|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online October 18, 2006
Journal of Experimental Biology 209, 4389-4397 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02518
Effect of speed on stride parameters in racehorses at gallop in field conditions
1 Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead
Lane, Hatfield, Hertfordshire, AL9 7TA, UK
2 Structure and Motion Laboratory, University College London, Royal National
Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex HA7 4LP,
UK
Author for correspondence at address 1 (e-mail:
awilson{at}rvc.ac.uk)
Accepted 29 August 2006
 |
Summary |
|---|
 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Key words: biomechanics, locomotion, horse, duty factor, speed, gallop, equine
|
Introduction |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
). All of these animals attain their top speed (40 mph) with a
similar minimum duty factor (0.2) and hence similar peak vertical force
(approximately 2x body weight). Although the determinants of maximum
running speed have been examined in detail in human athletes, who can achieve
speeds approximately half as fast (Weyand
et al., 2000; Usherwood and
Wilson, 2006), and in dogs
(Usherwood and Wilson, 2005),
similar studies of large galloping quadrupeds are sparse. This study therefore
set out to describe in detail the stride parameters of a group of elite
racehorses performing high-speed over-ground locomotion.
Elongated limbs should enable an athlete to achieve longer stance times and
take longer strides, and slender limbs combined with fast muscle fibres should
allow for more rapid repositioning of the limbs during the protraction phase
and hence higher stride frequencies. However, longer limbs do not
automatically result in longer strides
(Armstrong and Cooksey, 1983)
and protraction duration is unlikely to be greatly affected by muscle fibre
speed, given that limb protraction is, at least in horses, a largely passive
process, achieved through elastic recoil rather than active muscle work
(Heglund et al., 1982;
Wilson et al., 2003).
In humans, minimum protraction duration is the same regardless of subject
ability and within a subject, protraction duration is the same during declined
and inclined running, even though the maximum speed is markedly different.
Maximum attainable limb force, on the other hand, is significantly higher in
fast versus slow runners and is higher for declined versus
inclined running. Therefore, humans achieve faster top running speeds with
greater peak vertical ground reaction forces rather than more rapid leg
movements (Weyand et al.,
2000). This is not true for greyhounds
(Usherwood and Wilson, 2005)
and may not be true for large quadrupeds.
Direct measurement of ground reaction force during highspeed locomotion in
large animals is extremely difficult. Force measuring treadmills have been
used in horses (Weishaupt et al.,
2002) but treadmill gait is not completely normal and it would be
difficult to study fit top class racehorses at their maximum attainable speed
in that environment. Force shoes have been used in horses with some success
(Björk, 1958;
Frederick, Jr and Henderson,
1970; Hugelshofer,
1982; Kai et al.,
2000; Ratzlaff et al.,
1985; Ratzlaff et al.,
1990; Roepstorff and Drevemo,
1993) but their mass and size may influence locomotion. The linear
relationship between metacarpo-phalangeal joint extension angle and vertical
limb force can be used (McGuigan and
Wilson, 2003); however, this requires the collection of optical
motion capture data, which is very difficult for more than a few strides under
field conditions due to the resolution required for accurate angle
measurements and the protective boots worn by exercising horses.
As the speed of a running animal increases, the duration of the protraction
phase remains relatively constant (Pratt
and O`Conner, 1978), but stance time drops resulting in an
increase in stride frequency. The impulse applied to the animal's centre of
mass must remain constant for a given stride duration, therefore knowledge of
the duty factor (the fraction of the stride for which the limb is in stance)
provides the basis for the prediction of peak limb force during high-speed
over-ground locomotion (Alexander et al.,
1979; Witte et al.,
2004). This technique remains the easiest means of investigating
the relationship between limb force and running speed in all four limbs of
large animals during real-life activities. Stride timing variables (stance
duration and stride duration) can be measured in the horse using foot-mounted
accelerometers, which have been shown to be accurate to within 2.3 ms and 3.5
ms for the timing of foot-on and foot-off, respectively
(Witte et al., 2004). The
force estimated using this method is most accurate for animals performing
symmetrical gaits, such as trotting, where trunk mass is distributed evenly
between a pair of limbs. It may be less accurate, however, for the
asymmetrical gaits, where the assumption that paired legs apply the same
impulse is not necessarily true (Minetti,
1998; Witte et al.,
2004).
