|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online March 14, 2008
Journal of Experimental Biology 211, 1087-1092 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.012443
The effect of increasing inertia upon vertical ground reaction forces and temporal kinematics during locomotion
1 Wyle's Life Sciences Group, Houston, TX 77058, USA
2 NASA – Johnson Space Center, Houston, TX, USA
3 Department of Physical Therapy, The University of Texas Medical Branch,
Galveston, TX, USA
* Author for correspondence (e-mail: john.k.dewitt{at}nasa.gov)
Accepted 29 January 2008
 |
Summary |
|---|
 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Key words: biomechanics, locomotion, ground reaction force, inertia, gravity, weight, mass, human
|
INTRODUCTION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
;
LeBlanc et al., 2000;
Iwamoto et al., 2005).
Locomotive exercise is performed by astronauts, resulting in a ground reaction
force (GRF) that may create bone strains hypothesized to be an osteogenic
stimulus (Rubin and Lanyon,
1985). An external load is applied through a waist and shoulder
harness that anchors the astronaut to the treadmill and acts to replace
gravity. McCrory et al. (McCrory et al.,
2002) suggest that achieving an external load equivalent to 100%
of body weight is beneficial to generating GRF similar to those experienced in
normal gravity. However, in actual microgravity conditions, increasing
external loads may not be sufficient to recreate all components of GRF
trajectories experienced in normal gravity
(Schaffner et al., 2005).
Force is a product of mass and acceleration. The net GRF during locomotion
will be equal to the force between the foot and ground necessary to support
the body weight (gravitational force) and to accelerate the mass of the
subject (inertial force) and can be expressed as:
GRF=m(g+a), where m is the mass of
the subject, g is the force of gravity, and a is the
acceleration of the whole body center of mass (COM)
(Munro et al., 1987).
Therefore, the net GRF will be affected by gravitational and inertial
components.
In microgravity, g is the external load returning the
astronaut to the treadmill. The decreased GRF found by Schaffner et al.
(Schaffner et al., 2005) could
be due to reduced gravitational forces, altered locomotion mechanics, or a
combination of these factors. However, if gravity and mechanics remain
consistent, an increase in mass should result in an increase in GRF. Based on
this framework, the decreased GRF that occurs as gravity is reduced could be
compensated for with an increase in inertial mass if the mechanics of motion
remain unchanged as inertial mass is added. More specifically, if the motion
of the body does not change, the increase in GRF at a given gravitational
level should be equivalent to the increase in mass.
The added inertial mass hypothesis can be tested in the laboratory using overhead suspension. Gravitational weight can be increased by adding inertial mass, while simultaneously applying an equal but opposite force to relieve the additional weight. Past researchers have examined the effect of increasing inertial and gravitational force upon walking and running.
Increasing inertial and gravitational force has been shown to increase
metabolic cost and joint forces during walking
(Griffin et al., 2003).
Grabowski et al. (Grabowski et al.,
2005) found that increasing inertia without increasing
gravitational force during treadmill walking at 1.25 m s–1
resulted in an increased oxygen consumption, suggesting that the expenditure
of metabolic energy used to overcome inertial forces is independent of that
used to overcome gravitational forces. Chang et al.
(Chang et al., 2000) found that
during treadmill running at 3.0 m s–1, an increase in inertia
without a corresponding increase in gravitational force did not result in an
increase in GRF, and Teunissen et al.
(Teunissen et al., 2007) found
no changes in net metabolic cost.
There have been no studies examining the effect of added inertial force (AIF) without adding gravitational weight on GRF during walking, and none examining walking and running using identical methodologies. Since metabolic costs were affected differently with AIF between walking and running, the mechanical adaptations to AIF may be locomotion mode dependent.
The purpose of the current investigation was to examine the effect of AIF while maintaining gravitational force on GRF during walking and running. Specifically, we hypothesized that there will be increases in GRF that accompany increased inertial mass while maintaining gravitational force that are equivalent to the increase in mass, but that the adaptations will differ between the two modes of locomotion. The results of this investigation may help to better explain the effects of inertial forces on locomotion independent of gravitational forces.
|
MATERIALS AND METHODS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Instrumentation
Vertical GRF data were collected using a force-measuring treadmill (Kistler
Gaitway, Amherst, NY, USA). GRF data were sampled at a rate of 481.2 Hz with
two force plates beneath the running tread. Force plates were arranged so that
one plate rested in front of the other. Each plate contained four
piezoelectric load cells that measured vertical GRF and allowed for
determination of the center of pressure.
