Download Age Increases the Skeletal Versus Muscular Component of Lower

Journal of Gerontology: BIOLOGICAL SCIENCES
2000, Vol. 55A, No. 12, B593–B600
Copyright 2000 by The Gerontological Society of America
Age Increases the Skeletal Versus Muscular Component
of Lower Extremity Stiffness During Stepping Down
Paul DeVita and Tibor Hortobagyi
Biomechanics Laboratory, East Carolina University, Greenville, North Carolina.
Elderly adults step down with greater lower extremity stiffness than young adults. The purpose of this study was
to compare skeletal and muscular components of lower extremity stiffness between elderly and young adults during stepping down. Fourteen elderly (age, 70.1 years) and 16 young (age, 20.8 years) adults stepped down onto a
force plate from 10% and 20% body heights while being videotaped. Lower extremity stiffness was defined as the
ratio between the floor reaction force directed along the limb and limb compression. It was partitioned into skeletal and muscular components using the angular relationship (␾) between the direction of the force and the line
of the leg. Our results showed that ␾ was 21% smaller ( p ⬍ .03), the skeletal component was 48% larger ( p ⬍
.025), and the ratio of skeletal to muscular components was 32% larger ( p ⬍ .01) in elderly adults compared with
young adults. Elderly adults rely more on their skeletal and less on their muscular systems when stepping down
compared with young adults, producing a stiffer lower extremity.
F
LOOR contacts during daily locomotor movements apply forces onto the human musculoskeletal system that
tend to cause the person to collapse. Contact with a step during stair descent, for example, applies loads of approximately twice the bodyweight onto the foot; these loads are
transmitted upward through the skeletal system and tend to
cause dorsiflexion at the ankle and flexion at the knee and
hip joints (1). Initiating appropriate neuromuscular responses to these forces after the instant of floor contact will
be unsuccessful because they are often applied too rapidly
[i.e., in 100–150 ms (1–5)], particularly for elderly adults.
Elderly men and women have longer reaction times (6) and
neural modulation times (7), poorer joint proprioception (8),
and slower rates of torque development compared with
young adults (9–11). Thelen and colleagues (9), for example, showed that elderly women required 472 ms to reach a
functional level of 60 Nm of plantarflexor torque at the ankle while young women required only 311 ms to generate
the same torque. Since many adults—particularly elderly
adults—are not able to respond quickly enough after the application of ground reaction forces during locomotion, the
initial properties of their lower extremities at the onset of
the load are critical for successful movement.
Lower extremity stiffness, the intrinsic resistance of the
limb to compressive external loads, will therefore influence
how well a person locomotes (12–14). We recently reported
that age increased lower extremity stiffness with healthy
elderly adults using approximately 50% greater lower extremity stiffness while stepping down compared with
healthy young adults (15). Lower extremity stiffness can be
altered by changing the kinematics of the limb at touchdown and during the impact phase. Lower extremity stiffness is decreased with greater joint flexion, which alters the
direction of the ground reaction force vector in relation to
the limb segments and joints (14,16–21). Elderly adults
stepped down with 42% less ankle dorsiflexion and 57%
less knee flexion, maintaining a more erect posture com-
pared with young adults (15). The more erect posture
aligned the ground reaction force vector closer to the joint
centers, reducing external torques and the contribution of
the musculature to limb stiffness.
Lower extremity stiffness is thus a function of skeletal
and muscular components with stiffer limbs having a larger
skeletal component. If the knee is fully extended and the ankle dorsiflexed while descending stairs and a purely vertical
force is applied to the heel, the force passes directly along
the tibia and femur and through the centers of the ankle,
knee, and hip joints. Support would come entirely from the
skeletal system, and force from the musculature would not
be required (Figure 1). With greater knee flexion and ankle
plantarflexion, the skeletal segments are no longer vertical,
and the force vector would point across the foot, leg, and
thigh and away from the joint centers. In such a limb orientation the muscular component of limb stiffness increases
and the skeletal component decreases. The force applied to
the limb during stair descent also has a horizontal component (1) that decreases the force magnitude applied along
the longitudinal axis of the leg and thigh.
