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First published online April 18, 2008
Journal of Experimental Biology 211, 1368-1375 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014589
Lower-limb biomechanics during stair descent: influence of step-height and body mass
1 Institute for Biomedical Research into Human Movement and Health, Manchester
Metropolitan University, Alsager, UK
2 Research Institute MOVE, Faculty of Human Movement Sciences, VU University
Amsterdam, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands
* Author for correspondence (e-mail: m.spanjaard{at}fbw.vu.nl)
Accepted 8 March 2008
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: muscle mechanics, fascicle, ultrasound, musculotendon complex, stair descent
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Riener et al., 2002;
Protopapadaki et al., 2007;
Reeves et al., 2008), but
instead of shortening to provide propulsion as in level walking and stair
ascent for example (McFadyen and Winter,
1988; Fukunaga et al.,
2001; Lichtwark and Wilson,
2006; Spanjaard et al.,
2007a; Spanjaard et al.,
2007b), the active muscles lengthen to absorb kinetic energy that
is gained when descending a step (McFadyen
and Winter, 1988; Spanjaard et
al., 2007a; Spanjaard et al.,
2007b). During the landing phase of stair descent, the ankle joint
moment is first to peak, followed shortly after by a smaller knee joint moment
peak (McFadyen and Winter,
1988; Riener et al.,
2002; Protopapadaki et al.,
2007; Spanjaard et al.,
2007a; Spanjaard et al.,
2007b). The timing and magnitude of the ankle joint moment during
the initial landing phase of stair descent highlights the important role of
the plantarflexors during this task. Despite lengthening of the entire
muscle–tendon complex (MTC) during the initial landing phase of stair
descent, we have previously shown that the fascicles of the gastrocnemius
medialis (GM) muscle actually shorten
(Spanjaard et al., 2007a). The
energy stored in the tendon during this landing phase was then released at a
later stage of the stride cycle stretching the muscle fascicles.
To understand further the role of the ankle joint in stair descent when
real-world situations cause the task demands to alter, we recently manipulated
gait velocity (Spanjaard et al.,
2007b). We investigated three gait velocities (63, 88 and 116
steps min–1) during stair descent and we expected that joint
angle patterns would have been unaffected because the step dimensions remained
unaltered. In contrast, however, increases in gait velocity resulted not only
in ankle joint moment increases, but also in changes in joint angle patterns.
In addition, the GM muscle fascicles shortened more with increasing velocity.
Moreover, it was interesting to note that the consistent incremental pattern
of MTC lengthening and muscle fascicle shortening with increasing gait
velocity was not paralleled by the ankle joint moment, which did not increase
beyond 88 steps min–1.
To gain further insight into the role of the ankle joint and GM muscle when
the demands of stair-descent tasks are altered, we independently manipulated
two different parameters during stair descent: step-height and body mass. The
ankle joint and, in particular the GM muscle, are expected to be highly
influenced by these alterations since these are expected to accommodate the
change in force and negative work required
(McFadyen and Winter, 1988;
Riener et al., 2002;
Maganaris et al., 2006;
Protopapadaki et al., 2007;
Spanjaard et al., 2007a;
Reeves et al., 2008). The
underlying rationale for these experimental manipulations was to mimic
situations encountered habitually in the real world. For example, step-height
varies depending upon the location of the staircase, with typically higher
step-heights in private dwellings and lower step-heights in public places
(Roys, 2001). A greater total
body mass relative to the proportion of lean body mass is a typical
characteristic of a number of different populations such as obese people,
older adults and pregnant women.
Besides mimicking real world situations, changing task demands that alter
energetic requirements can also give us insight into how the GM muscle (the
fascicles of which are known to shorten during MTC stretch) copes with these
requirements. When step-height or body mass increases, the requirement for
negative work increases. The former increases the vertical distance that the
centre of mass (CoM) has to travel, while the latter likely increases the
force with which the CoM has to be decelerated. It is anticipated that the
ankle joint moment and negative ankle joint power will increase with both
increased step-height and body mass. For both increased demands, changes in GM
muscle fascicle behaviour could vary between two extremes: (1) muscle
fascicles will shorten more, performing more mechanical work, which will
increase the amount of negative work to be performed by other structures even
more; (2) the muscle fascicles will shorten less or even lengthen, which has
the additional advantage of acting at a more favourable position in the
force–velocity relationship (Hill,
1953), besides that of performing less positive mechanical work
where net negative work is needed. A disadvantage of the latter would be that
the initial state of the muscle and tendon might not be stiff enough to
effectively decelerate body mass, since the tendon could be close to slack
length. Therefore, it is hypothesized that increasing the demands by
increasing step-height or body mass involves more GM muscle fascicle
shortening. It is further hypothesized that, to withstand the increase in
demands, the ankle joint moment will increase.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Spanjaard et al., 2007b) from
our laboratory. Ethical approval was gained from the ethics committee of the
Institute for Biomedical Research into Human Movement and Health at the
Manchester Metropolitan University.
