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4389 The Journal of Experimental Biology 209, 4389-4397 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 T.

H.

Witte1,*, C.

V.

Hirst1,† and A.

M.

Wilson1,2,‡ 1 Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, Hatfield, Hertfordshire, AL9 7TA, UK and 2Structure and Motion Laboratory, University College London, Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex HA7 4LP, UK *Present address: Cornell University Hospital for Animals, Box 25, Ithaca, NY, 14853, USA † Present address: 25 Lodge Hill Road, Lower Bourne, Farnham, Surrey GU10 3QW, UK ‡ Author for correspondence at address 1 (e-mail: awilson@rvc.ac.uk) Accepted 29 August 2006 Summary Stride duration, stance duration and protraction the lead and non-lead limbs for either the fore or hind duration are key variables when describing the gaits of pairs of limbs.

Mean stance durations of 131 and 77·ms in terrestrial animals.

Together, they determine the duty the forelimbs and 143 and 94·ms in the hindlimbs were factor (the fraction of the stride for which the limb recorded at speeds of 9 and 17·ms–1, respectively.

Maintains contact with the ground surface), from which Equivalent values for protraction duration were 364 and the peak vertical force can be estimated.

When an animal 342 (fore) and 355 and 326·ms (hind).

Peak limb forces changes speed, these variables change at different (from duty factor) at 17·ms–1 were 24.7·N·kg–1 body weight proportions.

Limited measurements of these variables and (range 22.6 to 26.0·N·kg–1·body·weight) for the forelimbs predictions of peak limb force have been undertaken for and 15.3·N·kg–1 (range 13.7–16.2·N·kg–1·body·weight) for large mammals performing high-speed over-ground the hindlimbs.

The duration of the aerial phase of the exercise.

This study set out to make such measurements, stride (when no limbs are in contact with the ground) was employing a previously validated system consisting of independent of speed.

Overlap time (when more than one limb-mounted accelerometers and a Global Positioning leg is on the ground) dropped with speed and approached System data logger.

Measurements were made on nine zero at maximum speed.

Elite Thoroughbred racehorses during gallop locomotion over a range of speeds from 9 to 17·m·s–1.

No statistically Key words: biomechanics, locomotion, horse, duty factor, speed, gallop, equine.

Significant differences were seen in any variables between Introduction The top speeds achieved by a wide variety of cursorial quadrupedal animals from the hare to the horse are surprisingly similar, despite a 100-fold range in body mass (Garland, Jr, 1983).

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 2ϫ 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 highspeed 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 high- THE JOURNAL OF EXPERIMENTAL BIOLOGY 4390 T.

H.

Witte, C.

V.

Hirst and A.

M.

Wilson speed 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 metacarpophalangeal 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 (foreor 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 A 9 m s–1 Non-lead hind Lead hind Non-lead fore Lead fore Contact duration — 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, THE JOURNAL OF EXPERIMENTAL BIOLOGY Racehorse stride parameters at gallop 4395 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 To date, limited measurements of the speed dependence of stride-timing variables have been made in galloping animals travelling at a range of speeds over ground.

The strains in the limbs of buffalo and elephant have been estimated at one speed from duty factor (Alexander et al., 1979).

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 realworld 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 THE JOURNAL OF EXPERIMENTAL BIOLOGY 4396 T.

H.

Witte, C.

V.

Hirst and A.

M.

Wilson 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 nonlead 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.

This work was funded by the BBSRC and the HBLB.

T.H.W.

Is funded by the Horserace Betting Levy Board.

A.W.

Holds a BBSRC Research Development Fellowship and a Royal Society Wolfson Research Merit Award.

We thank racehorse trainer John Best for allowing the use of the horses under his care for this study, and for his patience and advice during testing.

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