A measurable limb-length discrepancy is a relatively common
problem. It is found in as many as 40%1 to
70%2 of the population,
and a retrospective study demonstrated that a discrepancy of >2
cm affects at least one of every 1000 people3.
The effects of limb-length discrepancy on function and the magnitude
of limb-length discrepancy warranting treatment have been subjects
of controversy for some time.
Studies have been performed to investigate the effects of limb-length
discrepancy on many functional parameters, including low-back pain4, osteoarthritis of the hip5, stress fractures6,
aseptic loosening of hip prostheses7,
standing balance8, forces transmitted
through the hips9, running economy10, and associated running injuries11. Although no study has identified
direct measures of gait economy with limb-length discrepancy, some
indirect measures include ground-reaction forces, total mechanical
work, and overall kinetic energy9,12-15.
To our knowledge, Vink and Huson16 are
the only authors who have used electromyography to examine the effects
of limb-length discrepancy on muscle activity.
On the basis of these studies, it appears that there is a breakpoint
between 2 and 3 cm below which measurable changes do not take place.
However, this information is largely anecdotal, and there has been disagreement
about it17,18. Some of the disagreement
may be due to the different populations (children and adults) used
in studies of the effects of limb-length discrepancy19-21. Another factor might be the
functional activities of the individual. For example, the amount
of limb-length discrepancy that can cause symptoms in athletes may
be substantially smaller than the amount that can cause symptoms
in nonathletes22. Whereas Siffert23 reported that a limb-length discrepancy
of 1.0 to 2.5 cm is rarely symptomatic in the general population,
Friberg6 found that Finnish Army
conscripts with as little as 1.0 cm of limb-length discrepancy had
a greater incidence of stress fractures during extensive training
than did those with no discrepancy. Subotnick1 proposed
that 0.25 in (0.64 cm) of limb-length discrepancy in an athlete
is as important pathologically as is 0.75 in (1.9 cm) in a nonathlete.
Trauma, various diseases, and surgical procedures can result
in a limb-length discrepancy. Perhaps the most common surgical procedure
resulting in a limb-length discrepancy is total hip arthroplasty. More
than 123,000 total hip arthroplasties were performed in the United
States in 1994 alone24. The most
common age of people undergoing total hip arthroplasty is fifty-five
to eighty-five years25. Although
there are a variety of complications associated with total hip arthroplasty,
one of the most common is postoperative limb-length discrepancy26. The prevalence of limb-length discrepancy
secondary to total hip arthroplasty has varied according to the
study, from 27% with discrepancies large enough to require
a shoe-lift27, to 18% with
discrepancies of 1.5 cm28, to
16% with discrepancies of any magnitude29.
If the more conservative figure of 16% were used, there
would have been nearly 20,000 people with a measurable limb-length
discrepancy secondary to total hip arthroplasty in 1994 in the United
States alone.
Investigators disagree regarding the magnitude of acceptable
postoperative limb-length discrepancy. Some investigators have tried
to quantify an important limb-length discrepancy, accepting as much
as 2.0 cm, whereas others have defined an important discrepancy
in terms of functional outcome30.
The average limb-length discrepancy secondary to total hip arthroplasty
has been reported to range from 3 to 19 mm31,
with the actual values ranging from 0 to 3.0 cm28.
Jasty et al.29 stated that, although
limb-length discrepancies of 2 cm are common in the general population
and may be asymptomatic, discrepancies after total hip arthroplasty
lead to more symptoms. Surveys of patients have demonstrated that
dissatisfaction after total hip arthroplasty is commonly due to
limb-length discrepancy26. Edeen
et al.31 interviewed patients
postoperatively and found that, although their average limb-length
discrepancy was only 0.97 cm, a substantial percentage (32%)
were aware of the discrepancy. In addition, 24% of the
patients, with an average limb-length discrepancy of 1.49 cm, overall
were "annoyed" by it. Hozack et al.32 stated that limb-length discrepancy
after total hip arthroplasty is one of the most common complaints in
lawsuits against orthopaedic surgeons.
Although investigators have studied the effects of limb-length
discrepancy on many functional parameters, we are not aware of any
study of how induced limb-length discrepancy affects older adults
in terms of walking economy or muscle activity in the lower extremities.