The influence of the skeletal system on lower extremity
stiffness has been investigated in young adults performing a
variety of movements (12–21). The influence of the musculature on upper and lower extremity stiffness has been reported for young adults. These studies have shown that
muscle stiffness is directly related to joint stiffness and to
the control of limb movements, limb length, and limb stiffness (17,22–26). McIntyre and colleagues (26), for example, investigated control strategies for maintaining stable
upper extremity postures under applied loads. Limb stiffness and limb stability were maintained by increasing joint
stiffness. Joint stiffness was increased by increasing muscle
force, which increased muscle stiffness. Grillner (24)
showed that gastrocnemius muscle stiffness can be used to
stabilize the length of the extremity during locomotion because of the stretch imposed on the muscle, which increases
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no subjects withdrew from the study because of an inability
to step down. All subjects provided written informed consent prior to testing.
Figure 1. If the knee is fully extended and the ankle is dorsiflexed
while descending stairs and a purely vertical force is applied to the
heel, the force passes directly along the tibia and femur and through
the centers of ankle, knee, and hip joints. Support would come entirely from the skeletal system, A. With greater knee flexion and ankle plantarflexion, the skeletal segments are no longer vertical, and
the force vector would point across the foot, leg, and thigh and away
from the joint centers. In such a limb orientiation the muscular component of limb stiffness increases and the skeletal component decreases, B.
muscle stiffness. Gottlieb (23) and Millner and colleagues
(25) showed that increasing cocontraction of muscle groups
on two sides of a skeletal joint increases joint stiffness and
therefore the stability of the extremity.
Little is known, however, about the combined contribution of each component to lower extremity stiffness during
daily activities in any population. The purpose of this study
was to compare the skeletal and muscular components of
lower extremity stiffness between elderly and young adults
during step-down movements. Along with the age-related
delays in neuromuscular function described above, most
elderly adults have lower muscle strength and power capabilities than young adults (27–31). Because of these declines in muscular capabilities with age, we hypothesize that
elderly adults will rely more on their skeletal system than
young adults and will therefore have a larger ratio of skeletal-to-muscular components of lower extremity stiffness
compared with young adults.
METHODS
Subjects
Fourteen elderly and 16 young adults volunteered. Elderly adults were, on average, 49.3 years older than young
adults (elderly mean ⫾ SD, 70.1 ⫾ 6.7 years; young mean ⫾
SD, 20.8 ⫾ 1.8 years). Mass and height were similar between the groups: Elderly adults averaged 69.9 ⫾ 16.1 kg
and 1.61 ⫾ 0.09 m, and young adults averaged 65.7 ⫾ 8.7
kg and 1.67 ⫾ 0.08 m in mass and height. All subjects were
apparently healthy, and elderly subjects provided a physician’s approval to participate in the study. The elderly subjects lived independently and reported that they performed
activities of daily living with little or no difficulty. We observed that the elderly subjects moved around the laboratory
with confidence and without balance-related problems. The
testing protocol was readily performed by all subjects, and
Experimental Set-up
One of two wooden platforms was positioned next to a
force plate (AMTI; model LG 6-4-1, Newton, MA). Heights
of the wooden platforms were 10% and 20% of the subject’s
standing height. The 10% height, which averaged 0.164 m,
was similar to a normal stairway step, and the 20% height
represented a relatively large step-down distance. The force
plate measured vertical and anteroposterior floor reaction
forces and the mediolateral force plate moment under the
forward stepping limb at 1000 Hz. Sagittal plane video recordings (SONY CCD-Iris camera, Tokyo, Japan, and JVC
VCR model HR S5100, Elmwood Park, NJ) were made of
the stepping-down movement at 60 Hz. The field of view
for the video image was approximately 1.5 m high and
2.5 m wide, which maximized the image size of the lower
extremity.
Testing Protocol
Subjects wore black spandex bike shorts, a tight fitting
t-shirt, and athletic shoes. Their height and mass were measured before the first trial. Reflective markers were placed
on the subjects’ right side on the lateral border of their fifth
metatarsal head, the heel of the shoe, lateral malleolus, lateral femoral epicondyle, and greater trochanter. All subjects
practiced the step-down movements until they were comfortable, after which five successful step-down trials were
recorded for each height. Each trial consisted of the subject
standing on the front edge of the platform, stepping down
with the right limb, and contacting the force plate with the
forefoot. The subject then lifted the left foot off the raised
platform and stepped forward past the force plate. The stepdown movements were generally easy to perform and did
not present difficult balancing problems. No subjects reported fatigue during the test session. The order of stepping
height tests was counterbalanced across subjects.
