Femoral loads during passive, active, and active-resistive stance after spinal cord injury: a mathematical model

Abstract

Objective: The purpose of this study was to estimate the loading environment for the distal femur during a novel standing exercise paradigm for people with spinal cord injury.

Design: A mathematical model based on experimentally derived parameters.

Background: Musculoskeletal deterioration is common after spinal cord injury, often resulting in osteoporotic bone and increased risk of lower extremity fracture. Potential mechanical treatments have yet to be shown to be efficacious; however, no previous attempts have been made to quantify the lower extremity loading during passive, active, and active-resistive stance.

Methods: A static, 2-D model was developed to estimate the external forces; the activated quadriceps forces; and the overall bone compression and shear forces in the distal femur during passive (total support of frame), active (quadriceps activated minimally), and active-resistive (quadriceps activated against a resistance) stance.

Results: Passive, active, and active-resistive stance resulted in maximal distal femur compression estimates of approximately 45%, approximately 75%, and approximately 240% of body weight, respectively. Quadriceps force estimates peaked at 190% of body weight with active-resistive stance. The distal femur shear force estimates never exceeded 24% of body weight with any form of stance.

Conclusions: These results support our hypothesis that active-resistive stance induces the highest lower extremity loads of the three stance paradigms, while keeping shear to a minimum.

Relevance: This model allows clinicians to better understand the lower extremity forces resulting from passive, active, and active-resistive stance in individuals with spinal cord injury.

Figures

Fig. 1

Schematic representations of six possible stance postures for individuals with spinal cord injury (SCI) using an external frame (not shown). The arrow at the hip and the vertical line at the knee represent a hip support belt and a knee support pad, respectively. The feet are placed on the ground.

Fig. 2

Schematic free body diagram of the lower body during (A) passive stance and (B) active (no added resistance, R = 0) and/or active–resistive stance (R = 16.7–67.7% BW). The internal forces from the quadriceps and patellar tendons (Fquad and Fpat, respectively) in (B) replace the Fpad in (A).

Fig. 3

Schematic free body diagram of the femur, sectioned at l = 0.85 of the total femur length, corresponding to the most common location of distal femur fractures after spinal cord injury. Distal femur shear (Fv) and compression (Fc) were estimated by this model, but the bending moments (Mb) were not calculated.

Fig. 4

The model estimates for the external belt (Fbelt), kneepad (Fpad), and ground reaction forces (N and Fx) during passive stance at each of the six stance postures (as shown in Fig. 1) are plotted in terms of percent body weight (% BW).

Fig. 5

Modeled quadriceps forces are shown in percent body weight (% BW) during active and active–resistive stance with resistances ranging from 13.7% BW to 67.7% BW for each of the six stance postures.

Fig. 6

Modeled distal femur compression (A) and shear (B) in terms of percent body weight are plotted for passive, active, and active–resistive stance at each of the six stance postures. Note the different scales between graph A and B.