Maintaining Femoral Bone Density from Daily Walking in Post-Menopausal Females:
How Many Steps per Day are Enough?
+1,2Boyer, K A; 3Kiratli, B J; 1,2Andriacchi, T P; 1,2Beaupre, G S
+1Bone & Joint RR&D Center, VAPAHCS, Palo Alto, CA; 2Mechanical Engineering, Stanford University, Stanford CA;
3
Spinal Cord Injury Center, VAPAHCS, Palo Alto, CA
[email protected]
INTRODUCTION
Osteoporosis is a major health concern for older adults. As bone
mineral density (BMD) is linked to skeletal loading and walking is the
most common weight-bearing activity in older adults, walking has been
extensively promoted for the prevention of osteoporosis. The amount
and intensity of walking to maintain a healthy skeleton, however, is
unknown and thus, in the context of skeletal health, evidence to support
a specific target of steps per day is lacking.
The Bone Density Index (BDI) [1, 2] offers an opportunity to
examine the influence of a daily walking regimen on bone health. With
this index, it is possible to investigate the effects and interactions of
body weight and habitual walking activity patterns, since the BDI
includes a weighting factor for the relative importance of the number of
loading cycles versus the magnitude of loading. The BDI has been
shown to correlate with calcaneus BMD in healthy women [1] and with
proximal femur BMD in a group of subjects with chronic stroke [2] .
The purpose of this study was to test for a correlation between the BDI
and BMD at the proximal femur in healthy, post-menopausal females.
This relationship between BDI and BMD was used to predict, for a
range of body weights and walking speeds, the number of steps per day
necessary to maintain a range of BMD values. These data were use to
test the hypothesis that 10,000 steps per day are sufficient to maintain a
healthy BMD for females with different body weights (BW).
METHODS
Gait mechanics, habitual walking activity and BMD were quantified
for 66 healthy post-menopausal females, (age 50 to 64 yrs). Subjects
provided informed consent prior to participation per an approved
Stanford University IRB protocol. Free-living steps per day and walking
speed were measured using an activity monitor (AMP231/331,
Dynastream Innovations, Canada) worn for five days. The loading
cycles for a given hip are one half of the total step count; therefore,
activity monitor totals were divided by 2 to obtain steps per leg per day.
Lower extremity kinematics and ground reaction forces (GRF) were
collected as subjects walked at slow, medium and fast speeds in a gait
laboratory. BMD was quantified for the total femur region of interest
using DXA (GE/Lunar iDXA). The T-scores (BMD normalized by the
young adult reference BMD value) were calculated by the scanner
software. A T-score ≥ -1 is considered normal, a T-score < 1 and > 2.5
indicates low bone density or osteopenia, and a T-score ≤ 2.5 indicates
osteoporosis.
The bone stimulus, BDI, was determined using a mathematical
model of bone density regulation [1]:
BDI  nsteps  (  GRF)m  2m ,
1
where m = 6 is an empirically determined constant; nsteps = average steps
per leg per day; = BW/mean BW of cohort. The GRF value was
estimated for each subject using the activity monitor walking speed
history and
a linear regression equation determined for walking speed
and GRF from the gait analysis. A linear regression model was used to
investigate the relationship between BDI and BMD. Using the
regression equation, the number of habitual daily steps was calculated
for different T-score values and different values of body weight ().
RESULTS
There was a significant correlation (p < 0.001; r2 = 0.19) between
BMD and BDI. (Fig. 1). The number of steps per leg per day in this
middle age female cohort ranged from 1,292 to 9,510, with average
free-living walking speeds ranging from 0.62 to 1.38 m/s.
Required step totals were calculated for the approximate range of
BMD values spanned by the regression shown in Fig. 1 assuming an
average free-living walking speed of 1.01 m/s. To maintain a T-score =
-1.00, an individual with the average weight from our cohort (mass =
65.1 kg, β = 1.00) would require 4,892 steps/day. Substantially more
steps per day are required for lighter individuals (18,568 steps/day for
mass 52.5 kg; β = 0.8) and fewer steps per day for a heavier individual,
(1,638 steps/day for mass 78.7 kg;  = 1.20) (Fig 2.)
Walking speed is one modifiable variable that could potentially be
used to increase the loading stimulus and decrease the number of steps
per day needed in an intervention. For the lower weight individuals
(BW 20% lower than average) only with an average walking speed
greater than 1.32 m/s is 10,000 steps per day sufficient to maintain a
T-score = - 1.0.
Fig. 1. Bone mineral density values for the total proximal femur
region of interest as a function of the Bone Density Index (BDI).
Fig. 2. Total steps per day required to maintain different target Tscores are given as a function of normalized body weight ().
DISCUSSION
The application of the BDI suggests a weight and speed
sensitivity of walking interventions for osteoporosis. In the context of
high risk persons (e.g., low body weight) the necessary steps per day
to maintain BMD can be substantially greater than the often-quoted
value of 10,000 steps per day [3] for most walking speeds and may be
an unreasonable goal for some individuals. Future research should
explore interventions to safely increase the GRF magnitudes during
everyday activities, which may provide significant benefit,
particularly for lower weight post-menopausal females, for healthy
skeletal aging.
ACKNOWLEDGMENTS
Funding: VA RR&D Service (grant A4067R). Data collection
assistance: C. Dairaghi, B. Elspas, J. Guerricabetia, and S. Williams.
REFERENCES
1. Bowley SM, Whalen RT. Orthop Res Soc, 26:63, 2001.
2. Worthen LC et al. J Rehab Res & Dev, 42:761-8, 2005.
3. Tudor-Locke C et al. MSSE, 40(7 Suppl):S537-43, 2008.
Poster No. 2288 • ORS 2011 Annual Meeting