Data Analysis
Cartesian coordinates of the reflective markers were derived from the video records starting with the frame of
forefoot contact with the force plate until the frame of maximum knee flexion using the Peak5 system (Peak Performance Technologies, Englewood, CO). High-frequency error was removed from the digitized coordinates with an
automatic, low-pass digital filter using an average cut-off
frequency of about 7 Hz.
The lower extremity was modeled as a linear spring and
was defined as the line between the metatarsal head and the
hip joint (Figure 2) (15). Lower extremity stiffness was
measured during the impact phase of stepping down, which
was the period between initial contact with the force plate
and the occurrence of the maximum ground reaction force.
Stiffness was computed as the ratio of the maximum force
applied to the lower extremity during the impact phase
(Fmax) and the resultant shortening of the lower extremity
(Xmax) (12,13,17,18). Fmax was the component of the
ground reaction force vector applied at the metatarsal head
AGE AND LOWER EXTREMITY STIFFNESS
and directed along the line to the hip joint (Figure 2). Xmax
was calculated as the difference in the length of the distance
between the metatarsal head and hip from initial contact until the occurrence of Fmax. The center of pressure was used
to validate the application of the force to the metatarsal head
and indicated that the floor reaction force was applied within 1
cm of this position.
Lower extremity stiffness represents the combined stiffness of all tissues in the extremity but is primarily an assessment of the skeletal and muscular contributions to support
against collapse. Fmax tends to cause the extremity to flex
at one or more joints, and therefore the distance between the
hip joint and metatarsal heads shortens. The amount of
shortening will be proportional to the muscular stiffness
crossing the joint or joints (17,24) and to the skeletal kinematics in relation to the point of application and direction of
the applied force (14,16,17,19). A model was developed to
partition the observed stiffness into skeletal and muscular
components on the basis of the position of the lower extremity segments and the direction of Fmax. Skeletal and
muscular components of lower extremity stiffness were calculated as a function of the angle between Fmax and the leg
with
2
2
k = k cos φ + k sin φ,
(1)
where k ⫽ observed lower extremity stiffness, ␾ ⫽ the angle between Fmax and the leg, k cos2␾ ⫽ the skeletal component, and k sin2␾ ⫽ the muscular component (Figure 2).
A completely extended and vertically oriented extremity
impacting the floor after a vertical descent would have ␾ ⫽ 0
and all stiffness due to the skeletal component. The muscular component increases as knee flexion and ankle plantar-
Figure 2. Stepping down begins with floor contact in a relatively
erect position, A, and proceeds to a more flexed position with an applied force, B. Skeletal and muscular components of lower limb stiffness were calculated as a function of the angle between Fmax and the
leg with: k ⫽ k cos2␾ ⫹ k sin2␾ , where k ⫽ observed lower extremity
stiffness, ␾ ⫽ angle between Fmax and the leg, k cos2␾ ⫽ skeletal
component, and k sin2␾ ⫽ muscular component. A completely extended and vertically oriented limb impacting the floor after a vertical descent would have ␾ ⫽ 0 and all stiffness due to the skeletal component. The muscular component increases as knee flexion and ankle
plantarflexion increase, and the descent trajectory includes a horizontal component, making ␾ ⬎ 0.
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flexion increase, and the descent trajectory includes a horizontal component, making ␾ ⬎ 0. The relative contribution
of skeletal to muscular components was assessed by a ratio
of the two components (skeletal/muscular). Our pilot data
indicated the angles between the leg and force vector and
between the thigh and force vector were within a few degrees of each other. The femoral and tibial influences to the
skeletal component are therefore both accounted for in the
model.
Validity of Stiffness Calculation
Both groups had similar kinematics at initial contact with
the force plate. The mean lengths of the lower extremity for
young and elderly subjects across both step heights were
0.916 m and 0.915 m, respectively, at floor contact. The
length of the lower extremity was reduced during the impact
phase of the step-down movement because of ankle dorsiflexion and knee flexion (Figures 3 and 4). The extremity
shortened during the application of the ground reaction
force, and maximum shortening was attained at about the
same time as the occurrence of Fmax. The displacement and
force characteristics observed during the impact phase of
stepping down therefore were fundamentally identical to
those observed during the impact phases of hopping and
running. The assessment of lower extremity stiffness in
stepping down seems appropriate.