This information may help to quantify how much limb-length discrepancy
can occur before functional gait is compromised. The purpose of
this study was to investigate the effects of artificially induced
limb-length discrepancies of 2, 3, and 4 cm on gait economy and
lower-extremity muscle activity in older adults.
Setting
All physiological testing was conducted from August to December
1999 at the Human Performance Laboratory at the University of New
Mexico in Albuquerque, New Mexico. The same well-lit, temperature-controlled
room was used for all testing. All data were collected from each
subject during one test session.
Subjects
Forty-four patients (nineteen men and twenty-five women)
ranging in age from fifty-five to eighty-six years were recruited
as a sample of convenience (Table I). They were screened for limb-length
discrepancy with use of a standard flexible tape measure, with each
limb measured from the medial malleolus to the anterior inferior
iliac spine as recommended by Beattie et al.33.
Individuals who had limb lengths within 1 cm of each other and who
did not have a neurological, orthopaedic, severe pulmonary, or cardiac
disorder or a history of falls were included in the study. After
being accepted as participants in the study, the subjects were randomly
assigned an order for the application of the shoe-lifts (the artificial
limb-length discrepancies).
This study was approved by the School of Medicine Human Research
Review Committee at the University of New Mexico. Prior to testing,
all subjects received explanations of the testing procedures and provided
written informed consent.
Measures
Heart rate: The subject’s heart rate
was measured with a heart rate telemetry unit (Polar Heart Rate
Monitor, model 1901201; Polar Electro, Woodbury, New York). Heart
rates were recorded at rest, at a steady state of each condition,
and during rest between each trial to establish adequate rest.
Rating of perceived exertion: Subjects received
verbal instruction regarding subjective scoring of perceived exertion
with use of Borg’s original rating scale of 6 to 20 points34.
Muscle activity: The subject’s skin
was prepared for the surface electromyographic electrodes by rubbing
for fifteen seconds with an alcohol preparation. Blue Sensor surface
electrodes (Medicotest, Rugmarken, Denmark) were applied parallel
to the muscle fibers on the muscle belly. In all cases, the electrode
placement was checked by asking the patient to contract the respective
muscle as well as other muscle groups in the region and inspecting
the electromyographic readout for cross talk. The electrodes were connected
to a Noraxon Myosystem-1200 sixteen-channel electromyographic
machine (Noraxon USA, Scottsdale, Arizona). Each rectified burst
of electromyographic activity had a trigger level of 2 V, an onset
time of twenty milliseconds, and a subsist time of twenty milliseconds.
Bursts of the electromyographic signal were then filtered with a Butterworth
low-pass filter at 250 Hz and were integrated with use of Myosoft
(Noraxon USA) software. A per-burst average was calculated from
a fifteen-second trial and was recorded in microvolts as the integrated
electromyographic signal. This measure was used to depict muscle
activity. The muscle activities of the right and left ankle plantar flexors,
quadriceps femoris, gluteus maximus, and gluteus medius were recorded
throughout all trials.
Oxygen consumption and minute ventilation: Expired
gas was collected every thirty seconds with use of a Jaeger ergo-oxyscreen
computerized expired-gas-analysis system (Erich Jaeger, Wurtzburg,
Germany) during the submaximal treadmill tests (Trackmaster Treadmill;
JAS Fitness Systems, Carrolton, Texas) at a self-selected
normal walking pace as subjects breathed through a T-configuration,
one-way valved mouthpiece (Hans Rudolph, Kansas City, Missouri).
When the values of absolute oxygen consumption remained within 75
mL O2/min for a minimum of three consecutive thirty-second
readings (the steady state), the values were recorded as the submaximal
oxygen consumption and the minute ventilation, and the subject was told
to stop walking. The error of the pneumotach is 1%, and
the analyzer had no measurable error; therefore, the sensitivity
of the measurements of minute ventilation and oxygen consumption
was 1%.
Shoe-Lifts
All shoes used by an individual subject were of the same make,
model, and size. A 2 or 4-cm shoe-lift constructed from crepe material
was glued into the midsole of the right shoe, and then the outsole
of the shoe was replaced. To attain 3 cm of artificial limb-length
discrepancy, a 1-cm insole insert was added to the 2-cm midsole
lift.
Physical Activity
Subjects were screened for their level of physical activity with
use of the modified Baecke questionnaire for elderly individuals35.