Inertial force was added to each subject using a weighted exercise vest (X-Vest; Perform Better, Cranston, RI, USA). The vest had pockets located around the upper and lower trunk for the addition of weights. Each pocket was fitted with slots in which up to 21 individual 0.45 kg masses could be placed. Slots for weights were located on both inner sides of each pocket (10 on the outer side; 11 on the inner side). During trials, masses were added equally to the front and rear of the vest. The masses were always added to the inner-lower slots first, followed by inner-upper, outer-lower and outer-upper slots.
Gravitational force was maintained with an overhead unweighting system (H/P/Cosmos Airwalk, Nussdorf, Germany). The system provided a constant upward force via a pneumatic pump that unweighted subjects through use of a harness worn about the waist and thighs (Fig. 1).
|
Treatment randomization occurred independently for each speed. To assure
that there was a balance of AIF conditions between subjects, a balanced Latin
square random assignment was used (Portney
and Watkins, 2000). The design allowed for a balance of treatment
orders so that no two testing sequences were the same for different subjects
within each speed. Each subject was randomly assigned a sequence from the
table, with only one subject completing each specific order. Trial order
assignment occurred separately for each speed. The subjects wore the
unweighting harness during all conditions, including the 0% AIF trial.
Upon arrival at the laboratory, each subject was provided with standardized running shoes (Xccelerator TR; NIKE, Inc., Beaverton, OR, USA) and completed a general health questionnaire. Once the unweighting harness had been donned, the subject's weight was measured by the force treadmill. This weight was used to compute the amount of AIF required to achieve each condition.
Data were collected at two speeds during five AIF treatments. Subjects walked at 1.34 m s–1 and ran at 3.13 m s–1. In addition to a control condition of no added inertial force (0% AIF), inertial force was added while body weight was maintained. We added an additional 10%, 20%, 30% and 40% of body weight and mass to each subject. For each AIF condition, the added weight was relieved with the unloading system so that the net force between the subject and treadmill remained equal to 100% body weight.
Subjects completed approximately one minute of treadmill locomotion at each AIF condition. Data collection began once the subjects had achieved a steady walking or running pace. Immediately following one minute of data collection, the weighted vest was removed and the unweighting harness was released. The subject then completed three minutes of walking at 1.34 m s–1 to eliminate any adaptation to gait that may have occurred during the test condition. The subjects were given additional rest of approximately three to four minutes until they felt that they were ready to continue with the next AIF condition.
Data processing
The first 10 strides of the left leg were analyzed in each one-minute
trial. The left side only was analyzed with the assumption that gait
kinematics were symmetrical within subjects
(Karamanidis et al., 2003).
The chosen epoch began with the first heel strike of the left foot and ended
with the eleventh heel strike of the left foot. Data analyses were performed
using software written in Visual Basic for Applications interfaced with
Microsoft Excel 2003 SP1 (Redmond, WA, USA) and MATLAB Version 7.2.0.232
(R2006a) (Mathworks, Natick, MA, USA).
Custom software converted the output from each force sensor to vertical GRF and center of pressure location. Raw voltage data from the eight load sensors in the treadmill force platforms were transformed into forces using calibration factors. The total vertical GRF during each sample was then calculated as the sum of the vertical forces measured by each sensor. Center of pressure during each sample was determined relative to the force platform reference frame using the force outputs from each sensor along with the dimensions of the force sensors relative to one another. Center of pressure locations were used to determine which foot was in contact with the treadmill during each step.
Data analyses
The instance of heel strike for each stride was determined using GRF data
according to the criterion of Chang et al.
(Chang et al., 2000). An
automated algorithm determined heel strikes as the samples at which a positive
change in the force greater than 1 N s–1 occurred when the
force magnitude was less than 100 N. The time of toe-off was computed in a
similar manner. The toe-off sample was defined as the sample at which a
negative change in the GRF less than 1 N s–1 occurred when
the magnitude of the force was less than 100 N.