We compared our method of calculating stiffness with
that of Farley and Gonzalez (13) during both running and
step-down movements. Running analyses were based on
Laboratory data (n ⫽ 12). The results from the present
method were in close agreement with those from Farley and
Gonzalez. Lower extremity stiffness during running was
highly correlated with both stiffness coefficients predicted
by the Farley and Gonzalez method (r for kleg ⫽ 0.827, r for
kvert ⫽ 0.838). The slope of the regression line between our
method and kvert was close to unity (1.09), and the y-inter-
Figure 3. Ankle and knee joint positions during the impact phase
of stepping down from representative trials in the 20% height condition. Positive ankle position is plantarflexion and negative knee position is flexion. Young and elderly subjects had similar amounts of ankle plantarflexion and knee flexion as they contacted the force plate.
Young subjects dorsiflexed more at the ankle and flexed more at the
knee during a longer impact phase compared with elderly subjects.
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Figure 4. Ground reaction force (GRF) and the resultant shortening of the extremity during the impact phase from the trials in Figure
3. Fmax values were similar between age groups, but the elderly subjects reached Fmax in less time. Young subjects shortened their lower
extremity more during the impact phase, and maximum shortening
occurred at about the same time as Fmax, as indicated by the arrows.
Xmax was calculated as the amount of shortening between initial
contact (time ⫽ 0.0 s) and Fmax. Xmax values for these trials were
0.075 m for young subjects and 0.040 m for elderly subjects.
cept (⫺2.6 kN/m) was close to zero. The mean lower extremity stiffness for the present method (24.9 kN/m) was
within 2% of kvert from Farley and Gonzalez’s method (24.6
kN/m) over all trials.
We had to modify Farley and Gonzalez’s (13) methods to
calculate the center of mass displacement during the stepdown movement. Trials from five subjects were redigitized
using 15 body points from which the vertical position and
displacement of the body center of mass were calculated
during the impact phase. Present lower extremity stiffness
(mean, 21.9 kN/m) and kvert (mean, 19.9 kN/m) were within
7% on a trial-by-trial basis. The correlation coefficient between these methods was excellent (r ⫽ 0.982), the slope of
the regression line was good (1.24), and the y-intercept (–2.9
kN/m) was relatively close to zero. We conclude that the
present methods for calculating lower extremity stiffness
were reasonable and valid.
Statistical Analysis
Four variables, ␾, skeletal and muscular stiffness components, and the ratio of the components, were averaged
across the five trials for each subject and condition set of trials. The mean values were then entered as the subject scores
into a two-way analysis of variance (ANOVA). The two
factors were age (independent factor) and step height (repeated factor),and the alpha level was set at .05.
RESULTS
We previously reported (15) that lower extremity stiffness was 50% larger and that lower extremity compression
was 28% lower (both p ⫽ .0001) in the elderly adults compared with the young adults tested in this study (Table 1). In
contrast, the maximum force was only 3% larger in the elderly subjects and not significantly different. We also reported that step height had statistically significant effects on
Xmax and Fmax. Xmax was 27% larger and Fmax was
about 50% larger (both p ⬍ .0001) with the 20% step height
compared with the 10% height. The increased force was offset by the larger amount of limb shortening such that lower
extremity stiffness was not significantly different between
step heights.
The important variable in the assessment of skeletal and
muscular components of lower extremity stiffness was the
angle between the leg and Fmax, ␾. ANOVA revealed no
significant interaction effect for the angle ␾, but it did reveal significant main effects for age and step height (Figure
5). Angle ␾ was 16% smaller (F ⫽ 5.64, p ⬍ .03) in elderly
adults (16.0 ⫾ 3.0⬚) compared with young adults (19.0 ⫾
4.3⬚), and it was 29% larger (F ⫽ 65.9, p ⬍ .0001) in the
20% step height (20.0 ⫾ 3.5⬚) compared with the 10% (15.5 ⫾
2.8⬚) step height.
There were no significant interaction or step height effects for the skeletal component to lower extremity stiffness, but there was a significant age effect (Figure 6). The
skeletal component was 48% larger (F ⫽ 7.63, p ⬍ .025) in
elderly adults (30.8 ⫾ 13.6 kN/m) compared with young
adults (20.8 ⫾ 5.4 kN/m). There were no significant interaction or age effects for the muscular component to lower extremity stiffness, but there was a significant step height effect (Figure 7). The muscular component was 78% larger
(F ⫽ 52.9, p ⬍ .0001) in the 20% step (3.2 ⫾ 1.1 kN/m)
compared with the 10% step (1.8 ⫾ 0.6 kN/m).