Measurements
The set-up has been described in detail previously
(Spanjaard et al., 2007a).
Subjects were asked to descend a custom-built steel staircase of four steps,
barefooted, in a step-over-step manner. The steps were independently mounted
on the floor. The height of the steps could be altered using individual,
purpose-built metal frames underneath the steps. The tread and width of the
steps were always the same: 280 mmx900 mm. The riser was set at four
different heights: standard height (170 mm), 50% decreased height (85 mm), 50%
increased height (255 mm) and 75% increased height (297.5 mm). The dimensions
of the standard height are most frequently encountered in semi-public places
(Roys, 2001).
Kinetic data were collected by force plates embedded in, and in front of, the staircase. Three force plates (Kistler 9286A, 27x52 cm) with built-in amplifiers were embedded in the first three steps (from the ground), and one force plate (Kistler 9253A, 40x60 cm) with an external amplifier (Kistler 9865C) was embedded in the floor, in front of the staircase.
A 9-camera VICON 612 system (VICON Motion Systems Ltd, Oxford, UK) was used to capture kinematic data. Retro-reflective markers were placed on bony landmarks, directly on the skin, or on appropriate tight-fitting clothing using double-sided tape. In total, 34 markers were placed on the body according to the standard `plug-in-gait' model of the VICON system. From the marker coordinates, knee and ankle joint angles were calculated.
GM muscle fascicle behaviour was assessed in vivo by ultrasound scans (Aloka SSD-5000, Tokyo, Japan) recorded in real-time during the stair-descent trials. A linear 7.5 MHz probe (UST-579T-7.5) with 60 mm field of view was tightly secured around the left lower leg in the mid-sagittal plane of the GM muscle with a custom-built fixation device. The fixation device was made of a plastic cast, moulded to fit the general contour of the calf, with a window for the probe. The probe was held rigidly by the cast, which was securely fixed on the calf using Velcro straps. The experimenter supported the probe cable throughout scanning. Sampling rate was 22 samples s–1 and image resolution was 768x576 pixels. The ultrasound scanning was synchronized with the kinematic, kinetic and EMG data using an external trigger.
A Bagnoli EMG system (Delsys Inc., Boston, MA, USA) was used to record the electrical activity of the GM muscle of the left leg. The recording electrodes were placed proximal to the ultrasound scanning probe in the mid-sagittal plane of the muscle. The sampling rate of the EMG recordings was set at 2000 samples s–1.
Protocol
Anthropometric measurements were taken for each subject to scale the
generic human plug-in-gait model in the VICON software (Oxford Metrics Inc.,
Oxford, UK). Subsequently, the markers and EMG electrodes were positioned and
data collection was initiated.
The subjects were tested at four different step-height and two different
body mass conditions. After practise trials and feeling comfortable with
descending the staircase (at that specific height), subjects performed one
stair descent at a predefined gait cadence, dictated by a metronome, which was
set at 88 beats min–1 [previously shown to closely match the
self-selected cadence in these subjects
(Spanjaard et al., 2007a)].
The measurement was repeated if the cadence of the subject did not correspond
with the beats of the metronome (as observed by the experimenter).
Furthermore, and only at the standard step-height, the subjects performed a
descent with added mass. Body mass was increased by 20% using a custom-made
jacket with the pockets filled with lead-covered pieces of known mass. The
jacket was secured tightly around the shoulders and waist of the subject
without interfering with the rest of the equipment. The pieces of lead were
placed such that the extra weight was distributed uniformly over the trunk. If
the descending cadence corresponded well with the metronome, the trial was
used for further analysis. In all trials, subjects stood still on top of the
staircase (a platform) and started the trial with their right foot. The trial
ended when the subject was on the ground, off the force plate.
