• View in gallery
    Figure 1—

    Photographs of 4 types of tactile stimulators (left to right: strap, tactile stimulators, limb weight, and limb weight plus tactile stimulators) used in a study of the biomechanical effects of stimulators on the hind feet of trotting horses.

  • View in gallery
    Figure 2—

    Mean peak hoof height over 3 trials in 8 horses; notice variability among horses.

  • View in gallery
    Figure 3—

    Flight arc of the right hind hoof without stimulators (control) and with 4 types of stimulators attached around both hind pasterns. Each curve represents 1 trial in 1 horse. Values on the x-axis represent time as a percentage of the swing phase of the horse's stride.

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Evaluation of biomechanical effects of four stimulation devices placed on the hind feet of trotting horses

Hilary M. ClaytonMary Anne McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Michael LavagninoMary Anne McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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LeeAnn J. KaiserMary Anne McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Narelle C. StubbsMary Anne McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Abstract

Objective—To compare effects of 4 types of stimulation devices attached to the hind feet on hoof flight, joint angles, and net joint powers of trotting horses.

Animals—8 clinically normal horses.

Procedures—Horses were evaluated under 5 conditions in random order: no stimulators, loose straps (10 g), lightweight tactile stimulators (55 g), limb weights (700 g), and limb weights with tactile stimulators (700 g). Reflective markers on the hind limbs were tracked during the swing phase of 6 trotting trials performed at consistent speed to determine peak hoof heights and flexion angles of the hip, stifle, tarsal, and metatarsophalangeal joints. Inverse dynamic analysis was used to calculate net joint energies. Comparisons among stimulators were made.

Results—Peak hoof height was lowest for no stimulators (mean ± SD, 5.42 ± 1.38 cm) and loose straps (6.72 ± 2.19 cm), intermediate for tactile stimulators (14.13 ± 7.33 cm) and limb weights (16.86 ± 15.93 cm), and highest for limb weights plus tactile stimulators (24.35 ± 13.06 cm). Compared with no stimulators, net tarsal energy generation increased for tactile stimulators, limb weights, and limb weights plus tactile stimulators, but only the weighted conditions increased net energy generation across the hip joint.

Conclusions and Clinical Relevance—The type and weight of foot stimulators affected the magnitude of the kinematic and kinetic responses and the joints affected. These findings suggest that different types of foot stimulators are appropriate for rehabilitation of specific hind limb gait deficits, such as toe dragging and a short stride.

Abstract

Objective—To compare effects of 4 types of stimulation devices attached to the hind feet on hoof flight, joint angles, and net joint powers of trotting horses.

Animals—8 clinically normal horses.

Procedures—Horses were evaluated under 5 conditions in random order: no stimulators, loose straps (10 g), lightweight tactile stimulators (55 g), limb weights (700 g), and limb weights with tactile stimulators (700 g). Reflective markers on the hind limbs were tracked during the swing phase of 6 trotting trials performed at consistent speed to determine peak hoof heights and flexion angles of the hip, stifle, tarsal, and metatarsophalangeal joints. Inverse dynamic analysis was used to calculate net joint energies. Comparisons among stimulators were made.

Results—Peak hoof height was lowest for no stimulators (mean ± SD, 5.42 ± 1.38 cm) and loose straps (6.72 ± 2.19 cm), intermediate for tactile stimulators (14.13 ± 7.33 cm) and limb weights (16.86 ± 15.93 cm), and highest for limb weights plus tactile stimulators (24.35 ± 13.06 cm). Compared with no stimulators, net tarsal energy generation increased for tactile stimulators, limb weights, and limb weights plus tactile stimulators, but only the weighted conditions increased net energy generation across the hip joint.

Conclusions and Clinical Relevance—The type and weight of foot stimulators affected the magnitude of the kinematic and kinetic responses and the joints affected. These findings suggest that different types of foot stimulators are appropriate for rehabilitation of specific hind limb gait deficits, such as toe dragging and a short stride.

