APPLIED SCIENCES
Biodynamics
Knee Biomechanics of Alternate Stair
Ambulation Patterns
SAMANTHA M. REID1, SCOTT K. LYNN1, REILLY P. MUSSELMAN1, and PATRICK A. COSTIGAN1,2
1
School of Kinesiology and Health Studies and 2School of Rehabilitation Therapy, Queen_s University, Kingston, CANADA
ABSTRACT
REID S. M., S. K. LYNN, R. P. MUSSELMAN, and P. A. COSTIGAN. Knee Biomechanics of Alternate Stair Ambulation Patterns.
Med. Sci. Sports Exerc., Vol. 39, No. 11, pp. 2005–2011, 2007. Purpose: This study compared the kinematics and kinetics of the knee
joint during traditional step-over-step (SOS) and compensatory step-by-step lead-leg (SBSL) and trail-leg (SBST) stair ambulation
patterns. Methods: Seventeen (M:9) healthy adults completed five trials of ascent and descent using three different stepping patterns: 1)
SBSL, 2) SBST, and 3) SOS. Kinematics and kinetics were collected with an optoelectronic motion-tracking system and a force plate
embedded into a four-step staircase. An inverse-dynamics link-segment model (QGAIT system) was used to calculate the net joint
kinetics. Results: During stair ascent, different peak anteroposterior (AP) forces were observed across all three stepping patterns (SOS 9
SBSL 9 SBST, P G 0.05). During ascent, the flexion moments of SOS (0.96 NImIkgj1) and SBSL (0.97 NImIkgj1) patterns were
similar and much larger than the SBST moments (0.14 NImIkgj1). In the descent conditions, the initial AP peak force for SOS was
larger than that of SBSL and SBST. However, the second peak force for SOS (4.92 NIkgj1) and SBST (4.68 NIkgj1) were larger than
SBSL (1.57 NIkgj1). During descent, the initial peak flexion moment for the SOS pattern was larger than SBSL and SBST, whereas
during the second peak, SOS (1.05 NImIkgj1) and SBST (1.11 NImIkgj1) were no different and larger than SBSL (0.18 NImIkgj1).
Conclusion: Overall, SBSL during ascent and SBST during descent had the highest loads. These results increase our understanding
of alternative stepping patterns and have important clinical (reduction of loading on injured/diseased leg) and rehabilitation implications. Key Words: STEP-BY-STEP, STEP-OVER-STEP, STAIRS, LOWER-LIMB KINETICS
B
Generally, healthy individuals use a traditional step-overstep (SOS) gait pattern during stair ambulation; however,
patients, older adults, and disabled populations may be
forced to adjust their stair gait pattern because of decrements in muscular strength (8,15,24), decreases in proprioceptive acuity (8), and altered balance mechanisms (15,24)
associated with age and disease (18,22). Therefore, those
populations with decrements in motor function often adopt
alternate gait patterns, such as increased handrail use,
sideways motion, or a step-by-step (SBS) pattern (placing
both feet on the same step before ascending or descending)
that deviate from the traditional SOS gait pattern (21,22).
These deviations in stair gait patterns result in higher energy
costs, lower efficiency, and an increased risk of falling,
particularly during stair descent (21–23) and SBS gait
pattern.
The majority of stair biomechanics research has focused
on stair ambulation as a tool for defining performance limits
during rehabilitation (14), to assess functional abilities (3),
or after surgical interventions such as knee replacements
(2,7,11). At present, no literature provides an in-depth
description of the biomechanics of alternate stair ambulation gait patterns. Much of the functional impairment during
ecause stair climbing is a common activity of daily
living, the ability to do it efficiently is important to
an individual_s quality of life (16). More demanding
than level walking, stair ambulation is performed with ease
by healthy individuals; however, it is more difficult to
perform for those with decrements in motor function,
balance problems, or reduced lower-limb function. The
difficulty with stair climbing is attributable to increased
muscular demands, which are reflected in larger forces (4),
angles (1), powers (14,19), moments (4,12,14), and ranges
of motion (ROM) (16), and these increased demands occur
consistently at the knee joint.