When travelling at high speeds, quadrupeds switch from symmetrical gaits,
where the footfalls of a pair of limbs (fore- or hind-) are evenly spaced in
time, to asymmetrical gaits, such as galloping, where the two limbs of a pair
strike the ground in couplets (Hildebrand,
1989). These gaits are analogous to a child skipping, and indeed a
galloping horse has been likened to two skipping bipeds linked by a trunk
(Minetti, 1998). The first
limb of a couplet to strike the ground is known as the non-lead limb and the
second the lead limb. For the forelimbs this means that the lead limb is the
last leg to leave the ground before the aerial, or flight, phase during which
there are no limbs in contact with the ground. The sequence of footfalls is
therefore non-lead hindlimb, lead hindlimb, non-lead forelimb and finally lead
forelimb prior to the aerial phase (Fig.
1). This sequence means that the function of the four individual
limbs of a galloping quadruped cannot be assumed to be equivalent. The ground
reaction force experienced by the non-lead limb at a slow canter is 25% higher
than that of the lead limb, although this difference declines toward symmetry
with increasing speed (Witte et al.,
2004). However, the effect of speed on the stance duration of the
lead and non-lead limbs of a galloping horse is unknown. As speed increases,
the degree of overlap between the limb stance phases decreases until the gait
takes on a true four-beat rhythm, with the lead and non-lead limbs functioning
almost independently. The degree of overlap between the stance phases of a
pair of limbs has been proposed to be a predictor of potential racing
performance and injury risk in Thoroughbred racehorses
(Pratt and O`Conner, 1978).
The ratio of forelimb to hindlimb impulses is 57:43
(Merkens et al., 1991) or
56:44 (Merkens et al., 1993)
during ridden canter and this ratio has been shown to be independent of speed
and gait across walk, trot and canter in unridden horses
(Witte et al., 2004). However,
the ratio of peak forces decreased slightly over a range of speeds at trot and
led canter.
|
This study set out to examine the speed dependence of fore- and hindlimb duty factor and predicted limb force during high-speed locomotion in the fit racehorse.
|
Materials and methods |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Each horse was equipped with four foot-mounted accelerometers and the
jockey with a stand-alone GPS data logger. A ±50 g accelerometer
(ADXL150, Analog Devices, USA) was mounted on the dorsal midline of each hoof,
ensuring that the sensitive axis of the sensor was orientated axially
(Witte et al., 2004). Data
were telemetered via a narrow band FM telemetry transmitter (ST500,
Wood and Douglas Ltd, Tadley, UK), which was secured with a battery to the
lateral aspect of the third metacarpal/metatarsal bone within an elasticated
exercise bandage (Fig. 2). The
data were received by an SR500 telemetry receiver (Wood and Douglas Ltd) and
were logged via a 12-bit A/D converter and PCMCIA card (DAQcard700,
National Instruments, Newbury, UK) into a laptop computer running custom
software written in MATLAB (The Mathworks, Natick, MA, USA). The accelerometer
output file was time-stamped to allow for subsequent synchronisation with the
GPS unit. The telemetry receivers and laptop computers were placed in a
vehicle, which was driven round the inside of the track level with the horses
as they were galloping. The receivers therefore stayed within range for the
duration of data collection.
|
A self-contained GPS data logger (modified G30-L, Laipac Technology Inc.,
Ontario, Canada) was configured to log the minimum recommended GPS data
(GPRMC) i.e. speed (knots), position (latitude and longitude), time (Universal
Time Constant) and date, once per second
(Witte and Wilson, 2004). The
device (dimensions 70 mmx50 mmx25 mm and mass 95 g) was mounted
securely on the rider's hat by means of a custom-made elasticated strap and
was powered on as the horses left the yard. Data were logged continuously from
this time for the duration of the exercise.
