SPINE Volume 33, Number 11, pp E342–E348 ©2008, Lippincott Williams & Wilkins The Influence of Pain-Related Fear on Peak Muscle Activity and Force Generation During Maximal Isometric Trunk Exertions James S. Thomas, PhD,* Christopher R. France, PhD,† DaoHang Sha, PhD,* and Nicole Vander Wiele, DPT* Study Design. A cross-sectional study of peak activa- tion of trunk muscles in 20 participants with chronic low back pain. Objective. To determine how pain-related fear influ- ences peak activation of abdominal and trunk extensor muscles during maximal isometric trunk exertions. Summary of Background Data. The pain adaptation model and the pain-spasm-pain model have been pro- posed to explain differences in trunk muscle activation between healthy participants and those with chronic low back pain. However, there is no clear consensus in the literature in support of either model, and there are no studies that have examined the role of pain-related fear on muscle activity in the context of these 2 models. Methods. Participants with chronic low back pain per- formed a series of maximal isometric exertions into ex- tension, flexion, rotation right and left, and side-bend right and left from a tall kneeling position with the spine in a neutral posture. A doubly multivariate MANCOVA was conducted with a between subject factor of group (high pain-related fear, low pain-related fear), a within subject factor of direction (extension, flexion, rotation left and right, side bend left and right), McGill Pain Question- naire ratings entered as covariates, and peak EMG for all 10 trunk muscles entered as dependent variables. Results. Averaged across all pull directions and all 10 trunk muscles assessed, participants with high pain-re- lated fear had peak EMG that was only 49.5% of partici- pants with low fear. Additionally, we found significantly smaller peak force for isometric exertions in flexion, side- bend right, and side-bend left directions. Conclusion. Pain-related fear is associated with in- creased disability, in theory, because of avoidance behav- ior and disuse. Our data suggest that participants high in pain-related fear specifically avoid activation of the ab- dominal muscles during maximal isometric trunk exer- tions. Key words: EMG, pain-related fear, isometric contrac- tions, low back pain. Spine 2008;33:E342–E348 Low back pain is a significant health problem that ac- counts for considerable medical costs, disability, and personal suffering. 1 Although the cause of recurrent and chronic low back pain is often unknown, it has been suggested that alterations of trunk muscle activation may precipitate acute episodes of low back pain 2,3 and possi- bly contribute to the development of recurrent or chronic symptoms. 4 Two prominent models that have been pro- posed to explain the associations of back pain and trunk muscle activation include the pain-spasm-pain model and the pain adaptation model. The pain-spasm-pain model 5 proposes that individuals with low back pain have increased muscle activity during rest and submaxi- mal activities as compared to healthy controls. Increased muscle activity then contributes to increased pain symp- toms, which, in turn, increases muscle activity. The pain adaptation model 6 asserts that agonist muscle activity decreases and antagonist muscle activity increases in or- der to limit joint velocity and subsequent joint loading. Thus this model suggests a functional benefit to altered patterns of muscle activation that serve to protect irri- tated or damaged spinal structures. Despite their differ- ences, both models predict that individuals with and without back pain will have equal muscle activation dur- ing maximal voluntary isometric contractions, 7 although this notion has not received extensive testing. The fear-avoidance model of low back pain and dis- ability has evolved over the last 2 decades in an attempt to explain why some people with acute musculoskeletal pain go on to develop chronic pain and disability whereas others do not. According to the fear-avoidance model, 8 pain-related fear can have a negative effect on physical performance over time by reinforcing reduc- tions in daily activity. Consistent with this notion, stud- ies have shown that individuals with chronic low back pain who scored high on pain-related fear are impaired on a variety of physical challenges. 9 –18 Pain-related fear has also been associated with decreased activation of the upper trapezius during submaximal isometric neck exer- tions, 19 increased reactivity of the erector spinae while sitting, 20 and increased peak EMG activity of the erector spinae at end-range trunk flexion. 21 In contrast, al- though increases in activation of the erector spinae has been reported during gait activities in those with back pain, pain-related fear does not seem to be related to activation of these muscles. 22,23 Because existing studies have focused primarily on trunk extensors muscles, it remains to be determined if pain-related fear may have From the *School of Physical Therapy, †Department of Psychology, Ohio University, Athens, OH. Acknowledgment date: September 19, 2007. Revision date: January 7, 2008. Acceptance date: January 16, 2008. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Supported by National Institutes of Health Grant RO1-HD045512. The study was conducted in accordance with the current American Psychological Association guidelines for human research and was ap- proved by the Ohio University Institutional Review Board. Address correspondence and reprint requests to James S Thomas, PhD, PT, Ohio University School of Physical Therapy, W277 Grover Center, Athens, OH 45701; E-mail: thomasj5@ohiou.edu E342