Address for correspondence: Patrick A. Costigan, Ph.D., Physical Education
Centre, Biomechanics Laboratory, Queen_s University, K7L 3N6; E-mail:
Pat.Costigan@queensu.ca.
Submitted for publication March 2007.
Accepted for publication June 2007.
0195-9131/07/3911-2005/0
MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ
Copyright Ó 2007 by the American College of Sports Medicine
DOI: 10.1249/mss.0b013e31814538c8
2005
Copyright @ 2007 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
stair negotiation is directly related to the knee (1,4,14,
19,20) and is manifest in healthy people as they age and
in those who live with knee osteoarthritis. Exploration of the
effects of alternate stair ambulation patterns on the knee joint
biomechanics is warranted to enhance gait rehabilitation
strategies and for comparison of knee mechanics of different
strategies. In evaluating stair climbing ability, it is important
to consider the method used to negotiate the stairs. What was
the stepping speed? Was a handrail used? Was the pattern the
typical SOS pattern or the modified SBS pattern? These
different stepping methods alter the mechanics of stair
stepping, and the way the mechanics are altered must be
known if we want to understand the additional compensations
that populations, such as older adults or those with knee
osteoarthritis, use in negotiating stairs. We need to be able
to discern whether differences from ‘‘normal’’ stepping
are attributable solely to changes in the ambulation pattern,
or whether it is possible that additional compensations are
at work.
APPLIED SCIENCES
METHODS
Seventeen (nine male and eight female) normal, healthy
adults without history or complaint of chronic pain, major
injury, or surgery to the lower limbs or back participated in
the study. Participants were 18–35 yr old (mean = 23.71,
SD = 2.37), and mean height and mass were 185.1 cm
(SD 11.7) and 82.1 kg (SD 14.3), respectively. The
university research ethics board approved the procedures,
and all participants provided informed written consent.
Motion was tracked with an Optotrak 3020 threedimensional tracking system (Northern, Digital, Waterloo,
Canada) while a force plate (AMTI, Newton, MA) measured
the ground-reaction forces. The stairs were of standard
dimensions, with each step being 15 cm high and 26 cm
deep, with a 56-cm tread (Fig. 1). The force plate was
mounted on concrete blocks and formed the middle of the
second step in the specially constructed four-step staircase.
Because the force plate (shaded area of the second step in
Fig. 1) was not attached to the staircase, it was possible for
subjects to step onto the stair beside the force plate while
the test leg stepped onto the force plate.
Radiographs. Before data collection, each participant
had three standardized radiographs taken of their test leg
(3). The radiographs included a frontal view of the knee and
hip and a sagittal view of the knee. During the procedure,
the participants stood on a rotatable platform in a test frame
behind two accurately spaced Plexiglas sheets. The
Plexiglas sheets in front of the lower limb contained lead
beads embedded at precise locations. The lead beads were
imaged along with the limb; because the configuration of
the beads was known, image magnification and distortion
could be corrected. The participant was positioned so that
the lateral view approximated the knee flexion plane, and,
by rotating the turntable 90- so the participant need not
move, the frontal view was orthogonal to the lateral view.
2006
Official Journal of the American College of Sports Medicine
FIGURE 1—Schematic drawing of the four-step staircase, with the
force plate mounted on concrete blocks to form the second step. The
stairs were of standard dimensions, with each step being 15 cm high
and 26 cm deep, with a tread of 56 cm. The force plate (shaded areas)
was not attached to the staircase; it was possible for subjects to step
onto the stair beside the force plate while the test leg stepped onto the
force plate.
Virtual markers. All radiographs were taken with lead
beads taped to the lateral aspect of the leg where the motion collection markers would be placed later. These beads
were imaged along with the knee_s internal structures and
were attached at the greater trochanter, lateral femoral
epicondyle, and the head of the fibula. Distances between
the imaged lead beads and specific bone landmarks were
measured and corrected using calibration information
derived from the radiograph procedure. This allowed
specific internal bone landmarks to be located with respect
to the local limb coordinate systems developed from the
surface markers. The specific internal landmarks were the
centre of the femoral head, the midpoint of the distal femur,
the midpoint of the proximal tibia, and the midpoint of the
ankle malleoli.