There was no significant interaction effect for the ratio of
skeletal to muscular components of lower extremity stiffness, but there were significant age and step height effects
(Figure 8). The ratio was 32% larger (F ⫽ 8.64, p ⬍ .01) in
elderly adults (12.7 ⫾ 4.4) compared with young adults
(9.6 ⫾ 3.9), and it was 38% less (F ⫽ 60.1, p ⬍ .0001) in
the 20% step height (8.5 ⫾ 3.2) condition compared with
the 10% step height condition (13.8).
DISCUSSION
The present study identified the contribution of skeletal
and muscular components to total lower extremity stiffness.
The basic premise was that through the combined action of
skeletal and muscular components, the lower extremity resists the compressive effect or tendency of external forces.
Partitioning lower extremity stiffness into the two components provides a method to assess a person’s reliance on
passive skeletal and active muscular components to support
the body during locomotion. This assessment is novel and
has not been reported previously. Such an analytical method
may provide a tool for quantitatively assessing decline in
functional performance with aging or with a sedentary lifeTable 1. Mean (⫾SD) Lower Extremity Stiffness Values From
Reference (15)
Group
Elderly adults
Young adults
10% Height
20% Height
Stiffness (kN/m)
Xmax (m)
Fmax (N)
33.4 ⫾ 14.3*
22.4 ⫾ 6.2
26.5 ⫾ 10.4
28.0 ⫾ 13.3
0.041 ⫾ 0.015*
0.057 ⫾ 0.016
0.043 ⫾ 0.015**
0.055 ⫾ 0.017
1255 ⫾ 476
1219 ⫾ 302
972 ⫾283**
1454 ⫾ 325
*Elderly adults significantly different from young adults, p ⬍ .05.
**10% height significantly different from 20% height, p ⬍ .05.
AGE AND LOWER EXTREMITY STIFFNESS
Figure 5. Group means (SD) for the angle ␾ between Fmax vector
and the leg. ␾ was 21% smaller (p ⬍ .03) in elderly adults (n ⫽ 14)
compared with young adults (n ⫽ 16), and it was 29% larger (p ⬍
.0001) in the 20% step height compared with the 10% step height. The
interaction effect was not significant.
style and improvements in functional performance in the
elderly population with training or rehabilitation. It is well
established that stair descent is difficult for some elderly
adults and that many accidents occur during this activity
(32,33). It is also well known that resistance training increases muscular strength in elderly adults (34–36) but that
strength improvements may or may not lead to improvements in functional tasks (34,35,37–39). The application of
the current methods to stair descent and other movements
may provide an assessment of the mechanism for any
improved or unaffected performance by identifying the
amount of increase or lack of increase in the muscular component to limb support. We propose that the optimal ratio of
skeletal and muscular components enables safe, successful
movement while an inappropriate ratio creates balance
problems and excessive stress on the skeletal or muscular
systems. This ratio has yet to be determined.
Elderly adults clearly relied more on their skeletal system
to support themselves during stepping down than did young
adults. The skeletal component to lower extremity stiffness
was nearly 1.5 times larger, and the ratio of skeletal to muscular components was about 1.3 times larger in elderly
Figure 6. Group means (SD) for the skeletal component of lower
extremity stiffness. Skeletal component was 48% larger (p ⬍ .025) in
elderly adults (n ⫽ 14) compared with young adults (n ⫽ 16). Step
height had no effect on the skeletal component, and the interaction
effect was not significant.
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Figure 7. Group means (SD) for the muscular component of
lower extremity stiffness. The muscular component was 74% larger
( p ⬍ .0001) in the 20% step compared with the 10% step (n ⫽ 30)
while age had no effect on the muscular component. The interaction
effect was not significant.
adults compared with young adults. This ratio identifies the
relative functional dependence each group had on their skeletal and muscular systems. Elderly adults increased the
skeletal component by keeping Fmax closer to or more in
line with the leg. They performed stepping down in a more
erect posture. This posture shortened the moment arm for
Fmax at the knee and therefore reduced the flexor torque
that tended to cause collapse (13,15,16,18,21). This strategy
enabled the elderly adults to step down with a smaller muscular contribution to lower extremity stiffness. The observed age-related adaptations are thought to have been
previously learned and used during daily step-down movements and are not due solely to the step-down task in the
study.