GRF data were used to find contact time, stride time, peak vertical impact force, loading rate, peak vertical propulsive force and impulse for each trial. All analyses were completed using raw GRF to ensure that peak values were not dampened. Visual inspection of each footfall was used to ensure that there were no anomalous data.
Contact time was the length of time that the left foot was in contact with the treadmill during each stride and was calculated as the duration between heel strike and toe-off for each left footfall. Stride time was the length of time between successive heel strikes of the left foot. Peak vertical impact force was the magnitude of the first distinct peak in the GRF trajectory. Peak vertical propulsive force was the magnitude of the second distinct peak. Loading rate was the peak vertical impact force divided by the time between heel strike and time of peak vertical impact force. The impulse for each footfall was computed as the integral of the GRF trajectory over contact time. Peak vertical impact force, loading rate, peak vertical propulsive force and impulse were all normalized to actual body weight found prior to the data collection session to allow for inter-subject comparisons.
|
|
RESULTS |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
|
|
|
|
During running, peak vertical impact forces and loading rate decreased as inertial force was added. There was no effect of AIF during running for peak vertical propulsive force. The impulse during the 40% AIF condition was greater than during the 0% AIF condition.
|
DISCUSSION |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Ground reaction force is a product of mass, acceleration due to gravity, and acceleration of the COM due to motion. Gravitational forces are those that are associated with weight, while inertial forces are those associated with the acceleration of a mass. Our experiment used overhead suspension to keep gravitational forces constant as mass increased. Therefore, unless adaptation in motion occurred, an increased mass should result in an equal increase in GRF.
Locomotive adaptations to AIF differ between walking and running
We found increases in the impact GRF with AIF during walking, but of lesser
magnitude than could be explained by the increase in mass. In addition,
propulsive GRF decreased with AIF while impulse was not affected. During
running, impulse was also unchanged except at the highest AIF by decreasing
impact GRF while maintaining propulsive GRF and increasing contact time.
The impulse–momentum relationship states that the change in momentum of the COM will be equal to the impulse applied. Momentum is the product of mass and change in velocity. Therefore, for a given impulse, an increase in mass will result in a decrease in the change in velocity during the time that the force is applied. The change in velocity of the COM during stance is equal to the final upward velocity minus the initial downward velocity. Since impulse did not change with AIF during either mode of locomotion, the net change in velocity of the COM during contact must have decreased. It is not clear if the impact velocity decreased, the takeoff velocity decreased, or if both were modified. However, the different responses of the GRF to the AIF indicate that adaptations in motion differ between walking and running but may be intended to produce the same result.
During walking, subjects maintained contact time and stride time, except
during the most extreme loading condition (40% AIF), where stride time
increased. Increases in peak impact GRF and loading rate were less than
predicted and were coupled with a decrease in propulsive GRF. The impact GRF
increases continued up to 30% AIF and may explain the increases in metabolic
costs with added inertia reported by Griffin et al.
(Griffin et al., 2003) and
Grabowski et al. (Grabowski et al.,
2005).
Because impulse did not change with AIF, the net change in the COM velocity during stance decreased. During the impact phase of walking, the increase in GRF with AIF, although less than predicted, decelerated the COM downward at a greater rate than when unloaded or loaded to lesser mass. However, because the increase in GRF was less than the increase in mass, the net change in downward velocity of the COM must have decreased. It is probable, therefore, given that the final downward velocity of the COM was zero, the impact COM velocity decreased with AIF. During the second half of stance, the decrease in propulsive GRF acting over the same time period resulted in a COM upward takeoff velocity that was slower than that occurring with less AIF. The net change, however, decreased given the increase in mass.
During running, the impact GRF decreased with AIF. The reduction in GRF
coupled with the increase in mass resulted in the reduction of the COM
downward deceleration when compared to unloaded conditions. Similar to
walking, it is probable that the decrease in net change of COM velocity was
caused by a decrease in COM velocity at impact. Since the propulsive GRF of
running did not change, but mass increased, the upward acceleration of the COM
was also less than during when unloaded, resulting in a lower COM takeoff
velocity. The lack of increase in GRF with AIF may also explain the lack of
increase in the net metabolic cost of running reported by Teunissen et al.