After an orthopedic injury has healed, gait deficits may persist for a prolonged period. In a study1 in which clinically normal horses had 1 metacarpophalangeal (ie, fetlock) joint immobilized in a cast for 7 weeks, the horses underwent 8 weeks of progressively increasing exercise following cast removal; however, after completion of the exercise period, the range of motion was still restricted. Reduced joint flexion may be seen by the clinician, but other locomotor deficits are not visible to the eye. These include kinetic or neuromuscular changes that do not have overt kinematic effects, although joint stability may be compromised, predisposing to further pathological changes as a consequence of micromotion within the joint.2–4 Human physical therapists use a range of techniques that are effective in restoring neuromotor function and muscle strength after injury or immobilization in human patients,5 but in the veterinary field, there is little evidence-based research that can be drawn upon to develop an appropriate and specific treatment plan for individual equine patients.

An example of the cascade of events that may follow an injury and the physiotherapeutic approach to rehabilitation is knee pain in humans. Patellofemoral pain is associated with altered knee joint proprioception6 and changes in the electromyographic firing sequence of the different heads of quadriceps femoris,7,8 including delayed onset of eccentric activity in vastus medialis obliquus.4 The consequences of altered muscular control include maltracking of the patella,4 and the effects are not limited to the knee; patients with patellofemoral pain often lose strength in the external rotators and abductors of the hip.3 There is evidence to support the use of physiotherapeutic treatment targeting vastus medialis obliquus by use of taping, bracing, and various forms of therapeutic exercise.5,9,10

A similar situation may pertain in horses in which stifle or tarsal joint pain may be associated with changes in activation patterns or atrophy of the lumbopelvic and hind limb musculature including the deep stabilizer muscles, such as vastus medialis, or hind limb stability muscles, such as biceps femoris and middle gluteal muscles, leading to joint instabilities that predispose to reinjury.11 One of the techniques used by equine physical therapists to restore or increase the range of joint motion, reestablish appropriate neuromuscular firing patterns, and strengthen the muscles that move and stabilize the joints of the hind limb involves the use of bracelets (tactile stimulators) secured around the region from the metatarsophalangeal or metacarpophalangeal joint to the hoof (ie, the pastern). The kinematic and kinetic effects of lightweight (55-g) tactile stimulators and heavier (700-g) inertial stimulators attached around the hind pasterns have been described.12,13 Lightweight tactile stimulators are associated with an elevated flight arc of the hoof mediated via a spinal reflex with the afferent arm originating in mechanoreceptors in the skin and the efferent arm acting via the tarsal musculature.12 Weighted (inertial) stimulators change the inertial properties of the distal portion of the limb and are thought to act by stimulating cutaneous mechanoreceptors that are sensitive to deeper pressure13 and also muscle spindles and Golgi tendon organs that sense the effects of the added weight especially at takeoff. The motor response involves increased concentric activity in the musculature of the tarsus and hip regions to pull the weighted limb off the ground and control the forward swing.

The purpose of the study reported here was to extend the previous investigations of the mechanisms and effects of different types of pastern stimulation devices by providing a direct comparison within the same group of horses of the kinematic and kinetic effects of tactile, inertial, and combination tactile-inertial stimulators attached to the hind pastern. The experimental hypotheses were that tactile and inertial stimulators are associated with significant increases in hind limb joint flexions and in height of the flight arc of the hoof and that a stimulator combining tactile and inertial effects results in a greater response than either type of stimulator acting alone.

