The Muscle Activation of the Erector Spinae During Hyperextension With and Without the Pelvis Restrained

Several researchers have reported that weak lumbar musculature may be a cause of low back pain (LBP) and others have demonstrated that strengthening the low back muscles can help relieve LBP. Although there is agreement that the back should be strengthened, isolating the back muscles is not easy. Back extension is often accompanied by posterior pelvic rotation resulting from contraction of the powerful hamstring muscles. Several studies conducted in a seated position have demonstrated that restraining the pelvis inhibits the contribution of the hamstring muscles and better isolates the lumbar extensors. The purpose of this study was to determine if pelvic restraint during hyperextension on a variable angle Roman chair (VARC) changes the muscle activation of the erector spinae musculature. Seventy volunteers (18 to 35 years) performed hyperextension exercises on the VARC with and without the pelvis restrained. Surface electromyography (EMG) was used to measure the muscle activation of the erector spinae at the third lumbar vertebra (L3). Total root mean square (RMS) for eccentric and concentric phases were amplitude normalized to a maximal voluntary isometric contraction (MVIC). A posttest questionnaire was used to determine the subjective feeling of localized fatigue. The EMG data were analyzed with a Multivariate Hotelling’s T² test and univariate t-tests. The questionnaire data were analyzed using McNemar’s test. Alpha was set at .05. A statistically significant (T²=10.47, F_{2, 68}=45.16, P=0.008) but small (approximately 3%) increase in the RMS of the erector spinae was demonstrated in the unrestrained condition for both independent variables: concentric and eccentric motion. Conversely, a greater proportion of participants subjectively reported greater effort in the lumbar extensors with the pelvis restrained (P=.025). It is unclear if these observed changes in muscle activation and subjective perceptions can direct changes in the clinical practice. The authors conclude from the EMG results that the BackStrong machine does elicit muscle activity in the lower back, but the back muscles are not targeted more by restraining the pelvis.