D-34 Free Communication/Poster - Musculoskeletal Mechanics II: MAY 28, 2009 1:00 PM - 6:00 PM ROOM: Hall 4F
PURPOSE: Koch‘s model of the hip indicates that during the single support phase of gait the hip abductor muscles must counter the varus torque created by the weights and inertial forces of the head, arms and torso. If these muscles become fatigued (especially gluteus medius) they may fail to counter the large varus bending moment in the neck of the femur and thus increase the stress on the superior neck. The purpose of this study was to quantify the increased stress in the superior femoral neck as a result of reduced gluteus medius tension during running.
METHODS: Ten experienced male runners (age 22.2 ± 3.2 yrs, height 1.8 ± 0.1 m, mass 69.8 ± 6.5 kg) ran ten trials at their preferred running velocity (4.4 ± 0.5 m/s). Motion-capture (120 Hz) and force platform data (1200 Hz) were collected and three dimensional moments were calculated at the hip, knee, and ankle joints using inverse dynamics. Kinematics were imported into a scaled SIMM model to estimate maximal forces, muscle orientations, and muscle moment arms for 44 lower extremity muscles. Estimations of actual muscle forces using a static optimization procedure found the set of forces that minimized the sum of the muscle stresses squared. Femoral neck geometry was assumed to be an effective hollow cylinder with radii that were chosen such that a scaled FEM model produced stresses that were matched to those calculated from beam theory (radius = 1.7 cm). Gluteus medius forces were then reduced in the beam model from about 2 body weights (BW) to 0 BW in 10% increments. Peak superior femoral neck normal stresses were noted.
RESULTS: Femoral neck stresses increased from 25.6 MPa with no reduction of the abductors to 43.3 MPa when the abductors were completely removed. This was a 1.8% increase in stress for every 10% reduction in abductor force.
CONCLUSIONS: Even though the hip contact forces were reduced from 14 to 12 BW‘s as gluteus medius forces were reduced, stress on the superior femoral neck was substantially increased. The gluteus medius produced a valgus bending moment at the neck that countered the varus moment caused by the upper body. A fatigued gluteus medius may have decreased force production and therefore femoral neck stress fracture potential may be increased.