Gait data. During the stair ascent and descent trials, the
participants had four infrared markers (IRED) affixed to the
lateral aspect of their test leg at the same locations as those
of the lead beads used in the x-rays (4). In addition, a single
IRED was attached to a forward-projecting probe strapped
securely to the midthigh, and another was strapped to the
upper tibia just below the tubercle (4).
After marker preparation, each participant completed five
trials of ascent and descent using three different stepping
patterns: 1) SBS, leading with the test leg (lead leg) (SBSL),
with placement of both feet on the same step, examining the
leading leg; 2) SBS, leading with the nontest leg (trail leg)
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coordinate system. The computed forces and moments are
the net external forces and moments, not their internal
counterparts. For example, a positive flexion moment
represents a net external flexor moment at the knee, which
is resisted by the knee extensors.
Data analysis. The QGAIT system calculated threedimensional knee angles, net forces, and net moments at
the knee, as well as standard time–distance parameters such
as stride length, time, velocity, and cadence. For each
participant and each stepping pattern, the knee force and
moment curves for the five trials were normalized to body
weight and 100% of the cycle, and then they were averaged
to give a single representative trial for each condition.
Specific curve locations where forces and moments were
maximal, as well as the angular range of motion, were
selected for analysis and were compared between stepping
patterns separately for ascent and descent, using one-way
analysis of variance. Ascent and descent conditions were
compared with independent t-tests.
RESULTS
FIGURE 2—A schematic illustrating the gait cycles of (A) step-overstep (normal reciprocal stepping pattern) and (B) step-by-step stepping
(placement of both feet on the same step before ascending or
descending) patterns analyzed during stair ambulation. Step-over-step
dotted leg is the lead leg, and step-by-step dotted leg is the trail leg.
ALTERNATE STAIR KNEE BIOMECHANICS
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APPLIED SCIENCES
(SBST), with placement of both feet on the same step,
examining the trailing leg; and 3) SOS, normal reciprocal stair
stepping (Fig. 2). To ascend the stairs, participants stood in
front of the staircase and took an initial step on level ground;
their next step was onto the staircase. To descend, participants
took an initial step on top platform before stepping onto the
staircase (Fig. 1). The gait cycle was defined from toe-off of
the test leg to subsequent toe-off of same leg, a swing-stance
sequence with the stance phase on the force plate step. Final
toe-off was determined using the force plate data, whereas the
initial toe-off used an algorithm that matched the motion at
initial toe-off to that of final toe-off.
Within the ascent and descent conditions, the order of the
stepping patterns trials was randomized. Participants wore
comfortable shoes and walked at their naturally chosen pace
for all conditions. Motion and force plate data were sampled
synchronously at 100 Hz.
Data processing. The Queen_s Gait Analysis ThreeDimensional (QGAIT) system protocol, described and
validated previously (5,6,9), was used to process both the
kinematic and kinetic data. QGAIT used information from
the standardized radiographs, participant-specific anthropometrics, and kinematic and kinetic data to calculate threedimensional angles, net forces, and net reaction moments at
the knee. The floating axis method (9) calculated threedimensional knee angles, and a standard link-segment
model calculated the three-dimensional net forces and net
reaction moments at the knee. The forces and moments
were defined using the right-hand rule in the tibia local
Time–distance parameters. Because the length of the
stair run was fixed, the step length for the traditional SOS
pattern (two stairs) was longer than that of either SBS pattern.
In addition, because the time to complete a step was not
different between conditions, the walking velocity for the SOS
conditions was faster than for the SBS condition (Table 1).
Stair ascent. For stair ascent, there were differences in
the sagittal-plane knee angle, net anteroposterior (AP) force,
and net flexion moment across conditions (Fig. 3). For the
knee flexion angle, both the peak and ROM were different
across all three stepping conditions (Table 1). Visual
examination suggested that the SOS condition was more
similar to the SBSL condition than to the SBST condition.
Similar statistical differences and visual similarities were
seen in the AP knee force results. The AP force peak at
50% of the gait cycle was different across conditions, but,
again, visual examination showed that the SOS and SBSL
conditions were more similar to each other than to the
SBST condition.