Testing
Subjects walked on the treadmill, without use of the handrails,
starting at 1 mph. The speed of the treadmill was increased at a
rate of 0.5 mph every fifteen seconds until the subjects reported
their self-selected normal walking pace. Subjects who were
unfamiliar with the use of a treadmill were allowed to practice
until they felt comfortable walking on the treadmill and the investigator
determined that their gait appeared normal. Subjects then walked
on the treadmill at their self-selected normal walking pace with
different amounts of artificial limb-length discrepancy applied
in a randomly assigned order. The subject’s oxygen consumption and
minute ventilation were measured after a steady state was achieved
at each level. Electromyographic data, heart rate, and rating of
perceived exertion were recorded throughout a fifteen-second trial
after a steady state was attained. The average duration of each
run was slightly more than six minutes. To prevent fatigue, subjects
rested between trials until their heart rate was within 5 bpm of
their resting heart rate.
Statistical Analysis
A repeated-measures analysis of variance was used to assess the
overall effect of artificial limb-length discrepancy on oxygen consumption,
minute ventilation, rating of perceived exertion, heart rate, and activity
of the right and left ankle plantar flexors, quadriceps femoris,
gluteus maximus, and gluteus medius during gait. Tukey post
hoc tests were then used on significant findings to compare
gait with no artificial limb-length discrepancy with gait with 2,
3, and 4 cm of artificial limb-length discrepancy for the above
parameters. In addition, the subject’s age, gender, height,
limb length, treadmill experience, and physical activity were analyzed
with use of repeated-measures analysis of variance to determine
whether they played a role in the effect of artificial limb-length
discrepancy on the above parameters. Significance was set at p £ 0.05.
Effect size was calculated for all significant findings.
Oxygen Consumption
Artificially induced limb-length discrepancy had a significant
overall effect on oxygen consumption (p < 0.0005, effect
size = 0.56). There was a significant increase in oxygen
consumption when subjects walked with a 4-cm discrepancy (mean,
10.49 mL O2/kg/min; p < 0.0005), a 3-cm
discrepancy (mean, 10.08 mL O2/kg/min; p < 0.0005),
and a 2-cm discrepancy (mean, 9.71 mL O2/kg/min;
p < 0.0005) compared with when they walked with no discrepancy
(mean, 9.26 mL O2/kg/min) (Fig. 1; Table II).
Rating of Perceived Exertion
Artificial limb-length discrepancy also had a significant overall
effect on the rating of perceived exertion (p < 0.0005,
effect size = 0.449). Subjects reported a higher rating
of perceived exertion when they walked with a 4-cm discrepancy (mean,
11.39; p < 0.0005), a 3-cm discrepancy (mean, 10.75; p < 0.0005),
and a 2-cm discrepancy (mean, 10.44; p = 0.013) compared
with when they walked with no discrepancy (mean, 9.67) (Fig. 2; Table II).
Minute Ventilation
Minute ventilation was significantly affected by artificial limb-length
discrepancy as well (p < 0.0005, effect size = 0.524).
Subjects had increased minute ventilation when they walked with
a 4-cm discrepancy (mean, 25.47 L/min; p < 0.0005)
and a 3-cm discrepancy (mean, 24.18 L/min; p = 0.013) than
when they walked with no discrepancy (mean, 22.69 L/min)
(Fig. 3; Table II).
Heart Rate
Artificial limb-length discrepancy had a significant overall
effect on heart rate (p < 0.0005, effect size = 0.459).
Subjects had a higher heart rate when they walked with a 4-cm discrepancy
(mean, 93.63 bpm; p < 0.0005) and a 3-cm discrepancy (mean,
92.12 bpm; p = 0.001) than when they walked with no discrepancy
(mean, 89.58 bpm) (Fig. 4; Table II).
Integrated Electromyographic Signal
Right quadriceps femoris muscle: There was a
significant overall effect of artificial limb-length discrepancy
on the activity of the right quadriceps femoris muscle (p < 0.0005,
effect size = 0.392). Subjects had higher activity when
they walked with a 4-cm discrepancy (mean, 22.72 V; p < 0.0005)
and a 3-cm discrepancy (mean, 20.50 V; p = 0.001) than
when they walked with no discrepancy (mean, 13.29 V) (Fig. 5; Table II).