The horses were also equipped with an inertial sensor, which forms the
basis of another paper (Pfau et al.,
2006).
The horses were ridden by their regular exercise rider during the study. They were exercised in groups of three, although data were collected from only one horse at a time. The horses were warmed up by walking and trotting for approximately 10 min on a sand based racetrack (Polytrack, Martin Collins, UK). They were then accelerated to canter over a few seconds, cantered at a steady speed for 600 m, before gradually accelerating to maximum speed over a further 400 m. Accelerometer data were collected only during canter and gallop exercise, which was approximately 2 min in duration. This encompassed the entire range of exercise speeds. Exercise duration was kept brief to ensure that the horses did not become fatigued during data collection (which could affect the results).
Data analysis
GPS data in the standard NMEA 0183 format were downloaded from the data
logger using GPS Wedge Software (CommLinx Solutions Pty Ltd, Lutana, TAS,
Australia). Speed and time data were extracted for each position fix using
custom software written in MATLAB.
Accelerometer data were imported into data transcription software (Audio
Transcriber, freeware,
http://www.etca.fr/CTA/gip/Projets/Transcriber/).
Features corresponding to `foot-on' and `foot-off' have been defined
previously by comparison to force plate data
(Witte et al., 2004). These
features were identified and the relative timing of these events was recorded.
The times of foot-on and foot-off were used to calculate stance duration,
protraction duration and duty factor for each stride of each leg. Stance
length was defined as the distance travelled by the trunk during the stance
phase of an individual limb and was calculated as stance duration multiplied
by horse velocity. Peak vertical ground reaction force was predicted for each
limb using the following equation
(Alexander et al., 1979):
 |
where Fzmax=peak vertical ground reaction force (N),
P=the relative impulse of the pair of legs in question (0.57 and
0.43, for the front and rear pairs, respectively
(Witte et al., 2004),
m=mass of animal (kg, including mass of jockey and tack),
g=gravitational constant (9.81 m s-1) and
ß=duty factor. Predicted forces were then normalised to body mass.
For each stride the fore- and hind-pairs of limbs were defined as lead and non-lead limbs from the relative timing of the footfalls for that pair of legs. Each limb-stride was therefore described as either `lead' or `non-lead'. The mid-point of each stride was determined and GPS speed was interpolated to calculate horse speed at that time.
Stride data below 8 m s-1 were discarded, as these speeds
constituted trot locomotion and locomotion through the trot-gallop transition.
In addition, low-pass filtering of speed data by the GPS receiver makes speed
measurements during rapid acceleration and deceleration inaccurate
(Witte and Wilson, 2004). The
strides thus discarded were few in number. Data above 8 m s-1 were
used for further analysis and categorised into 1 m s-1 speed bins
for the calculation of means, with the labels indicating the middle of the
range for each bin. The relationship of each variable to speed and the
influences of `lead' or `non-lead' and `fore-' or `hind-' were examined.
Influences were extracted using a general linear model with speed, lead or
non-lead and fore- or hind- as fixed factors, and horse identity as a random
factor, using SPSS (SPSS 12.0 for Windows, SPSS Inc., USA). Best-fit curves
were estimated for each variable for both fore- and hindlimbs for each
individual according to the second order polynomial function
[Y=b0+(b1t)+(b2t2)].
A second order polynomial function was chosen as the simplest model yielding
the best and most consistent fit to the variables of interest. Subsequently,
these models were used to predict values in each speed category for each
horse, and a population mean was determined from these data.