Data analysis
The first full stride cycle of the left leg was considered a steady-state
stride cycle (from the first touch-down point of the left foot on the second
step down, to the second touch-down point of the left foot on the floor), as
indicated by an earlier study (Andriacchi
et al., 1980). This steady state stride cycle was used for further
analysis. Kinematics and kinetics in 3-D for the ankle and knee joints were
calculated from marker positions and force plate data, using VICON software.
Only the sagittal plane information was used for further processing.
The GM muscle fascicle lengths were measured from the recorded ultrasonographic images. On each ultrasound image from the analyzed stride cycle, GM muscle fascicle length was measured manually using Matlab (The Mathworks, Inc., Natick, MA, USA). Muscle fascicle length was measured using the assumption that the fascicular trajectory was linear. The fascicle length measured in a standing position was the reference length for each subject. To account for individual differences, the fascicle length change was calculated as the difference between the reference length and the measured fascicle length during the analyzed stride cycle.
The GM MTC length change (muscle plus free tendon and aponeurosis in both
distal and proximal ends) was estimated using Menegaldo et al.'s equations
(Menegaldo et al., 2004),
taking the ankle and knee joint kinematics as input.
EMG signals were band-pass filtered (20–450 Hz) by the Delsys system,
then rectified and smoothed (2nd order low pass 5 Hz bidirectional filter)
using Matlab. EMG signals from previous studies
(McFadyen and Winter, 1988;
Riener et al., 2002;
Spanjaard et al., 2007a;
Spanjaard et al., 2007b;
Reeves et al., 2008) and the
present study revealed that the GM muscle was active during the touch-down
phase. In terms of GM muscle fascicle behaviour, the phase of interest started
with touch-down and ended when the muscle fascicles shortened maximally. For
this phase, joint moment peaks, the amount of GM muscle fascicular shortening,
fascicle shortening velocity, MTC length and the raw EMG root mean square
(RMS) were calculated. Repeated-measures ANOVA and Student's t-test
were used to statistically analyze the influence of step-height and the
influence of added body mass, respectively.
The position of the body CoM was calculated using a custom script in VICON
`bodybuilder', which was based on Dempster's regression equations
(Dempster, 1955). The script
allowed adjustment for the added body mass condition, where the extra 20% body
mass was ascribed to the trunk. Furthermore, an estimate of the net mechanical
power produced by all muscles during stair descent was calculated and termed
`locomotory power'. For this estimation, the human body was modelled as a
single point-mass (the CoM), which was influenced by a single force (the
ground reaction force). The locomotory power was calculated as the product of
the ground reaction force and the CoM derivative. Ankle and knee joint power,
calculated as the product of joint moment and joint angular velocity, were
compared with the locomotory power.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Step-height
There were no differences in step cadence between the step-height
conditions (P=0.671). However, relative foot contact times were
affected by step-height; the stance phase became shorter at higher
step-heights (P<0.005).
The main effects of step-height were found for peaks in ankle and knee joint moments. In agreement with our hypothesis, both increased with step-height (Fig. 1 and Table 1). The relative timing of the ankle moment peak occurred earlier in time for higher step-heights (P<0.001).
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The locomotory power during a step down for all heights is shown in Fig. 2. The maximal negative locomotory power, which occurred during the touchdown phase, increased with step-height (`minimal locomotory power' in Table 1). Exactly coincident with this negative peak, the ankle joint power also showed a negative peak (Fig. 3A), which also increased with step-height (Table 1). The relative contribution of the ankle power to the locomotory power during the negative power peak in touch-down increased with step-height (Table 1). Thus, the total amount of negative work on the whole body increased with step-height, as did the amount of negative work performed by the ankle plantarflexors.
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Muscle activation was increased at higher step-heights, as indicated by the RMS values of the GM EMG data (Table 1).
Added mass
There was no difference in step cadence between normal descent and descent
with 20% added body mass (P=0.144). The first double support stance
phase was longer in the added mass trials (P<0.005), while the
duration of the total stance phase was not statistically different
(P=0.068).