Left plantar flexor muscles: Artificial limb-length
discrepancy also had a significant overall effect on the activity
of the left plantar flexors (p = 0.042, effect size = 0.246).
When subjects walked with a 4-cm discrepancy, they had higher activity
(mean, 45.31 V) than when they walked with no discrepancy (mean,
32.17 V; p < 0.003) (Fig. 6; Table II).
Effects of Age, Gender, Height, Limb Length, Treadmill
Experience, and Physical Activity
With the numbers available, age, gender, height, limb length,
treadmill experience, and physical activity had no significant effect
on any of the above parameters.
Qualitative Observations of Gait with Artificial Limb-Length
Discrepancy
Observations of gait with the artificial limb-length discrepancies
revealed that the most common primary compensatory strategy was
steppage gait (increased hip and knee flexion), which was used by
twenty-one of the forty-four subjects. This was followed in frequency
by circumduction (increased hip abduction at swing phase), used
by eight subjects; vaulting (increased plantar flexion at step phase),
used by five; and hip-hiking (increased ipsilateral lumbar side
flexion at swing phase), used by two. Subjects often used more than
one, and as many as three, compensatory strategies simultaneously
while walking on the treadmill. A total of ten distinct gait-compensation
strategies were observed. Eight subjects did not demonstrate a consistent
observable gait compensation.
Problems Experienced by Subjects When Walking with
Artificial Limb-Length Discrepancy
On the whole, subjects had few problems when walking with an
artificial limb-length discrepancy. Several stated that they felt
as if they were walking on the side of a hill or with the right
foot up on a curb. One subject who had reported having arthritis in
the right knee complained of pain in that knee with the 4-cm artificial
limb-length discrepancy but insisted on completing the trial. Another
subject reported low-back pain when walking with both the 3 and
4-cm artificial limb-length discrepancy but also insisted on completing
both trials. Three subjects dragged the right foot when wearing
the 3 and 4-cm lifts, and two subjects lost their balance several
times when walking with the 4-cm limb-length discrepancy and had
to momentarily grab the handrail.
A large number of studies have been performed in an attempt to
determine the magnitude of limb-length discrepancy necessary to
manifest complications and, therefore, to warrant treatment3,6,19,22,23,36. However, controversy
still remains regarding the breakpoint between acceptable and unacceptable discrepancy,
especially in older adults.
The significant increases in oxygen consumption, minute ventilation,
heart rate, and rating of perceived exertion in this study were
probably a result, at least in part, of an increase in the subjects’ vertical
displacement of the center of mass37.
When a subject is in the stance phase of the longer limb, there
is a tendency to raise the center of mass of the head, abdomen,
and trunk. In fact, this increase in vertical displacement of the
head, abdomen, and trunk has been shown to be a strong predictor
(r2 = 0.91) of oxygen consumption
by normal subjects walking at variable speeds38. The increases seen
in quadriceps activity with 3 and 4 cm of artificial limb-length
discrepancy are consistent with the increases in ground-reaction
force of the longer limb reported by several authors12,14. The increased ground-reaction
force places a greater demand on the quadriceps at heel-strike.
It is probable that the subjects’ attempts to reduce the length
of the right (longer) limb by maintaining it in slight flexion during
stance phase could also account for the increase in the output of
the right quadriceps. The overall increase seen in the activity of
the left plantar flexors with the 4-cm artificial limb-length discrepancy
could be due to the ten subjects who demonstrated a vaulting gait
(five of whom used vaulting as the primary compensatory strategy
and five of whom used it as a secondary compensatory strategy).
These plantar flexors would be firing during the swing phase of
the right limb to gain additional clearance of the longer limb.
Although no study has identified direct measures of gait economy
with limb-length discrepancies, some indirect measurements have
been performed. Bhave et al.12 found
that limb-length discrepancy creates an asymmetry in ground-reaction
force and that surgical lengthening of the short limb to within
1 cm of the other limb reduced the asymmetry to below significance.
