Physiologically, bones adapt to the muscle forces. After spinal cord injury (SCI), muscle atrophy is common, and so is bone loss in the affected limbs. Spasms, which are very common in SCI patients may mitigate muscle atrophy. We therefore hypothesized that the remaining bone strength after SCI is in proportion to the persisting muscle strength.

Six patients, who had suffered a SCI 6.4 (SD 4.3) years earlier and 6 age matched, able-bodied control subjects were investigated. Bone strength was studied by peripheral quantitative computed tomography (pQCT)at the right knee by. Muscle strength was assessed for the knee extensor muscles by electrical stimulation with a triplet via the femoral nerve, and isometric knee extension torque was measured and patellar tendon force was estimated.

In the SCI group, tendon force and volumetric bone density were 75% and 50% lower, respectively, than in the control subjects (p < 0.01). Close curvilinear relationships were found between stress and volumetric density for the tibia epiphysis (r^2 = 0.90) and for the patella (r^2 = 0.91).

These data suggest that, under steady state conditions after SCI, epiphyseal bones are phusiologically adapted to the altered muscular forces after SCI.