SPINE Volume 28, Number 16, pp 1810–1820 ©2003, Lippincott Williams & Wilkins, Inc. Muscle Fatigue and Fatigue-Related Biomechanical Changes During a Cyclic Lifting Task P. Bonato, PhD,*† Gerold R. Ebenbichler, MD,‡§ S. H. Roy, ScD,* S. Lehr, MS, M. Posch, PhD, J. Kollmitzer, PhD,‡ and U. Della Croce, PhD¶ Study Design. Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task. Summary of Background Data. Recent advances in time-frequency analysis procedures for electromyograph- icic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue- related functional impairments in patients with low back pain. Objectives. To study the relationship between local- ized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue. Methods. A total of 14 healthy male subjects (26  5 years) cyclically lifted and lowered a 13 kg box (12 lifts/ min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyo- graphic signals from 14 muscle sites (including paraverte- bral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fa- tigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyo- graphicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gath- ered using a motion analysis system to study kinematic and kinetic changes during the lifting task. Results. During the cyclic lifting task, the electromyo- graphic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consis- tent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-spe- cific changes in electromyographicic Instantaneous Me- dian Frequency was not significantly correlated with elec- tromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Fre- quency changes of paravertebral back muscles. Signifi- cant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4 –L5 ver- tebral segment. Conclusions. Our results demonstrate correlation be- tween localized muscle fatigue and biomechanical adap- tations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be use- ful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle- specific findings indicates that they should not be consid- ered as equivalent assessment procedures. [Key words: surface electromyographicicy, dynamic muscle contrac- tion, biomechanics of lifting, muscle impairment, low back pain] Spine 2003;28:1810 –1820 Lifting has long been recognized as a source of low back injury, as well as various other musculoskeletal prob- lems. 1–3 Bending forward in order to move or lift an object generates a high bending moment on the osteoli- gamentous structures of the lumbar spine. Lifting creates reactive forces that are imposed on the spine, acting in synergy and opposite to the forces producing the move- ment. 4 The spine is particularly prone to the effects of these reactive forces due to its multisegmental structure and reliance on muscles to actively provide stability to the spine. 5–8 If the bending moment is high, even low compressive forces may increase the risk of disc hernia- tion or lesions of the osteoligamentous apparatus. Low back injury and low back pain (LBP) are thought to oc- cur when spinal load exceeds tissue tolerance. 9,10 Verte- bral tissue failure values derived from cadaver lumbar intervertebral joints range between 4000 N and 12,000 N. 11–14 The large variability in these results is likely re- lated to the fact that vertebral tissue failure values are From the *NeuroMuscular Research Center, Boston University, Bos- ton, †Department of PM&R, Harvard Medical School at Spaulding Rehabilitation Hospital, Boston, Massachusetts, ‡Department of PM&R, University Hospital of Vienna, §Rehabilitation Hospital “Weisser Hof”, Klosterneuburg, Department of Medical Statistics, University of Vienna, Austria, and ¶Dipartimento di Scienze Biomedi- che, Universita ` di Sassari, Italy. This work was partially supported by The Whitaker Foundation, Bio- medical Engineering Research Grants, under the project entitled “As- sessment of muscle function during cyclical lifting through the analysis of surface electromyographicic signals.” Acknowledgment date: July 16, 2002. First revision date: December 4, 2002. Acceptance date: January 8, 2003. 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. Address correspondence to Gerold R. Ebenbichler, MD, Department of Physical Medicine & Rehabilitation, University Hospital of Vienna, Waehringer Guertel 18 –20, 1090 Vienna, Austria; E-mail: Gerold.Ebenbichler@AKH-Wien.ac.at 1810