In contrast to our hypothesis, the ankle joint moment did not change when body mass was increased during the touch-down phase (Fig. 5A and Table 2). The timing of peak ankle moment did also not change with added mass (P=0.495). The peak knee joint moment during the touch-down phase was higher in the added mass condition, however, this occurred only after lift-off of the trailing leg (Fig. 5B and Table 2). The joint moments of the trailing leg (the second ankle and knee joint moment in Fig. 5A,B) did increase with added body mass (P=0.005 and P=0.008 for the ankle and knee joint, respectively).
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The locomotory power for the added body mass condition was similar to that of the standard condition (hence not shown). The negative peaks of both the locomotory power and the ankle joint power during touch-down were not statistically different between conditions (Table 2). Also, the contribution of the ankle power on the locomotory power during the negative power peak did not change with added body mass (Table 2). Although the locomotory power was not higher during the touch-down phase for the added mass condition, it was observed that higher values were maintained for a longer period, so that the total amount of work performed on the CoM was increased for the added body mass condition.
The effect of added body mass on the GM muscle fascicle length change during a single stride is shown in Fig. 6A and Table 2. The amount of GM muscle fascicle shortening and the timing of maximal shortening were not influenced by an increase in body mass (P=0.659 and P=0.769, respectively, N=4; Table 2). GM muscle fascicle shortening during touch-down did not differ between conditions. Fascicle shortening velocity was also not influenced by an increase in body mass (Table 2). MTC length, averaged over the analyzed period or at the time of maximal fascicle shortening, was not affected by an increase in body mass (P=0.332 and P=0.150, respectively, N=4; see Fig. 6B and Table 2). This indicates that the kinematics were hardly affected, which was confirmed after inspection of the ankle and knee joint angle traces (not shown). Added body mass did also not have an effect on the GM EMG during the analyzed phase, as shown by the RMS values (Table 2).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Step-height
The period in which the plantarflexor muscles are predominantly active
during stair descent is touch-down. The toe hits the next step first, then,
the heel touches down by a dorsi-flexion movement of the ankle, during which
the plantarflexor muscles are active to control this movement. Increased
step-height, in stair descent, requires more negative work during touch-down.
This is reflected in the negative locomotory power
(Fig. 2) and in the negative
ankle and knee joint powers, which all increased with increasing step-height
(Fig. 3,
Table 1). The ankle joint
moment is the most important actuator to perform negative work since the
contribution of the ankle joint to the locomotory power during touch-down
(where the locomotory power is most negative) was 59% at the standard height
and increased to 75% at the highest step-height
(Table 1). The knee and ankle
joint powers were consistent with previous studies
(McFadyen and Winter, 1988;
Riener et al., 2002;
Devita et al., 2007). The ankle
joint moment peak occurred earlier in time for higher step-heights
(Fig. 1), corresponding with
the shorter stance phase, and emphasizing the need to increase negative power
around the ankle with increasing step-height. We, and others, have argued
before that the ankle joint plays a key role in stair descent
(Riener et al., 2002;
Spanjaard et al., 2007a;
Reeves et al., 2008); this is
now supported by the large negative ankle joint power and its contribution to
the locomotory power during touch-down. When step-height is increased, the
relative contribution of the ankle joint to the locomotory power increases
even further (Figs 3 and
5,
Table 1).
The GM muscle is an important contributor to the ankle plantar flexion
moment. GM muscle activation was increased for higher steps, consistent with
an increase in the ankle joint moment. However, the ankle dorsi-flexed faster
at higher step-heights, while the plantar flexion moment was increased,
causing the ankle joint power to be more negative. The GM muscle fascicles
shortened during this phase, performing mechanical work, while the total MTC
lengthened, performing negative work. Earlier studies from our laboratory also
indicated shortening of fascicles during MTC lengthening while walking down
stairs (Spanjaard et al.,
2007a; Spanjaard et al.,
2007b). This behaviour of muscle fascicles is not shown when
humans or other species walk down a decline
(Gabaldon et al., 2004;
Lichtwark and Wilson, 2006).
Besides interspecies disparities, this difference is probably due to the toe
landing in stair descent where there is a heel landing during decline walking.
In species that walk on their toes (cats) it has been found that during level
walking, the GM muscle fascicles also shorten while the MTC is lengthening
(Griffiths, 1991).
Furthermore, the decline used in the experiments by Lichtwark and Wilson
(Lichtwark and Wilson, 2006)
was very small (10%).
|
;
Spanjaard et al., 2007a)], we
find that the tendon is stretched more at higher step-heights, despite initial
tendon lengths being similar (Fig.