The duration of the aerial phase of the stride, when no limbs are in contact with the ground, was calculated as the difference between lead forelimb foot-off time and non-lead hindlimb foot-on time for each stride. The duration of contact, when there was at least one limb in contact with the ground, was therefore the difference between the stride duration and the aerial phase duration. There were no double aerial phases recorded during the study so this gave an accurate representation of the contact duration. The duration of overlap (period when two feet were on the ground) was calculated as the difference between the sum of the individual limb stance phases and the total contact duration.
|
Results |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
|
|
|
|
|
|
|
|
Over the speed range 9-17 m s-1, stance duration decreased on average from 131 ms to 77 ms and 143 ms to 94 ms for the fore- and hind legs, respectively (Fig. 3, Table 1). Hindlimbs consistently showed significantly longer stance durations than forelimbs across the entire speed range (P=0.003) with no convergence or divergence at higher speeds.
Protraction duration was slightly longer in the forelimbs than in the hindlimbs at all speeds (P=0.007, Fig. 4). The overall population trend was a moderate, significant decrease in protraction duration from 364 ms to 342 ms and 355 ms to 326 ms for fore-(P<0.001) and hindlimbs (P<0.001), respectively. A slightly greater decrease in lead forelimb protraction duration was evident at very high speed (16 and 17 m s-1); however, this was not seen consistently in the individual horse data and was not statistically significant (P=0.67).
Stride frequency increased linearly with speed, showing no evidence of a plateau at the higher speeds (Fig. 5). At 9 m s-1 the mean stride frequency was 2.02 and it increased to 2.41 strides s-1 at 17 m s-1.
Duty factor decreased curvilinearly with speed (Fig. 6). Across the entire speed range hindlimb duty factor was significantly higher than that of the forelimbs, reflecting the longer stance duration (P=0.004). The proportionate difference between hind- and forelimbs was consistent. The mean individual horse duty factor recorded at 17 m s-1 ranged from 0.170 to 0.196 m s-1 (mean 0.182 m s-1) for the forelimbs and 0.205 to 0.246 m s-1 (mean 0.222 m s-1) for the hindlimbs.
The difference between forelimb and hindlimb duty factor resulted in a difference in predicted peak vertical ground reaction force (Fig. 7). Forelimb force increased by 47% and hindlimb force by 31% between speeds of 9 and 17 m s-1. At 9 m s-1 forelimb force was 44% higher than hindlimb force and at 17 m s-1 it was 61% higher. The mean peak force predicted at 17 m s-1 was 24.7 N kg-1 body weight (range 22.6 to 26.0 N kg-1 body weight) for the forelimbs and 15.3 N kg-1 (range 13.7-16.2 N kg-1) for the hindlimbs.
The distance travelled by the trunk during the stance phase (the stance
length) was significantly higher in the hindlimbs than in the forelimbs across
the entire speed range (1.28 m versus 1.18 m at 9 m s-1
and 1.58 m versus 1.31 m at 17 m s-1, P=0.002,
Fig. 8). Stance length
(SL) can be related to half the angle swept by the limb
(approximately contact angle if the sweep is symmetrical around the vertical)
during the stance phase (
) by the function
=sin-1(0.5SL/LL), where LL=leg
length (m). Leg length was estimated assuming that the point of attachment of
the scapula to the trunk was 0.1 m lower than the height of the horse and that
the hindlimb and forelimb were the same length. Mean leg length was therefore
1.5 m. ranged from 23° at 9 m s-1 to 26° at 17 m
s-1 for the forelimbs and from 25° at 9 m s-1 to
32° at 17 m s-1 for the hindlimbs.
The population mean duration of the aerial phase of the stride decreased between 9 and 17 m s-1 from 135 to 119 ms, although when individual horse mean values were compared at these two speeds using univariate analysis of variance (ANOVA) within the general linear model, there was no significant difference (N=8, P=0.16). The duration of the contact phase decreased significantly from 376 to 311 ms (P<0.001) (Fig. 9A). Since there was also a concomitant decrease in stride duration (from 495 ms to 415 ms), these decreases were not apparent when the variables were expressed as percentages of the stride duration (the aerial phase remained at approximately 27% and the contact phase at 73% over the entire speed range). The duration of overlap decreased from 183 ms at 9 m s-1 to 35 ms at 17 m s-1 (from 36% to 15% of the total stride duration, P<0.001).
|
Discussion |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
).