(Teunissen et al., 2007).
Running contact time increased with AIF. The increase in contact time allowed for the maintenance of impulse by allowing the GRF to act upon the COM for a longer time. The increase in contact time with no change in stride time suggests that subjects spent less time airborne as inertial forces were added. Less flight time is consistent with the explanation that impact and takeoff COM velocities decreased.
Since heel strike and takeoff velocities decreased with AIF, it is probable
that the trajectory of the COM changed as inertial forces increased. During
walking, the trajectory of the body's COM can be approximated as an inverted
arc (Alexander, 1976;
Lee and Farley, 1998). The COM
is highest during single-limb support and lowest during double-limb support
(Chou et al., 2001). The
decrease in takeoff velocity will result in a lower maximum height of the COM,
causing the arc to become flatter. The downward displacement during
double-support will be dependent upon the upward force applied and the time
the impact force acts. The flatter trajectory would also decrease the vertical
downward velocity of the COM at heel strike, explaining the lower than
expected GRF. While the same result occurs for walking and running, the manner
in which the subjects manipulate the GRF differs for each locomotion mode.
For both speeds, significant AIF effects were detected at 40% AIF, our highest inertial condition tested, for many of our dependent variables. Stride time increased for both walking and running, and there was a slight decrease in impact forces and loading rates from 30 to 40% AIF. A threshold effect may occur at increases in inertial forces greater than 30% of normal. The threshold might occur as a protective mechanism against injury, since increased loading could increase the risk of bone or muscle damage.
Adaptations to gravitational and inertial forces
Our findings suggest that adaptations during locomotion to altered inertial
forces differed from adaptations to altered net gravitational forces. We found
that adding inertial force while maintaining gravitational force did not
appreciably affect walking temporal kinematics, while during running, contact
time and stride time increased. Donelan and Kram
(Donelan and Kram, 1997) found
that contact time and stride time decreased during walking with decreased
gravitational force while maintaining inertial force. Griffin et al.
(Griffin et al., 1999) and
Finch et al. (Finch et al.,
1991) also found that stride time did not significantly change
during walking. He et al. (He et al.,
1991) and Millslagle et al.
(Millslagle et al., 2005)
found that, during running, contact time decreased and stride time increased.
Farley and McMahon (Farley and McMahon,
1992) found no increases in contact time with the reduction of
gravity during running. Both Chang et al.
(Chang et al., 2000) and Newman
et al. (Newman et al., 1994)
found decreases in peak vertical GRF during running, which makes sense since
there is a reduced need to decelerate and accelerate the COM.
Others have studied the effect of increasing gravitational and inertial
forces on locomotion. Contact time increased and stride time decreased during
walking and running (LaFiandra et al.,
2003; Chang et al.,
2000). Griffin et al. (Griffin
et al., 2003) found increases in peak GRF during walking, and
Chang et al. (Chang et al.,
2000) found similar increases in GRF during running. The increase
in GRF is intuitive because of the need to decelerate and accelerate a larger
mass during stance.
Taken together, these studies suggest that an increase in inertial and gravitational force results in kinematic adaptations during walking and running that include increased contact time, decreased stride times, and increased GRF. Decreases in gravitational force with constant inertia resulted in decreased contact time, no change in stride time, and decreased GRF. Contact time and vertical GRF during walking and running may be directly influenced by gravitational forces. Stride time, however, may only adapt when gravitational force is decreased.
Our results suggest that, during walking, gravitational force is a critical
factor utilized for selection and execution of the gait pattern, although
adaptations to increased inertia do occur. However, during running, inertial
forces play a larger role in the control process. Bernstein
(Bernstein, 1967) theorized
that motion requires the interaction between the central nervous system and
the state of the position, velocity and weight of the affected limbs. Our
findings are consistent with this theory, because if gravitational forces were
the main input when determining the motion patterns during locomotion, no
adaptations to increased inertial force on the trunk should occur, and GRF
will increase with AIF as predicted by mechanical equations of motion.
Limitations
While our original question was to investigate a potential enhancement to
exercise countermeasures performed in microgravity, we tested our hypothesis
in normal gravity using an overhead suspension system. Our testing location
allowed data collection from multiple subjects in a controlled environment.
