Trunk muscle responses to demands of an exercise progression to improve dynamic spinal stability

Objectives: To compare relative activation amplitudes among abdominal and trunk extensor muscle sites of healthy people performing a leg-loading exercise protocol aimed at progressively challenging spinal stability. Design: A prospective, comparative, repeated-measures design. Setting: Motion analysis research laboratory. Participants: Eighteen healthy male and female volunteers (mean age ± standard deviation, 23.8±3.4y). Interventions: Subjects performed 5 progression levels in random order, while surface electromyograms were recorded from 5 abdominal and 2 back extensor muscle sites. Levels 2 through 5 were of interest because they included a leg-extension phase. The root mean square (RMS) amplitude during the leg-extension phase was calculated and normalized to maximal voluntary isometric contractions (MVICs) for each muscle. A 2-factor repeated-measures analysis of variance tested the muscle-by-level interactions and the 2 main effects for the abdominal and trunk extensor sites separately. Bonferroni adjustments were performed on significant results. Main Outcome Measure: Normalized RMS (NRMS) amplitude for each muscle during the leg-extension phase. Results: There were statistically significant muscle-by-level interactions (P<.05) for both abdominal and trunk extensor sites. The relative amplitudes increased for the abdominal muscles and erector spinae muscles among levels, except between levels 3 and 4. Significant differences were found among the abdominal muscle sites for the lower-level exercises (levels 2-4), but not for level 5. The highest NRMS amplitude was at level 5, that is, 40% of MVIC. Conclusions: The patterns of activation amplitudes differed among levels; therefore, this was not a simple progressive loading protocol because muscles responded in a nonuniform manner to the demands associated with the various levels of progression. Based on the results, the protocol met some of the objectives of dynamic stability protocols.