Treadmill studies are an alternative to field studies; however, treadmill
locomotion has limited value as a representation of over-ground locomotion.
Gait patterns are altered during treadmill locomotion; for example, stance
duration is artificially lengthened
(Buchner et al., 1994;
Barrey et al., 1993). In
addition, the influence of surface could be considerable. The maximum speed
that can be attained on a standard equine treadmill without re-gearing
(
15 m s-1) is considerably below the top speed of elite equine
athletes [the record mean speed for a racehorse over 1/4 mile is 19 m
s-1 (Russell and McWhirter,
1988)]. Finally, it would be difficult to convince owners or
trainers to permit a treadmill study on fit racehorses due to the risk of
injury and the disruption to training. This study therefore represents the
first detailed investigation of the relationship of footfall timings and limb
force to running speed in all four legs of a horse galloping at high speed,
under real-world conditions.
The equipment employed during this study was lightweight and non-invasive
and did not hinder movement of either horse or rider. The mass of the
leg-mounted sensors was similar to the protective boots worn during routine
exercise. This means that the horses were able to perform a full training
session including maximal speed gallop under effectively normal training
conditions. The actual testing session was kept as short as possible in order
to ensure genuine high speed data without risking fatigue effects, for
instance a drop in stride frequency, which would alter the results
(Colborne et al., 2001).
The markedly higher peak vertical force on the forelimbs compared to the
hindlimbs, which has been reported at lower speed
(Merkens et al., 1993), is
indirectly confirmed here at high speed. The hindlimbs have a longer stance
duration but are a similar length and therefore sweep through a larger angle
during the stance phase. This results in the larger duty factor and therefore
a lower predicted force. This may be an adaptation to the major propulsive
function of the hindlimbs, and suggests that the limb should be less stiff
(Farley et al., 1993).
The forces predicted in our study are considerably higher than those
measured elsewhere. A force of approximately 9000 N was determined using an
instrumented horseshoe, which equates to 16.4 N kg-1 body weight
(Cheney et al., 1973). The peak
forelimb force predicted by our study is 24.7 N kg-1 body weight,
51% higher. The difference may be explained by the higher speed used here, the
presence of a rider and perhaps whether previous measurements were made on the
lead or non-lead limb. The mechanical roles of the lead and non-lead limbs
during high-speed asymmetrical gaits have not been fully defined. Certainly,
the forces experienced by lead and non-lead limbs are markedly different at
low speed. A 25% difference has been measured at slow ridden canter and a
similar difference has been predicted at 12 m s-1 during treadmill
locomotion (McGuigan and Wilson,
2003; Merkens et al.,
1993). However, it has also been shown during treadmill locomotion
that as speed increases, the peak lead and non-lead limb forces converge
(Witte et al., 2004),
suggesting that the predictions of mean force presented here will become more
accurate as speed increases. If the forces do not converge to symmetry as
anticipated the presented forces will represent an underestimate for the
non-lead limb and an overestimate for the lead limb, showing that previous
predictions of maximum limb load during galloping are indeed somewhat low.
Despite the increase in the stance length of the stride and hence the angle
through which the limb is swept with increasing speed, the protraction
duration fell. This could represent an active contribution to protraction, but
given the largely passive nature of the protraction process in the horse
(Wilson et al., 2003) this is
more likely to result, at least in the front legs, from the muscle-tendon unit
of biceps brachii being stretched further at the end of stance (due to higher
limb force and greater sweep of leg) storing more energy, and resulting in
greater protraction-phase limb acceleration.
The duration of the aerial phase of the stride was independent of speed.
