Active-resisted stance modulates regional bone mineral density in humans with spinal cord injury

Abstract

Objective: In people with spinal cord injury (SCI), active-resisted stance using electrical stimulation of the quadriceps delivered a therapeutic stress to the femur (∼150% of body weight) and attenuated bone mineral density (BMD) decline. In standard densitometry protocols, BMD is averaged over the entire bone cross-section. An asymmetric adaptation to mechanical load may be masked by non-responding regions. The purpose of this study was to test a novel method to assess regional BMD of the femur in individuals with SCI. We hypothesize that there will be regional bone-sparing changes as a result of active-resisted stance.

Design: Mixed cross-sectional and longitudinal.

Setting: Research laboratory.

Participants: Twelve individuals with SCI and twelve non-SCI controls.

Intervention: Individuals with SCI experienced active-resisted stance or passive stance for up to 3 years.

Outcome measures: Peripheral quantitative computed tomography images from were partitioned so that femur anatomic quadrants could be separately analyzed.

Results: Over 1.5 years, the slope of BMD decline over time was slower at all quadrants for the active-resisted stance limbs. At >2 years of training, BMD was significantly higher for the active-resisted stance group than for the passive stance group (P = 0.007). BMD was preferentially spared in the posterior quadrants of the femur with active-resisted stance.

Conclusions: A regional measurement technique revealed asymmetric femur BMD changes between passive stance and active-resisted stance. Future studies are now underway to better understand other regional changes in BMD after SCI.

Figures

Figure 1

(A) Schematic representation of the standing system. Quadriceps force during muscle stimulation (active-resisted stance) is transmitted to the distal femur as compression and shear. (B) Representative example of quadriceps force during active-resisted stance training. Substantial fatigue developed over the course of 60 contractions. (C) Magnetic resonance image showing training-induced hypertrophy in a subject who performed unilateral active-resisted stance training for 6 months (subject 1, Table 1).

Figure 2

Longitudinal pQCT images from a subject who performed unilateral active-resisted stance training for > 2 years (subject 1, Table 1). This individual's contra-lateral limb performed passive stance and experienced extensive deterioration of the trabecular lattice. The plots at right depict the relative BMD decline for each limb compared to this subject's baseline BMD values.

Figure 3

Longitudinal BMD for distal femur quadrants, normalized to the non-SCI BMD value for each quadrant. Values are mean (SE). The slope of longitudinal BMD decline for the quadrants is listed.