Biomedical Engineering Reference
In-Depth Information
2. The effects of endurance training on human bone: results and limitations
Conventionally, the extrinsic parameters of bone, such as BMD, BMC, and size-related
measurements (
e.g.
, bone dimension, bone geometry), have been widely accepted as
indicators of bone strength as well as predictors of fracture risk. Unfortunately, endurance
running is usually considered an exercise mode that confers no benefit in terms of bone
mineral accretion.
1, 2
Moreover, distance runners reportedly have low BMD and are often
candidates for osteoporosis or stress fracture.
1, 2, 5-8
This section reviews studies on the effects
of distance running training on both BMD and bone metabolism.
2.1 Results of human studies
2.1.1 BMD and BMC in distance runners
For references to human studies, the NIH website (http://www.ncbi.nlm.nih.gov
/sites/entrez/) was searched by subject (adolescent, young adult runners) and research
type (cross-sectional studies). Additionally, the major purpose of this article is to describe
the long-term effects of ERT on bone in runners without concomitant health problems.
Hence, reports describing energy deficiencies and/or serious menstrual cycle disorders in
runners were excluded. The summary of previous cross-sectional studies in Table 1
indicates that distance runners usually reveal lower BMD and BMC values than those who
engage in higher impact sports.
9
According to Frost's theory,
10
the slenderer body
dimensions (Body Mass Index = 20 ~ 22) of runners who have a relatively lower body
weight (BW) might partially contribute to a lower BMD and BMC. However, when
compared to body-size matched control groups or another non-weight bearing exercise
group, runners still do not seem to have much advantage on whole body, lumbar spine or
regional cortical bone BMD.
11-18
Although oligomenorrhea or amenorrhea has been
considered the cause of low BMD in female runners, even healthy female runners with
normal menstrual cycles had lower BMD when compared to their size-matched control
subjects.
12-14
Thus, ERT is usually concluded to be profitless for bone mineral accretion and
bone health as well. However, if the analysis is limited to weight-bearing sites, runners do
reveal higher site-specific regional BMD and/or BMC (
e.g.
, femoral neck, distal tibia,
calcaneus) than do controls.
15, 19-21
Therefore, ERT is not entirely non-beneficial for bone
mineral accretion when considering BMD and/or BMC as the major predictors of bone
health.
Table 1 shows the findings of several studies indicating that distance runners have
absolutely higher BMD values than do control groups.
3, 22-24
In the research publicized by
Brahm and associates,
3
the runners showed only a slightly higher total-body BMD (3.6%
higher, p=0.03), and no significant difference from the control group in total-body BMC was
revealed. Interestingly, this study found that runners had distinctly higher BMD values in
the legs and in the proximal femora. Regarding subject specificity, the training level of
subjects or the normality of control subjects would be a major concern. Compared to elite
distance runners, high school or club level runners may be trained at a more moderate
intensity. Thus, these subjects did not really have typical body dimensions (e.g. slender
body shape, low BMI) of elite distance runners.
22, 24
On the other hand, the BMI of 20.7 in the
control group recruited in Kemmler
et al.
may have been too low for a normal control
group.
23