The flexion moment curves were analyzed using the
distinctive peak at the beginning of the stance phase. Here,
the results show that the SOS (0.96 NImIkgj1) and SBSL
(0.97 NImIkgj1) conditions were similar, whereas the SBST
(0.14 NImIkgj1) peak was smaller. Unlike angles, forces,
and moments in the sagittal plane, there were no differences
in the frontal-plane angles, forces, or moments across
conditions (Fig. 4).
During stair ascent, the majority of the muscle power
absorbed and generated at the knee joint occurs in the
sagittal plane (Table 1 and Figs. 3 and 4), and a large
generation peak occurred during the swing phase for both
SOS and SBSL (Figs. 3 and 4); however, the magnitudes of
these peaks were not significantly different (P 9 0.05). This
peak is absent for SBS trail leg and exhibits no apparent
TABLE 1. Peak knee joint flexion angles, forces, moments, stepping velocity, and stride length for both step-over-step (SOS) and step-by-step (SBS) ascent and descent stepping
patterns.
Ascent
SOS
SBSL
SBST
Descent
SOS
SBSL
SBST
Stride Length
(m)
Velocity
(mIsj1)
0.62 (0.03)a
0.29 (0.01)b
0.30 (0.01)b
0.61 (0.02)a
0.29 (0.01)b
0.29 (0.01)b
Anteroposterior Force
(NIkgj1)
Flexion Moment
(NImIkgj1)
Ab/Adduction Moment
(NImIkgj1)
Power (W)
Flexion Angle
Peaks (-)
Peak 1
Peak 2
Peak 1
Peak 2
Peak 1
Peak 2
0.46 (0.06)a
0.22 (0.03)b
0.21 (0.03)b
j91.6 (51.7)a
j101.5 (26.0)a
j5.2 (4.9)b
83.5 (4.9)a
78.8 (5.0)b
57.3 (6.6)c
4.45 (0.59)a
3.92 (0.55)b
1.85 (0.51)c
2.02 (0.59)a
1.49 (0.37)b
1.88 (0.58)b
0.96 (0.13)a
0.97 (0.20)a
0.14 (0.13)b
N/A
N/A
N/A
0.25 (0.19)
0.29 (0.14)
0.30 (0.11)
0.17 (0.11)
0.20 (0.09)
0.20 (0.08)
0.49 (0.07)a
0.23 (0.03)b
0.24 (0.04)b
177.0 (40.8)a
16.6 (17.6)b
149.9 (28.9)c
83.3 (6.1)a
43.4 (7.1)b
73.7 (5.2)c
2.78 (0.63)a
2.04 (0.53)b
1.96 (0.37)b
4.92 (0.79)a
1.57 (0.40)b
4.68 (0.81)a
0.50 (0.17)a
0.27 (0.23)b
0.34 (0.17)b
1.50 (0.24)a
0.18 (0.17)b
1.11 (0.23)b
0.34 (0.10)
0.33 (0.10)
0.32 (0.10)
0.29 (0.12)
0.25 (0.10)
0.29 (0.10)
All numbers are presented as means (SD). Values with similar superscripts are not significantly different at P G 0.05 according to Duncan_s post hoc test.
SBSL, lead leg of step-by-step; SBST, trail leg of step-by-step.
and, visually, the curve profiles of the SOS and SBST
conditions were more similar to each other than to the curve
profile of SBSL condition (Fig. 3).
Knee AP force had two peaks. The first peak was larger
in the SOS (2.78 NIkgj1) condition than in the two SBS
(SBSL, 2.04 NIkgj1; SBST, 1.96 NIkgj1) conditions. For
the second peak, the SOS (4.92 NIkgj1) and SBST (4.68
NIkgj1) values were not different, whereas the SBSL (1.57
NIkgj1) peak was smaller than the other two. During the
second half of the stance phase, the SOS and SBST curve
profiles were more similar to each other than to the SBSL
curve profile.
For the knee flexion moment, there were also two main
peaks. The first peak was larger in the SOS (0.50 NImIkgj1)
condition than in the two SBS (SBSL, 0.18 NImIkgj1;
APPLIED SCIENCES
power generation or absorption phase throughout stance,
indicating that there is a ‘‘working’’ and ‘‘supporting’’ limb
during the SBS gait pattern and that these roles do not
change, unlike in the case of the SOS pattern, where both
limbs perform the same functions at different times.
