Novel biomechanics demonstrate gait dysfunction due to hamstring tightness.

BACKGROUND: Children with cerebral palsy and hamstring tightness often demonstrate limited terminal swing knee extension. The conventional clinical measure of popliteal angle describes static hamstring tightness, but is not consistent with dynamic limitation. We hypothesize hamstring tightness, determined via modification of the conventional popliteal angle measure, is directly related to decreased terminal swing knee extension in children with cerebral palsy and normal magnitude knee flexion moments. METHODS: Six patients with cerebral palsy and six normal subjects were evaluated via physical examination and instrumented gait analysis. Physical examination included popliteal angle measures at first hamstring resistance to passive extension (R(1)), and end-range extension (R(2)) with the hip in varying degrees of flexion. Passive R(1) data were used to calculate regression equations to predict R(1) during gait, resulting in a novel measure of Available Knee Extension. Hamstring EMG was also compared. FINDINGS: R(1) during physical examination was significantly correlated with Available Knee Extension at terminal swing (Pearson r = -0.7251, P < 0.0001). Patients walked with significantly decreased velocity (0.959 vs. 1.27 m/s, P = 0.0002) and decreased knee extension at terminal swing (25.6 vs. 2.05 degrees, P < 0.0001), in the presence of normal knee flexion moments (-0.289 vs. -0.306 Nm/kg, P = 0.5009), and significantly decreased power absorption (-0.821 vs. -1.43 W/kg, P < 0.0001). Eleven of 12 patient knees demonstrated negative Available Knee Extension at terminal swing, with markedly limited knee extension. Five of 12 normal knees demonstrated negative Available Knee Extension, but this was near full extension. Hamstring EMG onset times were not significantly different. INTERPRETATION: We believe Available Knee Extension, defined on the basis of clinical measures of first resistance to hamstring stretch, provides a biomechanical link between physical examination findings and dynamic limitations in terminal swing knee extension.

Performance of an inverted pendulum model directly applied to normal human gait.

BACKGROUND: In clinical gait analysis, we strive to understand contributions to body support and propulsion as this forms a basis for treatment selection, yet the relative importance of gravitational forces and joint powers can be controversial even for normal gait. We hypothesized that an inverted pendulum model, propelled only by gravity, would be inadequate to predict velocities and ground reaction forces during gait. METHODS: Unlike previous ballistic and passive dynamic walking studies, we directly compared model predictions to gait data for 24 normal children. We defined an inverted pendulum from the average center-of-pressure to the instantaneous center-of-mass, and derived equations of motion during single support that allowed a telescoping action. Forward and inverse dynamics predicted pendulum velocities and ground reaction forces, and these were statistically and graphically compared to actual gait data for identical strides. FINDINGS: Results of forward dynamics replicated those in the literature, with reasonable predictions for velocities and anterior ground reaction forces, but poor predictions for vertical ground reaction forces. Deviations from actual values were explained by joint powers calculated for these subjects. With a telescoping action during inverse dynamics, predicted vertical forces improved dramatically and gained a dual-peak pattern previously missing in the literature, yet expected for normal gait. These improvements vanished when telescoping terms were set to zero. INTERPRETATION: Because this telescoping action is difficult to explain without muscle activity, we believe these results support the need for both gravitational forces and joint powers in normal gait. Our approach also begins to quantify the relative contributions of each.

Challenges in hip joint modeling for a patient with proximal femoral focal deficiency.

We were presented with a technical challenge driven by a clinical need. A patient with proximal femoral focal deficiency required gait analysis, but our typical biomechanical model [Vicon Clinical Manager (VCM)] would not have correctly identified his abnormal right hip center (RHIP). His underdeveloped right femur was fused to his ileum, his anatomical knee functioned as his right hip, and an above-knee prosthesis provided functional knee and ankle joints. During a special calibration, we estimated the global location of RHIP as the center of the femoral epicondyles, also identifying the global location of pelvic markers. These data were used in equations after Davis et al. to establish local coordinates for RHIP. We used a system of three simultaneous equations to solve for input to VCM that would reproduce this location for RHIP. This procedure allowed for inverse dynamics in VCM, and showed the emergence of an abduction moment at the right hip postoperatively, that exceeded changes predicted by sensitivity analyses. Although our clinical need was met, we concluded that a better approach would have involved full implementation of custom models to reflect abnormal patient anatomy.