Elderly adults probably selected this strategy for several
reasons. First, the elderly subjects had lower muscle
strength compared with the young subjects. These subjects
were tested isokinetically as part of another study, and the
peak concentric and eccentric torques were 47% and 23%
Figure 8. Group means (SD) for the ratio of skeletal-to-muscular
components of lower extremity stiffness. The ratio was 32% larger (p ⬍
.01) in elderly adults (n ⫽ 14) compared with young adults (n ⫽ 16),
and it was 38% less (p ⬍ .0001) in the 20% step height condition compared with the 10% step height condition. Elderly adults had a
greater relative reliance on their skeletal system compared with their
musculature for lower extremity stiffness during stepping down compared with young adults. The interaction effect was not significant.
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lower in the elderly group (unpublished data). Along with
this difference, present elderly subjects probably had lower
muscle power (40–42) and a lower rate of muscle force and
torque production (9–11) compared with the young subjects. The stiffer movement strategy reduced the load on the
muscular component and therefore reduced the possibility
of collapse in the elderly subjects. Second, stepping down
requires mostly eccentric muscle contraction to control
and stop the downward motion of the body (1). Elderly
adults have been shown to retain eccentric muscle function
more so than concentric function [present subjects and
(10,43,44)]. Porter and colleagues (10), in fact, showed only
a 3% reduction in peak eccentric plantarflexor torque in a
group of 67-year-old subjects compared with a group of 27year-old subjects. Although elderly adults may have reasonable eccentric muscle capabilities, the present data suggest
that elderly subjects may not fully employ them in functional activities. The increased limb stiffness may be the result of the aged neuromuscular system’s attempt to reduce
the range of flexion and dorsiflexion in order to resolve the
control complexity of eccentric contractions by the extensor
and plantarflexor muscles (15,45).
Third, elderly adults may have selected this movement
strategy to reduce compressive forces inside the knee joint
even though these subjects did not have histories of knee
joint degeneration. Most of the compressive, bone-on-bone
force inside lower extremity joints is due to the force developed by muscular contraction (46–48). The elderly subjects
may perceive greater comfort and less joint stress with the
stiffer strategy and a greater reliance on skeletal rather than
muscular components to lower extremity stiffness. Finally,
elderly adults may select a stiffer, more skeletal-oriented
strategy to reduce the contractile effort of the muscles and
the development of muscle fatigue. Alexander (21) demonstrated that humans and other animals would perform less
mechanical work and expend less metabolic energy if they
were to keep the ground reaction force vector closer to the
leg and hip and if they were to walk erect with little joint
flexion. DeVita and Skelly (16) demonstrated that joint
torques generated by the musculature do less work in stiffer,
more erect landings than in less stiff landings with more
joint flexion. We do not mean that elderly subjects became
fatigued during the present tests but that they used a previously learned strategy in this study.
One negative consequence of these age-related adaptations is that they may increase the incidence of falling accidents during stepping down or while descending stairs. The
reduced reliance on muscles may provide safer movements
for elderly adults in well-performed step-down movements.
However, in a single movement that is performed poorly,
the stiffer, skeletal-oriented strategy may reduce the ability
of the person to take corrective actions before falling. For
example, a single step may be lower than others in the stairway and cause a larger-than-normal and off-balance impact
onto the person. Stepping down with a larger muscular component may enable the person to absorb the unexpected impact successfully (16,19,20), whereas a person using a
greater skeletal component may be forced off balance because impacts to stiffer lower extremities occur in shorter
times compared with impacts to less stiff limbs (13,14,18).
Indeed, the elderly subjects performed the movement from
initial contact to maximum force in 18% less time (p ⬍ .05)
compared with the young subjects (elderly, 0.095 ⫾ 0.024 s;
young, 0.116 ⫾ .027 s), showing that the elderly subjects
have less time to adapt to any unexpected forces.