When horses increased speed they reduced the overlap between legs, resulting
in the limbs functioning more sequentially, rather than synchronously
(Fig. 1). Reducing the overlap
duration enables horses to achieve speed increases, and accommodate the
concomitant decreases in individual limb stance durations, without increasing
the aerial phase duration. This is advantageous since a longer aerial phase
requires a greater vertical oscillation of the trunk and greater fluctuations
in potential energy, which may be energetically expensive. Extrapolating the
overlap-speed relationship upwards would predict that overlap would reach zero
at a speed of about 20 m s-1. Duration of limb overlap has been
suggested as a limit to maximum gallop speed and an indicator of injury risk
(Pratt and O'Conner, 1978),
though the mechanism is not clear.
The experiment was undertaken on a typical horse-racing surface. The
surface over which an individual locomotes acts in series with the leg and the
stiffness of the surface would therefore be expected to have an effect on the
data collected (McGuigan and Wilson,
2003). The limited data of McGuigan and Wilson show that a soft
surface skews the GRF curve to the right, delaying the time of peak force.
However, there was little change in the curve `fatness' that would affect the
force prediction. Although the track used was designed and maintained to
achieve constant racing conditions under all weather conditions, jockeys have
reported local variations in track stiffness. This may account for some of the
variation in the data collected.
This study employs GRF predictions from previous studies carried out during
ridden and non-ridden locomotion over force plates and on treadmills with
higher surface stiffness than those on which this study was undertaken, and
these represent a potential source of error. It has previously been shown that
a softer surface, such as Polytrack, acts as a plastic element in series with
the limb springs, reducing leg spring stiffness and hence rate of force rise
(Ferris and Farley, 1997;
Wilson et al., 2001). However,
the unloading curve is less affected due to lack of return from the plastic
rather than elastic deformation of equestrian surfaces
(Zebarth and Sheard, 1985). In
addition, vertical impulse must remain constant irrespective of surface
properties unless stride frequency changes. Therefore, it seems unlikely that
differences in surface properties will substantially affect the results
presented here, unless the shape of the GRF-time curve is dramatically
altered.
Overall the error of GRF peak force relative to the peak predicted by a
sine wave of the same base and area has been shown to be 7% at trot and 3% and
5% for the lead and non-lead limbs at canter, respectively
(Witte et al., 2004). An
additional error would result from the possibility that the impulse generated
by the lead and non-lead legs was different. These errors were 19 and 16%,
respectively (Witte et al.,
2004) at low speed canter, but this error declined with speed and
is likely to be small at the speeds considered here.
The potential influence of a rider on our data, when compared to the
un-ridden state used in some of the studies on which our predictions are
based, would be to alter the front:hind ratio of forces. However, this appears
unlikely due to the jockey's position directly over the centre of mass.
Indeed, the data of Merkens et al. indicate that a rider has little or no
effect on vertical limb force distribution at canter
(Merkens et al., 1991;
Merkens et al., 1993).
The techniques employed during this study offer the potential to study large cursorial animals travelling at high speed under field conditions. Studies of the influence of surface and incline on high-speed locomotion can be easily performed. The high peak forces predicted here suggest that previous estimates of the load on the musculoskeletal elements may have been underestimates. Therefore these tools may present the means by which the most appropriate methods of training racehorses can be investigated. The musculoskeletal structures of young racehorses respond to their mechanical environment. Appropriate training regimes that generate realistic stimuli to these structures will reduce the incidence of injury in these animals.
Significantly lower duty factors were measured for the forelimbs compared to the hindlimbs at all galloping speeds, which in combination with the front-back weight distribution, resulted in higher predicted forces in the front legs. There were no statistically significant differences between the lead and non-lead limbs in any of the variables examined. As speed increased stance time and duty factor dropped but flight duration remained constant. This was achieved by reducing the period of the stride where more than one leg was on the ground.
|
Acknowledgments |
|---|
|
Footnotes |
|---|
Present address: 25 Lodge Hill Road, Lower Bourne, Farnham, Surrey GU10
3QW, UK
|
References |
|---|
|
TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Alexander, R. M., Maloiy, G. M. O., Hunter, B., Jayes, A. S. and Nturibi, J. (1979). Mechanical stresses during fast locomotion of buffalo (Syncerus caffer) and elephant (Loxodonta africana). J. Zool. Lond. 189,135 -144.
