The effects of stance width and foot posture on lumbar muscle flexion-relaxation phenomenon.
Level 4 - case-series / case-control
Within-subject laboratory biomechanical study in healthy volunteers
PubMed 24411694 · doi:10.1016/j.clinbiomech.2013.12.009
What was done
Thirteen volunteers performed trunk flexion movements across six distinct conditions combining three stance widths with two foot orientations (toe-forward and toe-out). Paraspinal electromyography was recorded at the L3 and L4 levels alongside motion tracking sensors at S1, T12, and C7 to measure lumbar and trunk kinematics. Investigators identified the lumbar flexion angle where muscle activity dropped to resting levels (the flexion-relaxation phenomenon) and the threshold where internal moment shifted from active muscles to passive spinal tissues.
What was found
Increasing stance width reduced the flexion-relaxation lumbar angle by 1.3° for the L3 paraspinals. Switching from a toe-forward to a toe-out posture reduced the flexion-relaxation angle by 1.4° at L3 and 1.1° at L4. The systemic load-shifting boundary between active and passive spinal structures did not change across conditions.
Why it matters
Small alterations in stance and foot posture alter lumbar muscle relaxation timing during forward bending. Clinical evaluations using the flexion-relaxation phenomenon to evaluate low back neuromuscular control need consistent foot placement to avoid measurement artifact.
Limits
The abstract reports on a very small sample (n = 13) without detailing participant characteristics such as sex, age, or health status. No measures of statistical dispersion, confidence intervals, or significance testing values (p-values) are reported in the abstract. The observed angular differences were small (1.1° to 1.4°), and the study evaluated immediate biomechanical changes rather than clinical outcomes.
Cited by
- supports The lumbar paraspinal muscles function to control forward trunk flexion to protect the spine.