Muscle activation in therapeutic exercises to improve trunk stability.

OBJECTIVE To evaluate the relative activation amplitudes from 3 abdominal and 2 trunk extensor muscle sites in healthy subjects performing the pelvic tilt, abdominal hollowing, and level 1 of the trunk stability test (TST level 1) exercises and to compare the activation amplitudes among muscle sites and exercises. DESIGN Prospective comparative study. SETTING Motion Analysis Research Center, Dalhousie University, Canada. PARTICIPANTS Twenty-four healthy men (mean age, 30 +/- 8.1 yr [SD]) without low back pain. INTERVENTIONS Subjects performed 3 exercises in a balanced order, repeating each exercise 5 times while surface electromyography (EMG) was recorded from the 5 muscle sites. Exercises were divided into 2 phases: movement and stabilization. The root-mean-square amplitude of the EMG for each phase was calculated and normalized to the maximal voluntary isometric contraction (MVIC) amplitude for each muscle. A 2-factor repeated-measures analysis of variance (ANOVA) tested the muscle by exercise interaction and the main effects for each phase separately. MAIN OUTCOME MEASURES Normalized activation amplitude was the main dependent variable. Ensemble-average curves were calculated to examine the phasing of activation. RESULTS ANOVA showed a statistically significant interaction (p < .05) for both phases, indicating the 3 exercises recruited the 5 muscle sites using different patterns of relative activation. The external oblique (EO) muscle site was activated to higher amplitudes than the other 4 sites in all 3 exercises for both phases. The highest activity was recorded from the EO during the pelvic tilt, just more than 25% of MVIC. The only exercise to recruit the erector spinae to significantly higher amplitudes than the multifidus site was the TST level 1. CONCLUSIONS Study exercises were not interchangeable for the patterns of trunk muscle activation amplitudes. The exercises did not recruit the abdominal muscles to adequate levels for strengthening for this healthy sample; however, all 5 muscle sites were activated, forming the basis of a stabilizing exercise approach.