Armstrong, L. E. and Cooksey, S. M. (1983). Biomechanical changes in selected collegiate sprinters due to increased velocity. Track Field Q. Rev. 3, 10-12.
Barrey, E., Galloux, P., Valette, J. P., Auvinet, B. and Wolter, R. (1993). Stride characteristics of overground versus treadmill locomotion in the saddle horse. Acta Anat. Basel 146,90 -94.[Medline]
Björk, G. (1958). Studies on the draught forces of horses: development of a method using strain gauges for measuring forces between hoof and ground. Acta Agric. Scand. 8, Suppl. 4.
Buchner, H. H., Savelberg, H. H., Schamhardt, H. C., Merkens, H. W. and Barneveld, A. (1994). Kinematics of treadmill versus overground locomotion in horses. Vet. Q. 16,S87 -S90.
Cheney, J. A., Liou, S. Y. and Wheat, J. D. (1973). Cannon-bone fracture in the thoroughbred racehorse. Med. Biol. Eng. 11,613 -620.[CrossRef][Medline]
Colborne, G. R., Birtles, D. M. and Cacchione, I. C. (2001). Electromyographic and kinematic indicators of fatigue in horses: a pilot study. Equine Vet. J. Suppl. 33, 89-93.
Farley, C. T., Glasheen, J. and McMahon, T. A. (1993). Running springs: speed and animal size. J. Exp. Biol. 185,71 -86.[Abstract]
Ferris, D. P. and Farley, C. T. (1997).
Interaction of leg stiffness and surfaces stiffness during human hopping.
J. Appl. Physiol. 82,15
-22.
Frederick, F. H., Jr and Henderson, J. M. (1970). Impact force measurement using preloaded transducers. Am. J. Vet. Res. 31,2279 -2283.[Medline]
Garland, T., Jr (1983). The relation between maximal running speed and body mass in terrestrial mammals. J. Zool. 199,157 -170.
Heglund, N. C., Fedak, M. A., Taylor, C. R. and Cavagna, G.
A. (1982). Energetics and mechanics of terrestrial
locomotion. IV. Total mechanical energy changes as a function of speed and
body size in birds and mammals. J. Exp. Biol.
97, 57-66.
Hildebrand, M. (1989). The quadrupedal gaits of vertebrates. The timing of leg movements relates to balance, body shape, agility, speed, and energy expenditure. BioScience 39,766 -775.[CrossRef]
Hugelshofer, J. (1982). Vergleichende Kraft-und Belastungszeit-Messungen an den Vorderhufen von gesunden und an Podotrochlose erkrankten Pferden. PhD thesis, University of Zurich, Switzerland.
Kai, M., Aoki, O., Hiraga, A., Oki, H. and Tokuriki, M. (2000). Use of an instrument sandwiched between the hoof and shoe to measure vertical ground reaction forces and three-dimensional acceleration at the walk, trot, and canter in horses. Am. J. Vet. Res. 61,979 -985.[CrossRef][Medline]
McGuigan, M. P. and Wilson, A. M. (2003). The
effect of gait and digital flexor muscle activation on limb compliance in the
forelimb of the horse Equus caballus. J. Exp. Biol.
206,1325
-1336.
Merkens, H. W., Schamhardt, H. C., van Osch, G. J. V. M. and van den Bogert, A. J. (1991). Ground reaction force analysis of Dutch warmblood horses at canter and jumping. In Equine Exercise Physiology. Vol. 3 (ed. S. G. B. Persson, A. Lindholm and L. B. Jeffcott), pp. 128-135. Davies, California: ICEEP Publications.