The subjects could familiarize themselves with the testing environment, and
testing sessions were not limited by factors that influence experiments in
microgravity, such as limited sample size and availability to collect data.
However, because we tested subjects in normal gravity, it is possible that our
results would differ in a microgravity environment. The AIF were applied in a
manner that could be used during spaceflight. However, in our experiment, the
limbs were subject to normal gravity, and the gravitational forces resulting
from the AIF were reduced by suspension with a harness. The harness could have
influenced the adaptations that we measured.
It must be noted that we were unable to measure changes in horizontal
forces, potentially affecting our results. It was impossible to determine if
the reduction in vertical GRF was accompanied by an increase in horizontal
GRF. If this were to occur, the magnitude of the GRF would be unaffected, but
the orientation of the GRF vector would change. This could be another
explanation for the reduced vertical GRF with increased inertial forces.
However, Chang et al. (Chang et al.,
2000) found that during running with increased inertial forces,
the orientation of the GRF vector did not change. Since our experimental setup
was similar to theirs, we have no reason to believe that a change in GRF
vector occurred. In addition, given that we tested all subjects in the same
manner, we believe any effects of the horizontal GRF would be systematic.
Chang et al. (Chang et al.,
2000) did find an increase in horizontal impulse when inertial
forces were increased and gravitational forces were held constant. However,
the increases were not linearly related to the amount of added inertia.
Applications to spaceflight exercise
One of our intents in this investigation was to determine if locomotive
exercise performed in reduced gravity could be enhanced with the addition of
inertial force. We hypothesized that adding inertial force may help to
increase the vertical GRF that occurs during treadmill exercise, and thus
enhance the current countermeasure. Our findings in normal gravity suggest
that the addition of inertial force may increase walking GRF during space
flight. However, adaptations in the gait pattern during running would likely
mitigate increases in GRF. Confirmation of these suppositions could be gained
only during a microgravity experiment.
Conclusion
We investigated the effects of adding inertial force while maintaining
gravitational force upon temporal kinematics and GRF during walking and
running. Perhaps the most interesting finding is that the adaptations to AIF
during walking were different than during running, suggesting that walking and
running should be thought of as two distinct tasks, rather than alternate
forms of locomotion. However, the control strategies utilized as a result of
increased inertial forces may attempt to maintain impulse by adapting motion
kinematics. Furthermore, changes in motion that occur due to increases in
inertial forces are not the same as those occurring due to increases in
gravitational forces. Researchers should differentiate between adding mass and
weight during biomechanical investigations.
|
Acknowledgments |
|---|
|
References |
|---|
|
TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
|---|
Alexander, R. McN. (1976). Mechanics of bipedal locomotion. In Perspectives in Experimental Biology. Vol. 1 (ed. P. S. Davies), pp.493 -504. Oxford: Pergamon Press.
Bernstein, N. A. (1967). The problem of the interrelation of coordination and localization. In The Coordination and Regulation of Movements (ed. N. A. Bernstein), pp.15 -59. Oxford: Pergamon Press.
Chang, Y. H., Huang, H. W. C., Hamerski, C. M. and Kram, R. (2000). The independent effects of gravity and inertia on running mechanics. J. Exp. Biol. 203,229 -238.[Abstract]
Chou, L. S., Kaufman, K. R., Brey, R. H. and Draganich, L. F. (2001). Motion of the whole body's center of mass when stepping over obstacles of different heights. Gait Posture 13,17 -26.[CrossRef][Medline]
Donelan, J. M. and Kram, R. (1997). The effect of reduced gravity on the kinematics of human walking: a test of the dynamic similarity hypothesis for locomotion. J. Exp. Biol. 200,3193 -3201.[Abstract]
Farley, C. T. and McMahon, T. A. (1992). Energetics of walking and running: insights from simulated reduced-gravity experiments. J. Appl. Physiol. 6,2709 -2712.
Finch, L., Barbeau, H. and Arsenault, B.
(1991). Influence of body weight support on normal human gait:
development of a gait retraining strategy. Phys. Ther.
71,842
-855.
Grabowski, A., Farley, C. T. and Kram, R.
(2005). Independent metabolic costs of supporting body weight and
accelerating body mass during walking. J. Appl.
