AN OBJECTIVE ASSESSMENT OF A CLINICAL FATIGUE TEST FOR TRUNK FLEXORS AND EXTENSORS.

Cheng C, Fox MB, McHugh MP

Nicholas Institute of Sports Medicine and Athletic Trauma, New York, NY

Introduction: Patients with low back pain are known to have insufficient muscle endurance in the trunk flexors and extensors. Most tests of trunk endurance involve expensive equipment and are time-consuming. Inexpensive clinical tests have been developed but are relatively subjective. The objectivity of these clinical endurance tests may be improved by the addition of EMG analyses. EMG analyses during instrumented tests have demonstrated greater fatigue in patients with LBP. However, these EMG techniques have not been applied to simple clinical tests of muscle fatigue. Therefore, the purpose of this study was to (1) determine if the erector spinae, rectus abdominus, and external oblique musculature are specifically targeted during frequently used clinical fatigue tests; (2) determine if the EMG indices of fatigue are reproducible over time.

Methods: For measuring extensor endurance subjects lay prone while holding the sternum off the table. For measuring flexor endurance subjects lay supine with the knees and hips held at 90º and the trunk flexed to keep the scapulae of the table .The subjects maintained the test positions for as long as possible up to five minutes. Surface electrodes were placed over the right and left erector spinae muscles at the L4 level, rectus abdominis muscles at the level of the umbilicus and external oblique muscles at the level of the ASIS. The rate of decrease in median frequency (MF) was computed for each muscle for the respective tests. Ten healthy subjects (6 women, 4 men; age 21-35 yrs) without a history low back injury were tested two weeks apart.

Results: Trunk extensor and flexor endurance was similar (203+/-95 s vs. 201+/-100 s) with 5 subjects lasting 5 min in both tests. MF declined by 2.8 Hz/min in the rectus abdominus and by 1.7 Hz/min in both the internal oblique and erector spinae muscles. These values were extremely low compared to previous studies which typically showed a 15-25 Hz/min decline. Poor reproducibility was found for the decline in MF for the three muscles tested (ICC 0.58-0.17). However, trunk flexor endurance time was correlated with the decline in rectus abdominus MF on both the right (r=0.78, p<0.01) and left (r=0.70, p<0.05) sides.

Conclusion: The small declines in MF of the erector spinae, rectus abdominus, and external obliques indicate that these muscles were not sufficiently targeted during the fatigue tests. This is also supported by the poor reproducibility of the EMG measurements. The time to fatigue was correlated with the decline in rectus abdominus MF indicates specific involvement of this muscle group. The lack of correlation between time to fatigue and MF decline of the external obliques or erector spinae muscles indicate that these muscles were not specifically involved in the task.

Relevance: This common clinical test may not be a true indicator of muscle fatigue in the primary trunk extensors and flexors.

Journal of Orthopaedic and Sports Physical Therapy, A-56, PO33, 2001.