Stair descent. As was the case with the stair ascent
results, the sagittal-plane knee angles, net AP forces, and
net flexion moments were different across conditions,
whereas the adduction moment did not differ across the
stepping conditions (Table 1 and Figs. 3 and 4). Knee
angles during stair descent were analyzed using the
maximal flexion angle and the total ROM. The three gait
patterns were different from each other on both of these
measures. The results revealed that the SOS and SBST were
larger than the SBSL for both peak and total ROM angles,
FIGURE 3—Sagittal-plane curve profiles of step-over-step and step-by-step stepping patterns during stair ascent and descent. Forces and moments
are defined using the right-hand rule in the tibia local coordinate system. Positive powers indicate concentric phases; negative powers indicate
eccentric phases. The gait cycle begins with toe-off.
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FIGURE 4—Frontal-plane curve profiles of step-over-step and step-by-step stepping patterns during stair ascent and descent. Forces and moments
are defined using the right-hand rule in the tibia local coordinate system. Positive powers indicate concentric phases, and negative powers indicate
eccentric phases. The gait cycle begins with toe-off.
ALTERNATE STAIR KNEE BIOMECHANICS
controlling the lowering of the body. The lead leg during
ascent and the trail leg during descent were the working
limbs in that they experienced high AP forces, powers, and
flexion moments, similar in magnitudes to those measured
in the SOS ascent and descent patterns. Conversely, the
support limbs were the trail leg during ascent and the lead
leg during descent, because the AP forces, powers, and
moments were lower than either the working limbs in the
SBS pattern or the SOS pattern. The SBS working limbs
during both ascent and descent were similar to the SOS
profiles (Fig. 3).
DISCUSSION
Simply on the basis of the range of motion that you
travel, you would expect that the traditional SOS stair
ambulation pattern would be very different from the
compensatory SBS pattern. Indeed, the step length is longer
for the SOS pattern and results in an increased stride
velocity. The resultant stride length and stride velocity
during ascent and descent were comparable with values
reported by Nadeau et al. (16).
During the SOS pattern, each limb performs the same
function at different times. As with previous investigations
(14,19), initiation of the SOS stance-phase ascent in our
study produced large sagittal-plane power-generation peaks,
suggesting that the knee of the lead leg is responsible for
pulling the body up from the lower step. Once the trail leg
clears the stair, it takes over as the lead leg and continues
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APPLIED SCIENCES
SBST, 1.11 NImIkgj1) conditions. For the second peak, the
SOS (1.05 NImIkgj1) and SBST (1.11 NImIkgj1) were not
different, whereas the SBSL (0.18 NImIkgj1) peak was
smaller. Again, the SOS and SBST condition curve profiles
were more similar to each other than to the SBSL curve
profile.
As in the case of the stair ascent, during stair descent the
majority of power absorbed and generated at the knee joint
occurs in the sagittal plane. During descent, a large
absorption peak occurs in late stance for both SOS and
SBST (Table 1 and Figs. 3 and 4). However, the lead leg of
SBS exhibits no absorption or generation peak throughout
the entire stance phase. Additionally, the absorption peak
for SOS is significantly greater than SBST (P G 0.05).
Again, similar to stair ascent, the working and supporting
limbs during the SBS gait pattern have similar roles, in
contrast to the SOS pattern, where both limbs perform the
same functions at different times.
Unlike the SOS pattern, during the SBS pattern each leg
(lead or trail) performs a different function. During SBS
ascent, the trail leg provides support, indicated by the lack
of sagittal-plane power generation, whereas during stance
the lead leg raises the body up to the next step. These
functions are the same as those performed during SOS
ambulation, with the difference being that in the SBS
ambulation the legs do not change roles; the trail leg
remains the trail leg, and the lead leg remains the lead leg.
