Relationship between pelvic incidence and hip, pelvic, and lumbar motion during squatting in healthy individuals with high and low pelvic incidence.

Pelvic incidence (PI)—the sacral angle relative to the pelvis—is a posture-independent spinopelvic parameter influencing pelvic and lumbar alignment. Because PI affects sagittal spinal configuration, it may reflect susceptibility to posterior pelvic tilt and reduced femoral head coverage. However, its influence on pelvic, lumbar, and hip motion during dynamic tasks is unclear. We examined whether individuals with high and low PI have differing time-series hip, pelvic, and lumbar angles during squatting. Twenty-six healthy male individuals were classified into high (≥ 53.8°) and low (≤ 43.1°) PI groups (n = 13 each, based on a prior power analysis). Trunk and hip angles in the local coordinate system, lumbar curvature based on spinous process markers, and thorax, pelvis, and thigh angles in the global system were analyzed. Time-series data were normalized over 100% of the squat descent. Lumbar angle time-series during 47%–53% (transition between lordosis and kyphosis) and 72%–87% (transition to maximum kyphosis) of the squat motion differed between the groups (P < 0.05). Lordosis-to-kyphosis transition occurred later in the high PI group (P = 0.02; Cohen’s d = − 0.97). PI is a key structural factor that influences lumbar spine’s contribution to the squatting movement strategy more than the pelvic contribution.