Physiol. 98,579
-583.
Griffin, T. M., Tolani, N. A. and Kram, R.
(1999). Walking in simulated reduced gravity: mechanical energy
fluctuations and exchange. J. Appl. Physiol.
86,383
-390.
Griffin, T. M., Roberts, T. J. and Kram, R.
(2003). Metabolic cost of generating muscular force in human
walking: insights from load-carrying and speed experiments. J.
Appl. Physiol. 95,172
-183.
He, J. P., Kram, R. and McMahon, T. A. (1991).
Mechanics of running under simulated low gravity. J. Appl.
Physiol. 71,863
-870.
Iwamoto, J., Takeda, T. and Sato, Y. (2005). Interventions to prevent bone loss in astronauts during space flight. Keio J. Med. 54,55 -59.[CrossRef][Medline]
Karamanidis, K., Arampatzis, A. and Bruggemann, G. P. (2003). Symmetry and reproducibility of kinematic parameters during various running techniques. Med. Sci. Sports Exerc. 35,1009 -1016.
LaFiandra, M., Wagenaar, R. C. and Obusek, J. P. (2003). How do load carriage and walking speed influence trunk coordination and stride parameters? J. Biomech. 36, 87-95.[CrossRef][Medline]
LeBlanc, A., Lin, C., Shackelford, L., Sinitsyn, V., Evans, H.,
Belichenko, O., Schenkman, B., Kozlovskaya, I., Oganov, V., Bakulin, A. et
al. (2000). Muscle volume, MRI relaxation times (T2), and
body composition after spaceflight. J. Appl. Physiol.
89,2158
-2164.
Lee, C. R. and Farley, C. T. (1998). Determinants of the center of mass trajectory in human walking and running. J. Exp. Biol. 201,2935 -2944.[Abstract]
McCrory, J. L., Baron, H. A., Balkin, S. and Cavanagh, P. R. (2002). Locomotion in simulated microgravity: gravity replacement loads. Aviat. Space Environ. Med. 73,625 -631.[Medline]
Millslagle, S., Levy, M. and Matack, N. (2005). Comparative kinematic measures of treadmill running with or without body weight support in runners. J. Sports Sci. Med. 4, 583-589.
Munro, C. F., Miller, D. I. and Fuglevand, A. J. (1987). Ground reaction forces in running: a reexamination. J. Biomech. 20,147 -155.[CrossRef][Medline]
Newman, D. J., Alexander, H. L. and Webbon, B. W. (1994). Energetics and mechanics for partial gravity locomotion. Aviat. Space Environ. Med. 65,815 -823.[Medline]
Portney, L. G. and Watkins, M. P. (2000).Foundations of Clinical Research: Applications to Practice (1st edn) . Upper Saddle River, NJ: Prentice Hall.
Rubin, C. T. and Lanyon, L. E. (1985). Regulation of bone mass by mechanical strain magnitude. Calcif. Tissue Int. 37,411 -417.[Medline]
Schaffner, G., De Witt, J. K., Bentley, J. R., Yarmanova, E., Kozlovskaya, I. and Hagan, R. D. (2005). Effect of subject loading device load levels on gait. NASA Technical Report TP-2005-213169, http://ntrs.nasa.gov/search.jsp.
Schneider, V., Oganov, V., LeBlanc, A., Rakmanov, A., Taggart, L., Bakulin, A., Huntoon, C., Grigoriev, A. and Varonin, L. (1995). Bone and body mass changes during space flight. Acta Astronautica 36,463 -466.[CrossRef][Medline]
Teunissen, L. P. J., Grabowski, A. and Kram, R.
(2007). Effects of independently altering body weight and body
mass on the metabolic cost of running. J. Exp. Biol.
210,4418
-4427.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
Related articles in JEB:
This article has been cited by other articles:
|
|
 |
|
  C. J. Arellano, D. P. O'Connor, C. Layne, and M. J. Kurz The independent effect of added mass on the stability of the sagittal plane leg kinematics during steady-state human walking J. Exp. Biol., June 15, 2009; 212(12): 1965 - 1970. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Phillips TACKLING SPACE FLIGHT BONE LOSS J. Exp. Biol., April 1, 2008; 211(7): i - i. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