We previously reported that step height had no significant effect on lower extremity stiffness (15). Step height did
affect the biomechanics of stepping down, however, and the
components of lower extremity stiffness. Stepping down
from the taller step caused the subjects to flex 45% more at
the knee (10%: 18.2 ⫾ 4.5° vs 20%: 26.4 ⫾ 5.8°; F ⫽ 109,
p ⬍ .0001) to compensate for the 50% increase (F ⫽ 95.8,
p ⬍ .0001) in Fmax (15). This altered strategy increased the
muscular component 74% and decreased the skeletal-tomuscular ratio 38% in the taller step. Young and elderly humans respond to increased impact forces in stepping down
by altering their lower extremity kinematics and by increasing the muscular contribution to lower extremity stiffness
and support. Stepping down from taller steps, therefore, can
be problematic for elderly adults because they rely more on
their skeletal component. The results suggest that stairway
design can be improved for elderly adults if it incorporates
lower step heights compared with standard heights used in
homes and buildings.
The present analytical approach is new, and its assumptions warrant discussion. It is generally accepted that muscles, and not other passive tissues, are mainly responsible
for the generation of net muscle moments around joints.
This assumption has been accepted in gait and other fullbody movement studies (1,16,49,50), in isokinetic tests of
joint torques (51), in motor control studies on joint stiffness
(25), and in studies on muscle stiffness and its relationship
to joint torques (22,24). Vrahas and colleagues (50) directly
investigated the contribution of various tissues to torque
generation at the hip joint. They reported that passive structures such as joint capsules and ligaments contribute a small
portion to the total torque, usually less than 10%, when
compared with the torque produced by the muscular system.
Because the muscles are the main torque producers that
control joint rotations and the overall orientation of the
lower extremity, it is reasonable to propose that the muscular component contributes to lower extremity stiffness.
The skeletal component to lower extremity stiffness has
been indirectly demonstrated in several studies (12–17,19).
Each of these studies identified the relationship between
lower extremity stiffness and the kinematics of the skeletal
system during movement. The fundamental result was that
as joint flexion decreased, lower extremity stiffness increased. Farley and colleagues (17) demonstrated this relationship in hopping as did DeVita and Skelly (16) in landing
from a jump. Lower extremity stiffness was increased by reducing the amount of knee flexion and by reducing the moment arm of Fmax. This kinematic adjustment brought the
force vector more parallel to the leg, shortening its moment
arm and reducing the need for muscular contribution. This
adjustment effectively increased the skeletal component to
lower extremity stiffness. If the knee joint were completely
extended, the muscular component would be eliminated,
and the skeletal component would be maximized. However,
this strategy is not used in normal human movement be-
AGE AND LOWER EXTREMITY STIFFNESS
cause the skeleton would reach yield strength and fail (fracture). As knee flexion increases and the skeletal segments
are no longer vertically positioned, the muscular component
will be increased and the skeletal component will be decreased with a vertically applied force. On the basis of the
biomechanics literature concerning many types of impacts
to the lower extremities, it seems reasonable to attribute
lower extremity stiffness to skeletal and muscular components.
11.
12.
13.
14.
15.
Conclusions
We previously reported that lower extremity stiffness
was 50% greater in elderly adults compared with young
adults (15). The present results identify one mechanism
used by elderly adults to increase their lower extremity stiffness. Changes in lower extremity stiffness with age were
due to a shift in the relative contribution of the components
of lower extremity stiffness. Elderly adults increased their
dependence on the skeletal system and reduced their dependence on the muscular system leading to a greater skeletalto-muscular ratio. This strategy was achieved by decreasing
the amount of flexion in the joints and by reducing the angle
between the maximum force vector and the leg. The results
supported the hypothesis that elderly adults will have
greater relative skeletal and lower relative muscular components of lower extremity stiffness compared with young
adults. The age-related adaptations allowed the elderly subjects to successfully step down from a height used in stairways and from a taller height despite having reduced neuromuscular capabilities compared with young adults.
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Acknowledgments
This work was supported in part by Research and Creative Activity
grants from East Carolina University (P.D. and T.H.), a North Carolina Institute on Aging grant (P.D.), and a National Institute of Child Health and
Human Development Grant 30422 (T.H.). We thank Mr. Jeff Money and
Mr. Jason Barrier for their work in the data collection and analysis portions
of the study. We also thank Dr. Robert Hickner and Dr. Kevin O’Brien for
their constructive comments on the procedures used in this work.
Address correspondence to Paul DeVita, PhD, Department of Exercise
and Sport Science, East Carolina University, Greenville, NC 27858. E-mail:
[email protected]
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Received March 12, 1999
Accepted June 9, 2000
Decision Editor: Jay Roberts, PhD