7). This suggests that the GM muscle fascicles produce more force
at higher step-heights, as was also expected from ankle joint moment and EMG
data, but they do this at shorter lengths, and thus probably further away from
the optimal fascicle length (Gordon et al.,
1966; Maganaris,
2003). Presumably, the GM muscle fascicles need to shorten this
much to increase the stiffness of the total MTC and control the dorsi-flexion
movement. Part of the kinetic energy from the touch-down and the extra
mechanical work performed by the fascicle shortening appear to be stored in
the tendon. Part of this energy is dissipated in mid-stance, while the other
part of this energy seems to be used for ankle plantarflexion during
lift-off.
In a previous experiment, we investigated the influence of gait velocity on
GM muscle fascicle behaviour during stair descent
(Spanjaard et al., 2007b) and
found an increase in the amount of fascicle shortening at higher gait
velocities. This is similar to the results for the influence of step-height
from the present study. However, the kinematics, and therefore the MTC lengths
were quite similar between all velocities, unlike in the present study for the
different step-heights. Also, fascicle-shortening velocity was increased for
faster gaits, which was different than at higher steps, where we did not find
any differences in fascicle shortening velocity. It seems that for both of the
approaches employed to increase the task demands (increase in step-height and
increase in gait velocity), the ankle joint moment and the GM muscle fascicle
shortening were increased. Although the increase in shortening with increased
joint moments does not seem unexpected, we stress that the GM muscle fascicles
shortened in all conditions, while the MTC was lengthened by increasing
external forces.
Added mass
An increase of 20% body weight was expected to cause higher impact forces
during the touch-down phase in the leg positioned on the step below. In
contrast, however, the joint moments of the trailing leg increased
(Fig. 5, between 35 and 60%
stride cycle). This suggests that to descend the stairs with added body mass a
strategy was employed whereby the extra load was carried by the trailing leg.
The leading leg was loaded relatively less
(Fig. 5, between 0 and 15% of
stride cycle). The knee joint moment did increase, which occurred only after
the touch-down phase, when the trailing leg had just started its swing
phase.
Because of the change in strategy with added mass, the plantarflexor muscles were not loaded more during touch-down with added mass than during normal stair descent, and the GM muscle fascicle behaviour was the same in both conditions.
General effects of increased demands on lower-limb mechanics
The shortening of the GM muscle fascicles during touch-down of stair
descent controlled the dorsi-flexion movement, while the MTC was lengthening.
The GM muscle fascicles shortened more when step-height was increased, which
corresponded with the increase in ankle joint moment. However, 20% extra body
mass did not lead to extra shortening of the GM muscle fascicles. Due to a
change in strategy, the weight on the leading leg was not increased, but
instead, the trailing leg supported the extra weight. It would appear that
these results of increasing the demands of stair descent would be predictable
once ankle moments are known; higher ankle joint moments require more GM
muscle fascicle shortening (increase in step-height), whereas when the ankle
moment does not increase there is no extra GM muscle fascicle shortening
(increased body mass). However, in a previous experiment in which we changed
gait velocity to alter task demands
(Spanjaard et al., 2007b), we
showed that when the peak ankle moment reaches a plateau and does not rise any
further (with increasing gait velocity), the GM muscle fascicle shortening did
increase further. This shows that the relation between fascicle shortening and
ankle joint moment cannot be generalized to all situations where the task
demands are altered. The non-linear elasticity of the tendon and the actions
of other muscles are likely to play a role here. Also, the activation ratio
between the three muscles of the triceps surae is known to change with
movement velocity (Herman,
1967; Hof and van den Berg,
1977; Vandervoort and McComas,
1983; Duchateau et al.,
1986; Tamaki et al.,
1997), which may explain the difference between the influence of
gait velocity and the influence of step-height on GM muscle fascicle
behaviour. When the demands of the task were increased by adding body mass,
the strategy was altered such that the leading leg, especially the ankle
joint, was not loaded further. Therefore the amount of GM muscle fascicle
shortening was similar between added mass and normal stair descent.
Methodological considerations
In the present study, the fascicular trajectory was approximated as a
straight line, neglecting the slight curvature of the fascicles
(Maganaris et al., 1998). The
difference between the two measurement approaches is, however, small [<3%,
as estimated in a previous experiment
(Spanjaard et al., 2007a)] and
falls within the observed variation (5.9%) of muscle fascicle length
measurements performed by Narici et al.