Kaufman et al.14 found that a
discrepancy of more than 2 cm was necessary to result in asymmetry
of the ground-reaction force. Liu et al.15 also
measured ground-reaction force and concluded that gait asymmetries
were not evident until a limb-length discrepancy was in excess of
2.33 cm. Brand and Yack9 found
that a 2.3-cm artificial limb-length discrepancy did not change
ground-reaction force at the hip but a 3.5-cm discrepancy did. Delacerda
and Wikoff13 found that use of
a corrective shoe by a woman with a 2.87-cm limb-length discrepancy
significantly reduced the kinetic energy expended during walking
(p < 0.05). We know of only one study in which electromyography
was used to measure the effects of limb-length discrepancy on gait.
Vink and Huson16 found that there
was a significant increase in the electromyographic activity of
the erector spinae only when the limb-length discrepancy was 3 cm. On
the basis of these studies, it appears that a limb-length discrepancy
of slightly >2 cm is a breakpoint above which there are
measurable changes in gait. In fact, it is generally accepted that
a 2-cm limb-length discrepancy represents the breakpoint between
treatment and no treatment. However, this information is largely
anecdotal and there has been disagreement about it. For example,
Song et al.21 found that asymmetries
in mechanical work performed by the long extremity were observable when
the limb-length discrepancy was as small as 1.64 cm.
In our study, overall increases in physiological parameters were
not equal between successive incremental increases in artificial
limb-length discrepancy. Figure 7 shows the overall average percent
increases in all significant physiological variables compared with the
values associated with no artificial limb-length discrepancy. The
5.6% overall increase in physiological parameters between
0 and 2 cm of artificial limb-length discrepancy was small when
compared with the 13.3% increase between 0 and 3 cm and the
24.1% increase between 0 and 4 cm. It is noteworthy that
the percent increase in physiological parameters between 0 and 2
cm of artificial limb-length discrepancy averaged 2.8%/cm,
whereas it averaged 7.7% between 2 and 3 cm and 10.8% between
3 and 4 cm of artificial limb-length discrepancy. These findings
are in agreement with those of several authors who reported that
changes in gait did not take place until limb-length discrepancy
was in excess of 2 cm2,12-15.
These findings should be interpreted with caution, however, since
in our study oxygen consumption and, perhaps most importantly, the
rating of perceived exertion were significantly increased with 2 cm
of artificial limb-length discrepancy (p < 0.0005 and p = 0.013,
respectively). We found that most increases seen with 4 cm of artificial
limb-length discrepancy were large (average increase, 24%), which
is contrary to the findings of Phelps et al.17,
who reported that alterations in oxygen consumption outside the
range of normal were not seen in young athletes even with limb-length
discrepancies of 6 cm, and also to those of Richter18, who observed no changes in heart
rate until his subjects had 4 cm of artificial limb-length discrepancy.
The fact that our study dealt with older adults could account for
the difference between our findings and those of Phelps et al.,
who performed their study on young adults. In Richter’s
study, the only outcome was heart rate, which does not "tell
the whole story" because increases in energy expenditure
can be absorbed by increases in stroke volume and/or peripheral
extraction as well as heart rate.
According to Waters et al.39,
unlimited endurance requires that the energy cost for walking be <50% maximum
oxygen consumption. It is at about 50% maximum oxygen consumption
that individuals who are not physically fit experience lactate threshold36. Therefore, from an oxygen-consumption
perspective, an otherwise healthy individual would be able to walk
with a 4-cm limb-length discrepancy indefinitely. The same would
be the case for minute ventilation. In our study, the minute ventilation
seen with a 4-cm artificial limb-length discrepancy was 25.47 L/min,
which is 24% of the maximum minute ventilation of a typical
seventy to seventy-nine-year-old man40.
The average maximum oxygen consumption of older people (sixty-five
years and older) with a poor fitness status is about 25 mL O2/kg/min41. The average oxygen consumption
of our subjects walking with a 4-cm artificial limb-length discrepancy
was 10.49 mL O2/kg/min, or 42% of maximum
oxygen consumption.
If an older person has compromised cardiac and/or pulmonary
status, however, a limb-length discrepancy as small as 2 cm could
pose a problem. The maximum oxygen consumption of a person with severe
congestive heart failure can be as low as 10 to 15 mL O2/kg/min42,43. In fact, Meyer et al.44 found that, on the average, the
lactate threshold occurred at 9.3 ± 0.4 mL
O2/kg/min in eighteen patients with severe congestive
heart failure. In our study, a 2-cm artificial limb-length discrepancy
raised oxygen consumption from 9.26 to 9.71 mL O2/kg/min.