Materials and Methods

Horses—The study was performed with approval of the university's animal ethics committee. The subjects were 8 horses (mean ± SD height, 150.0 ± 1.4 cm; mass, 448 ± 36 kg; age, 13.4 ± 4.3 years) that were assessed to be functionally not lame (lameness grade < 1 on a 5-point scale).14 Prior to the start of the study, the horses had been accustomed to the runway used for data collection. Horses trotted back and forth until trials were performed consistently with an even rhythm at a speed that did not require the handler to urge the horse forward or restrain its movement. This was established as the target speed, and each horse was required to trot at within 0.25 m/s of its own target speed during data collection. Target speeds ranged from 3.09 to 3.68 m/s in individual animals. Horses were then trained to match their speed to that of the handler by responding to movements of the handler's shoulders so the lead shank was loose during data collection. To assist in maintaining consistent speed across trials, the handler was informed of the mean speed of each trial immediately after its completion. It was important to maintain a consistent speed within each horse across the different conditions because speed affects spatio-temporal kinematics, angular kinematics, and kinetic variables in the hind limb.15,16

Stimulators—Data were collected at a trot under 5 conditions (Figure 1): no stimulators on any limb (control); a lightweight (10-g) nylon strap attached loosely around the hind pasterns (strap); braided nylon straps (55 g) with 7 double-stranded oval brass links, 6 cm in length, loosely attached around the hind pasterns so the links brushed lightly against the pastern, coronet, and proximal portion of the hoof wall as the horse moved12 (lightweight tactile stimulators); flexible human wrist weights (700 g) wrapped in tape for protection and attached snugly around the hind pasterns by use of fabric hook-and-loop fastener straps and adhesive tape13 (limb weights); and lightweight tactile stimulators added to the limb weights (combination stimulator). Sufficient weight was removed from the limb weights so that the total mass was maintained at 700 g. Combination stimulators were attached snugly to both hind pasterns by use of fabric hook-and-loop fastener straps and adhesive tape.

Figure 1—
Figure 1—

Photographs of 4 types of tactile stimulators (left to right: strap, tactile stimulators, limb weight, and limb weight plus tactile stimulators) used in a study of the biomechanical effects of stimulators on the hind feet of trotting horses.

Citation: American Journal of Veterinary Research 72, 11; 10.2460/ajvr.72.11.1489

Data collection—Horses were accustomed to the sound and feel of the different stimulators the day before data were collected. They were prepared for data collection by applying reflective, 6-mm markers over the following bony landmarks: on the dorsal midline overlying the dorsal spinous process of the 10th thoracic vertebra (T10); bilaterally on the hind limbs to the lower part of tuber coxae, cranioventral part of greater trochanter, lateral femoral condyle, lateral aspect of the talus, and lateral condyle of third metatarsus; and bilaterally to the hind hoof wall on the dorsal midline 4 and 6 cm above the ground and midlaterally 2 cm above the ground. Lateral radiographic views of the hind hooves were obtained with the hoof markers in place and with the horses standing on wooden blocks. Custom software was used to determine the position of the center of rotation of the distal interphalangeal joint on the basis of the hoof marker positions.17

Motion capture was performed by use of an 8-camera motion analysis systema recording at 120 Hz. A wand technique was used to calibrate a data collection volume measuring 5 × 3 × 2.5 m. The error in a linear measurement of 1 m within the calibrated volume was < 0.8 mm. A stationary file was recorded prior to data collection to define the kinematic model. Six consecutive trotting trials were recorded for each stimulator condition in each horse in a predetermined random order.

Data analysis—The data were analyzed by use of proprietaryb and custom software. On the basis of the speed of the T10 marker, the 3 trials that were closest to the target speed were analyzed for each stimulator condition. This was a sufficient number of trials to collect representative kinematic data.18

After filtering with a Butterworth lowpass digital filter at cutoff frequency of 12 Hz, correction algorithms for skin displacement19 were applied to the tuber coxae, greater trochanter, and lateral femoral condyle marker coordinates. The markers on 1 hind limb (randomized among horses) were tracked and used in the analysis. The following spatiotemporal variables were calculated: stride duration, hind limb stance duration, hind limb swing duration, maximal height of the distal hind limb toe marker, and peak swing phase flexion angles of the hip, stifle, tarsal, and metatarsophalangeal joints. Net energy generated or absorbed across each joint in the swing phase was calculated by use of an inverse dynamics approach20 with the mass of the distal segments being corrected for addition of stimulator mass.21

Statistical analysis—Mean and SD were calculated for the gait variables, and the Kolmogorov-Smirnov test was used to determine whether the variables were normally distributed. Values for peak hoof height were log transformed, after which the distribution was normal. Normally distributed variables were analyzed by use of a 2-factor (condition and horse), repeated-measures ANOVA with Tukey B post hoc tests to detect differences between stimulator conditions. For spatiotemporal variables that were not normally distributed, a Friedman test and Wilcoxon signed rank test were used to detect differences between stimulator conditions. Values of P < 0.05 were considered significant for all statistical tests.