(Narici et al., 1996). The
reliability of the muscle fascicle length measurements in the present study
was calculated from the reference fascicle lengths, which were measured during
upright standing, on 4 different days. The intraclass correlation between
these measurements was 0.8, indicating high reliability.
Although the probe was securely fixed on the skin, it is not known by how
much the muscle shifted in relation to the scanning plane, therefore we are
unable to determine the precise magnitude of fascicle length measurement error
introduced by scanning the muscle in 2-D. Data reported elsewhere
(Klimstra et al., 2007)
indicate that a combined probe rotation by 5° in the longitudinal
direction and 5° in the sagittal-frontal direction from the original
scanning plane results in an average fascicle length error of no more than 8%.
However, in the present study the ultrasound probe was securely fixed around
the lower leg and no observable movement of the probe in relation to the leg
could occur without manual application of external force to the probe.
Moreover, it is highly unlikely that the GM muscle would rotated by as much as
the experimental probe rotations examined by Klimstra et al.
(Klimstra et al., 2007).
Hence, we are confident that our fascicle length measurement errors introduced
by 2-D scanning are much smaller than 8%. Errors in fascicle length
measurements will be propagated in the calculation of fascicle shortening
velocity.
To optimize image quality, the ultrasound sampling rate in the present
study was set to 22 samples s–1, which means that only 3.5
ultrasound frames were recorded in the phase of interest. We considered this
compromise reasonable and we trust that our results and conclusions are valid
since we were interested in general patterns of fascicle length changes.
Furthermore, we believe that the frequency content of the fascicle length data
is well below half the sampling frequency (11 Hz)
(Bawa and Stein, 1976;
Hidler et al., 2002).
The model by Menegaldo et al.
(Menegaldo et al., 2004) used
to calculate MTC length changes from ankle and knee joint kinematics in the
present study shows similar results to those obtained using other models
available. In fact, MTC length change values calculated according to Menegaldo
et al.'s model (Menegaldo et al.,
2004) are in between the values calculated using other models
(Grieve et al., 1978;
Hawkins and Hull, 1990). The
pattern of variation for MTC length is the same regardless of the model used.
However, we opted for the model by Menegaldo and colleagues because this was
based on detailed 3-D measurements of bone geometry and muscle–tendon
origin and insertion positions from a living subject rather than cadavers.
Another point of discussion is the contribution of the different muscles in
the triceps surae to the tendon stretch (as a measure of force applied) and
the total joint moment. It is possible that a large part of the plantarflexion
moment is produced by the soleus muscle (due to its large physiological
cross-sectional area). However, previous work has shown that the soleus muscle
is already fully activated when the ankle joint moment reaches 
70% of
maximal voluntary contraction (MVC)
(Maganaris et al., 2006). Any
further increase in joint moment can thus be ascribed to both heads of the
gastrocnemius muscle. Earlier studies also showed that the ankle joint moment
during stair descent (at standard height) can increase up to 75% of MVC
(Reeves et al., 2008). This
means that any modulation around this peak in joint moment can be ascribed to
the gastrocnemius muscle. Furthermore, the ankle joint moment increased even
further at higher step-heights, so it is expected that the relative
contribution of the gastrocnemius muscle as compared to the soleus muscle will
only increase in these circumstances. Nevertheless, we are presently unable to
confirm this notion experimentally because we cannot measure the force
contribution of different muscles converging in a common tendon.
Conclusion
The GM muscle fascicles shortened during the touch-down phase of stair
descent while the GM MTC was lengthening. This indicates that the muscle
fascicles were performing positive work while the whole GM MTC was performing
negative work, consistent with the negative ankle joint power. With increased
step-height, the requirements for net negative work of the lower extremity
increased, which resulted in an increase in GM MTC lengthening, suggesting an
increase in negative work performed by the GM MTC. In contrast, the GM muscle
fascicles shortened more at higher step-heights, performing more positive
work, which was consistent with our hypothesis. Adding 20% body mass altered
the movement strategy in such a way that the extra load was supported by the
trailing leg during touch-down, which resulted in the leading leg being loaded
as in the normal situation. Therefore, GM muscle fascicle behaviour was not
altered under the influence of extra body mass, in contradiction to our
hypothesis.
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Acknowledgments |
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