Merkens, H. W., Schamhardt, H. C., van Osch, G. J. and Hartman, W. (1993). Ground reaction force patterns of Dutch Warmbloods at the canter. Am. J. Vet. Res. 54,670 -674.[Medline]
Minetti, A. E. (1998). The biomechanics of skipping gaits: a third locomotion paradigm? Proc. R. Soc. Lond. B Biol. Sci. 265,1227 -1235.[Medline]
Pfau, T., Witte, T. H. and Wilson, A. M. (2006). Centre of mass movement and mechanical energy fluctuation during gallop locomotion in the Thoroughbred racehorse. J. Exp. Biol. 209,3743 -3757.
Pratt, G. W. and O'Connor, J. T. (1978). A relationship between gait and breakdown in the horse. Am. J. Vet. Res. 39,249 -253.[Medline]
Ratzlaff, M. H., Frame, J., Miller, J., Kimbrell, J. and Grant, B. (1985). A new method for repetitive measurements of locomotor forces from galloping horses. Proceedings of the 9th Equine Nutrition and Physiology Symposium,260 -265.
Ratzlaff, M. H., Hyde, M. L., Grant, B. D., Balch, O. K. and Wilson, P. D. (1990). Measurement of vertical forces and temporal components of the strides of horses using instrumented shoes. J. Equine Vet. Sci. 10,23 -25.
Roepstorff, L. and Drevemo, S. (1993). Concept of a force-measuring horseshoe. Acta Anat. Basel 146,114 -119.[Medline]
Russell, A. and McWhirter, N. D. (1988). The Guinness Book of Records 1988. London: Guinness Books.
Usherwood, J. R. and Wilson, A. M. (2005). No force limit on greyhound sprint speed. Nature 438,753 -754.[CrossRef][Medline]
Usherwood, J. R. and Wilson, A. M. (2006).
Accounting for elite indoor 200m sprint results. Biol.
Lett. 2,47
-50.
Weishaupt, M. A., Hogg, H. P., Wiestner, T., Denoth, J., Stussi, E. and Auer, J. A. (2002). Instrumented treadmill for measuring vertical ground reaction forces in horses. Am. J. Vet. Res. 63,520 -527.[CrossRef][Medline]
Weyand, P. G., Sternlight, D. B., Bellizzi, M. J. and Wright,
S. (2000). Faster top running speeds are achieved with
greater ground forces not more rapid leg movements. J. Appl.
Physiol. 89,1991
-1999.
Wilson, A. M., McGuigan, M. P., Su, A. and van den Bogert, A. J. (2001). Horses damp the spring in their step. Nature 414,895 -899.[CrossRef][Medline]
Wilson, A. M., Watson, J. C. and Lichtwark, G. A. (2003). Biomechanics: a catapult action for rapid limb protraction. Nature 421,35 -36.[CrossRef][Medline]
Witte, T. H. and Wilson, A. M. (2004). Accuracy of non-differential GPS for the determination of speed over ground. J. Biomech. 37,1891 -1898.[CrossRef][Medline]
Witte, T. H., Knill, K. and Wilson, A. M.
(2004). Determination of peak vertical ground reaction force from
duty factor in the horse (Equus caballus). J. Exp.
Biol. 207,3639
-3648.
Zebarth, B. J. and Sheard, R. W. (1985). Impact and shear resistance of turf grass racing surfaces for Thoroughbreds. Am. J. Vet. Res. 46,778 -784.[Medline]
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
This article has been cited by other articles:
|
|
 |
|
  K. J. Parsons, T. Pfau, and A. M. Wilson High-speed gallop locomotion in the Thoroughbred racehorse. I. The effect of incline on stride parameters J. Exp. Biol., March 15, 2008; 211(6): 935 - 944. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. J. Parsons, T. Pfau, M. Ferrari, and A. M. Wilson High-speed gallop locomotion in the Thoroughbred racehorse. II. The effect of incline on centre of mass movement and mechanical energy fluctuation J. Exp. Biol., March 15, 2008; 211(6): 945 - 956. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
