Understanding the Muscle Activity Pattern of the Hip Flexors during Straight Leg Raising in Healthy Subjects

The aim of this study was to elucidate the activities of the hip flexor muscles during straight leg raising (SLR) in healthy subjects. We also investigated the activities of these muscles during SLR with deep flexion, abduction, and external rotation.

The ten dominant right legs of ten male volunteers were analyzed in this study. Twelve SLR motion tasks were performed; these comprised combinations of hip flexion at 30°, 45°, and 60°; abduction at 0° and 20°; and external rotation at 0° and 30°. The activities of the psoas major (PM) and iliacus (IL) were measured using fine-wire electrodes, whereas the activities of the rectus femoris, sartorius, adductor longus, and tensor fasciae latae muscles were measured using surface electrodes. The percentage of the maximal voluntary isometric muscle contraction (%MVC) during SLR was calculated for each muscle and used for data analyses. The Friedman test and the Wilcoxon signed-rank test were performed for statistical analyses. The significance level was set at P <0.05.

The %MVCs for the PM and IL at 60° flexion were significantly larger than those at 30° or 45° flexion. Moreover, for a constant hip flexion, the %MVC values for the PM and IL showed no significant changes when hip abduction and external rotation were added. For the other muscles, the %MVC values showed no significant change with increasing hip flexion with or without added abduction and external rotation.

Our findings suggest that subjects who perform SLR of up to 60° mainly activate the PM and IL at larger hip flexion angles, whereas the other muscles included in the analysis do not contribute greatly to increased flexion angles during SLR.