity in children. Biofeedback and Self-Regulation, 16, 1-21. Kotses, H., Harver, A., & Schnitter, P. (1983). Equating subjective experience between biofeedback training and control groups. Psychophysiology, 20, 453 (Abstract). Schnitter, P., Harver, A., & Kotses, H. (1984). VOLTWIN: A computer program to establish criteria for reinforcement of stable levels of EMG activity. Behavior Research Methods, Instruments, and Computers, 16, 320-322.