This happens in SBS descent as well. In SBS descent, the
lead leg provides support and the trail leg is responsible for
APPLIED SCIENCES
through to the next step. During the SOS ascent gait cycle,
there are no other power-generation phases at the knee, so
the movement of the contralateral limb to the subsequent
step is the responsibility of the ankle and the contralateral
hip flexors (14,19). During SOS descent, the knee_s
sagittal-plane power profiles suggest that the lowering of
the body is controlled by the trail leg while the lead leg
reaches for the lower step. Once on the lower step, the lead
leg becomes the trail leg, and the next cycle begins. Clearly,
this is not the case with the SBS pattern.
SOS ascent peak flexion moments were similar to those
reported in other investigations (4,12), and the SBS working limb had similar values to SOS during both ascent and
descent. Supporting limb flexion moments were lower by
comparison with the working limb. The reduced flexion
moments seen in the SBS support limb could have clinical
implications, because individuals with chronic knee pain
typically demonstrate reduced peak flexion moments during
stair ambulation (10,20,17).
It is likely that patients with knee osteoarthritis and
patellofemoral pain adapt their gait to minimize knee flexion moments to reduce joint reaction forces at the knee joint
(10,20), and results from this study suggest that knee pain
may be reduced during stair ascent by using the more
painful leg as the support or trail leg. Furthermore, the
literature has shown that effective stair climbing requires
approximately 1.0 NImIkgj1 of extensor muscle moment
(16), whereas in this study, SBS peak flexion moment of the
support limb was less than 0.3 NImIkgj1, a much greater
reduction. Thus, one potential reason for using the SBS
pattern is the increased difficulty in stair ambulation
associated with decrements in motor function (i.e., muscular
strength, proprioception). The SBS pattern allows for a
shorter and slower stride, which may increase stability and
compensate for lower-limb weaknesses. However, this must
be taken with the realization that individuals using this gait
pattern will have to take twice as many strides to cover the
same distance.
The most interesting feature seen by examining the joint
angle, force, and moment curve profiles in this study is that
the profiles for the working legs are very similar whether
you consider ascent or descent. Although there are some
statistical differences, the curves are surprisingly similar at
the time of maximum load. In hindsight, this might seem
obvious, because it does not matter whether you use the
traditional stepping pattern or a modified stepping pattern:
you must still move your body to the next step, which,
regardless of the stepping pattern, requires the same amount
of work.
The current study used young, healthy volunteers to
determine the effect of an SBS stair ambulation pattern on
the biomechanics of the knee. Thus, an important caveat is
that the results reflect changes in the stepping pattern, and
not additional compensations attributable to aging or
disease. When alternate stepping patterns are performed
by healthy participants, any differences in the mechanics
between the two stepping patterns can be attributed to the
patterns, rather than to the individual_s ability to perform
the pattern. However, this may be quite different when
comparing the SOS pattern of one group against the SBS
pattern of another group that is not capable of performing
the SOS pattern. The data presented here can be used to
highlight compensations that occur in addition to a change
in the stepping pattern. We now know that the compensatory SBS ambulation pattern should resemble the traditional
SOS ambulation pattern, if we consider the support and
working legs. Significant deviations from the typical pattern
would suggest additional compensations (i.e., muscle weakness, balance, etc.) that must be considered over and above
the altered stepping pattern. The moments and forces are
similar between the SBS and SOS patterns, and it is likely
that the increased physical demands placed on the knee
joint, coupled with the reduced physical capacity of select
populations, are responsible for these alterations. Future
studies are needed to determine the reasons, both actual and
perceived, why an alternate stepping pattern is chosen.
The angle, force, power, and moment patterns at the knee
joint of normal reciprocal SOS stepping pattern and
alternate SBS stepping pattern have been presented for a
group of young, healthy adults. The resulting curve profiles
establish normative profiles for stair ambulation patterns
that can be used to evaluate whether changes in knee joint
mechanics are attributable solely to ambulation patterns, or
whether additional compensations are present. Moreover,
the results have clinical implications for the reduction of
knee pain in populations with chronic knee pain. This will
be important when evaluating stair ambulation performance
in patients, older adults, and disabled populations.
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APPLIED SCIENCES
ALTERNATE STAIR KNEE BIOMECHANICS
Medicine & Science in Sports & Exercised
2011
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