This was the first study known to the authors to report prevalence of hyperkyphosis in men and women with racial differences. In addition, the extensive covariate measurements available in Health ABC provided a unique opportunity to assess the relationship of spinal muscle area and density to hyperkyphosis. We found that lower spinal extensor muscle density but not cross-sectional area is associated with hyperkyphosis, independent of other significant risk factors including age, trunk fat and lumbar spine BMD. This novel finding suggests that spinal muscle density contributes to kyphosis, over and above the effects of age and osteoporosis. These findings are consistent with studies that report lower thigh muscle density but not cross-sectional area as increasing risk of hospitalization in older adults (23
). In addition, lower spinal muscle density but not area was associated with poorer physical function (24
). Furthermore, thigh muscle density appeared to account for the association between muscle strength, physical function, and muscle mass and hip fractures in older adults (25
). The burden of these poor health outcomes is attributed to muscle density rather than to muscle mass. The 4.2 HU difference in average spinal muscle density between normal participants and those with hyperkyphosis is clinically significant. Previous studies have shown that resistance exercise over a 12-week period improves thigh muscle density by 2.7 HU (26
). Spinal extensor muscle resistance exercises performed by older adults (27
) may likely have similar effects.
Other independent risk factors for hyperkyphosis in our analysis included increased age, lower lumbar spine BMD, and less trunk fat. The association with lumbar spine BMD was consistent with the known association of hyperkyphosis with osteoporosis (30
In the Pittsburgh site of the Health ABC sample, prevalence of hyperkyphosis was higher in white women than in men and black women. Most previous studies of kyphosis have been restricted to women, and none have investigated differences in hyperkyphosis between racial groups. In contrast to our results, one study of hyperkyphosis reported that men were more likely to have hyperkyphosis than women (9
). However, the measure of kyphosis in that study was not based upon the Cobb angle but a clinical measure of inability to lie flat with the head in a neutral position. It is likely that the clinical kyphosis measure used in that study captures cervical and thoracic kyphosis and that exaggerated cervical kyphosis may be more common in older men versus women.
The unexpected finding that higher spinal muscle area was associated with increased risk of hyperkyphosis in black men, after controlling for muscle density, also warrants further investigation. Although black men have considerably higher average spinal muscle area, and especially muscle density, compared with other groups, this does not imply that these factors have different effects on kyphosis in this group. Because evidence for interaction was weak, we think that this association is most likely due to chance.
Lower density of the spinal extensor muscles may in part be explained by the flexion-relaxation phenomenon (FRP), described in several studies of healthy adults (32
). FRP denotes silencing of the spinal extensor muscles during trunk flexion maneuvers. In FRP, posterior spinal ligaments, discs, and vertebrae passively support the upper body without active support from the spinal extensor muscles. Two previous studies reported that during short duration slumped sitting, FRP was observed in the thoracic and lumbar erector spinae muscles (35
). FRP has not been studied in persons with hyperkyphosis; however, it is possible that the constant slumped posture characteristic of hyperkyphosis could reduce muscle signaling of the lumbar spinal extensor muscles. Reduced muscle signaling could result in disuse and eventual fatty infiltration of the spinal extensor muscles. Additionally, sustained loads on the vertebrae in the elderly are known to cause progressive anterior wedge deformity (37
). These wedge deformities could also result in reduced muscle signaling and have a detrimental effect on spinal muscle density.
This study had several limitations. First, the participants in Health ABC were independent and high functioning at recruitment, and so, our results may not be generalizable to frailer men and women. However, prevalence of hyperkyphosis in the analytic sample was 21%. Second, vertebral fractures were not adjudicated in Health ABC, so we were unable to control for this potential confounder of muscle area and density. However, we did adjust for lumbar spine BMD, and results were unchanged. Third, although measuring kyphosis in the supine rather than standing position underestimates kyphosis angle by approximately 4 degrees (22
), likely resulting in misclassification of some participants with hyperkyphosis as normal, our results were robust in a sensitivity analysis defining hyperkyphosis as >36 rather than >40 degrees. Finally, we made inferences about the relationship of muscle composition in the lumbar region to kyphosis measurements made in the thoracic region. However, muscle composition was not measured in the thoracic region in Health ABC, and previous research has reported measurements of trunk extensor muscle strength, not thoracic muscle strength, in relationship to kyphosis (12
We found that lower spinal muscle density is associated with hyperkyphosis among healthy community-dwelling older men and women. This association between spinal muscle density and hyperkyphosis suggests that targeting this risk factor with strengthening exercises may potentially reduce risk for hyperkyphosis. Randomized trials are needed to determine whether an exercise program targeting spinal muscle density reduces hyperkyphosis and in turn improves adverse health outcomes.