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A Possible Methodological Flaw in Comparing Dominant and Nondominant Sided Lumbar Spine Muscle Responses Without Simultaneously Considering Hand Dominance

Sung, Paul S., PT, DHSc, PhD*; Spratt, Kevin F., PhD; Wilder, David G., PhD, CPE*

doi: 10.1097/01.brs.0000137071.47606.19

Study Design. Different factorial designs analyzing the same data were compared for consistent interpretation.

Objective. To assess hand dominance as an explanatory factor when comparing dominant and nondominant side back muscle response time delay and fatigability.

Summary of Background Data. A number of studies have evaluated lumbar spinal muscle responses to sudden load and fatigue. Although the dominant side of the back is usually determined by considering hand dominance, few studies explicitly consider hand dominance as part of the explanatory model and assume that right- and left-handed patients are not unique cohorts.

Methods. The erector spinae and multifidi lumbar muscle groups in patients with chronic low back pain (CLBP) were investigated. For the response time to a sudden load, the biceps brachii muscle was compared with erector spinae and multifidi muscles to understand any compensatory upper extremity movement patterns. For fatigue, the erector spinae and multifidi muscle median frequency (MF) and its slope (MFS) were measured during a 1-minute isometric back extension maneuver.

Results. Altogether, 46 patients with CLBP were studied. Right- and left-handed subjects did not differ in reported disability based on the Oswestry Disability Index, (F1, 44 = 2.11, P < 0.153). The dominant and nondominant side response times were significantly different for left-handed patients but not for right-handed patients. The nondominant side back muscles, located on the contralateral side of the dominant hand, were faster for the right-hand dominant patients and slower for the left-hand dominant patients. In the muscles on the nondominant side of the back, the left-hand dominant patients demonstrated significantly more fatigue than right-hand dominant patients. However, in the muscles on the dominant side of the back, neither set of patients demonstrated fatigability.

Conclusions. When examining differential lumbar muscle responses, failing to consider hand dominance explicitly in conjunction with the side assessed resulted in confounding effects. The results of the study indicated a delayed back muscle response time on the nondominant side. This delayed response time raises the possibility that unbalanced muscle activity could prompt a decreased, uncoordinated, asymmetric bracing effect, thereby increasing the risk of lumbar segment buckling. Further studies are needed to investigate the characteristics of the back muscles on the same side of the dominant hand and the factors mediating neuromuscular differences in patients with CLBP. Understanding the effects of hand dominance on fatigue in certain back muscles should lead to better treatments thereby improving neuromuscular control in patients with CLBP.

The effect of hand dominance has not been considered in great detail regarding low back muscles in patients with chronic low back pain. The different response characteristics, in which left-sided spinal muscle responses for right-handed patients will be the same as right-sided spinal muscle responses for left-handed patients, should be considered. Statistically significant and clinically relevant differences in muscle response times and muscle fatigue were observed for right- and left-handed patients. Importantly, these differences were not observed when subject handedness was not considered in the model. The long-term preferential use of the dominant side of the back may alter physiologic properties of the back muscles.

From the *Iowa Spine Research Center, Department of Biomedical Engineering, University of Iowa, Iowa City, IA; and †Iowa Spine Research Center, Department of Orthopaedic Surgery, University of Iowa Hospitals & Clinics; Psychological & Quantitative Foundations, College of Education, University of Iowa; and Iowa Spine Research Center, Department of Biomedical Engineering, University of Iowa, Iowa City, IA.

Acknowledgment date: August 14, 2003. Revision date: November 25, 2003. Acceptance date: December 16, 2003.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Institutional 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 and reprint requests to Paul S. Sung, Pt, DHSc, PhD, Physical Therapy Program, Department of Health Sciences, Cleveland State University, 2501 Euclid Avenue, HS 122, Cleveland, OH 44115; E-mail:

© 2004 Lippincott Williams & Wilkins, Inc.