If patients with congestive heart failure had a 2-cm limb-length
discrepancy, they would have to have a maximum oxygen consumption
of at least 19.42 mL O2/kg/min (50% maximum
oxygen consumption) to walk for a prolonged period of time. The maximum
oxygen consumption would have to be 20.16 mL O2/kg/min
if the limb-length discrepancy were 3 cm and 20.98 mL O2/kg/min
if the limb-length discrepancy were 4 cm. The average oxygen consumption
expenditures were measured at an average walking pace of only 1.6
mph in this study.
Similarly, patients with severe chronic obstructive pulmonary
disease or restrictive lung disease can have a maximum minute ventilation
as low as 25 to 30 L/min45.
These individuals would almost reach their maximum minute ventilation
(97% to 81%) if they had a 3-cm limb-length discrepancy,
and they would be affected by even this moderate increase in minute ventilation.
With a 4-cm limb-length discrepancy, patients with a maximum minute
ventilation of 25 L/min would exceed their maximum value
whereas individuals with a maximum minute ventilation as high as
32 L/min would be at 80% of their maximum minute
ventilation.
The 54% increase in the activity of the right quadriceps
muscle seen with the 3-cm artificial limb-length discrepancy could
be a problem for patients with compromised limb strength, as might
result from a stroke. Patients who have had a stroke are more likely
to fall onto their affected side (chi square = 22.5, p < 0.001)46, and a patient who has done this
and has sustained a subcapital hip fracture necessitating a total
hip arthroplasty would be especially susceptible to the problems
created by a limb-length discrepancy after surgery.
It is generally accepted that overfatigue of the muscles of people
with multiple sclerosis47, Guillain-Barré syndrome48, and postpoliomyelitis syndrome49 can be detrimental. Since individuals
with these disorders have substantially compromised endurance with
repeated high-intensity contractions, a limb-length discrepancy
could limit the distance that these individuals are able to travel
before fatigue. The 41% increase in the activity of the
left plantar flexors and the 71% increase in that of the right
quadriceps muscle seen with the 4-cm artificial limb-length discrepancy
in our study could be clinically relevant.
Gait Compensation Strategies
The fact that artificial limb-length discrepancy of as much as
4 cm resulted in no significant increases in the activity of any
of the other muscle groups deserves some consideration. At least
four distinct compensation strategies (steppage, circumduction, vaulting,
and hip-hiking) emerged during gait, with some subjects using more
than one, and as many as three, strategies simultaneously. In all,
ten different combinations of strategies were apparent. In addition,
three subjects were observed dragging the right foot repeatedly,
and two subjects lost their balance several times. The result of
this variety of strategies would be an equal variety of muscle recruitment
patterns. Therefore, no one muscle group had greater activity in
more than a few individuals. This probably explains the lack of
significant increases in the activities of the other muscle groups.
Limitations of the Study
This study addressed the acute effects of limb-length discrepancy.
Chronic adaptations are possible and may reduce the amount of functional
loss over time. An example of an adaptation is pelvic obliquity,
which is an efficient way to reduce increases in vertical center-of-mass
oscillations. However, in an older adult, this compensation may not
be possible because of stiffness in the spine and decreased spinal
motion. In addition, the shoe-lifts added an average weight of 72
g to the shoe. This added weight might have been responsible for
some of the increases in the physiological parameters. Even though
subjects were allowed as long as twenty minutes to adapt to treadmill
walking, thirteen of the subjects had limited experience with walking
on a treadmill and, for them, it may not have been representative
of walking on the ground.
Conclusions
An artificial limb-length discrepancy of 2 cm had a significant
effect on oxygen consumption and the rating of perceived exertion.
Older adults appear to have a breakpoint between 2 and 3 cm of artificial limb-length
discrepancy that determines the effect on most other physiological
parameters, although 3 cm appears to be the breakpoint for significant quadriceps
fatigue in the longer limb as it causes a 54% increase
in quadriceps activity. Elderly patients with substantial compromise
of pulmonary, cardiac, or neuromuscular function may have difficulty
walking with a limb-length discrepancy as small as 2 cm.
It would be instructive to repeat this study with the use of
a portable gas-analysis system and to expand it to activities of
daily living.