Results

Speed and stride duration did not differ across stimulator conditions, indicating that horses maintained their target speeds and kept a consistent rhythm. Although stride duration did not change, stance duration was shorter and swing duration was longer when horses wore tactile stimulators, limb weights, and combination stimulators, compared with the control and strap conditions (Table 1).

Table 1—

Mean ± SD values of velocity and temporal variables for 5 stimulator conditions in a study of the biomechanical effects of stimulators on the hind feet of 8 trotting horses.

VariableNo stimulatorStrapTactile stimulatorLimb weightLimb weight and tactile stimulator
Speed (m/s)3.44 ± 0.233.46 ± 0.253.52 ± 0.223.51 ± 0.273.50 ± 0.25
Stride duration (s)0.67 ± 0.050.66 ± 0.020.67 ± 0.020.67 ± 0.030.68 ± 0.04
Stance duration (s)0.26 ± 0.02a,b,c0.26 ± 0.02d,e,f0.25 ± 0.02a,d0.25 ± 0.02b,e0.24 ± 0.03c,f
Swing duration (s)0.40 ± 0.05a,b,c0.40 ± 0.02d,e,f0.42 ± 0.02a,d,g0.43 ± 0.03b,e0.44 ± 0.04c,f,g

Within a row, values with the same superscript are significantly (P < 0.05) different.

Peak hoof height did not differ between the control and strap conditions, but it increased significantly with tactile stimulators and limb weights and further increased significantly for the combination of limb weights with tactile stimulators (Table 2). Individual horses varied in their responsiveness to different stimulators (Figure 2), which resulted in a significant effect of horse and an interaction between horse and condition, indicating that there were differences between horses in their response to specific types of stimulators.

Figure 2—
Figure 2—

Mean peak hoof height over 3 trials in 8 horses; notice variability among horses.

Citation: American Journal of Veterinary Research 72, 11; 10.2460/ajvr.72.11.1489

Table 2—

Mean ± SD values of variables measured during the swing phase of the stride at a trot for 8 horses without stimulators and with 4 types of pastern stimulators.

VariableNo stimulatorStrapTactile stimulatorLimb weightLimb weight and tactile stimulator
Peak hoof height (cm)5.4 ± 1.4a,b,c6.7 ± 2.2d,e,f14.1 ± 7.3a,d,g16.9 ± 15.9b,e,h24.4 ± 13.1c,f,g,h
Peak flexion angle (°)     
   Hip73.2 ± 9.674.6 ± 6.472.2 ± 9.671.1 ± 11.071.3 ± 9.9
   Stifle118.3 ± 10.2a,b,c117.7 ± 6.1d104.5 ± 15.8a95.9 ± 21.2b95.2 ± 13.8c,d
   Tarsus116.5 ± 5.0a,b114.6 ± 2.5c96.0 ± 15.1a87.7 ± 21.784.8 ± 13.5b,c
   MTPJ133.8 ± 8.7a131.2 ± 9.0b116.7 ± 18.1c107.8 ± 20.8101.5 ± 15.4a,b,c
Net joint energy (J/kg)     
   Hip0.160 ± 0.099a,b0.259 ± 0.026c0.246 ± 0.0460.367 ± 0.029a,c0.320 ± 0.016b
   Stifle−0.141 ± 0.057a,b,c−0.184 ± 0.018d−0.211 ± 0.030a,e−0.320 ± 0.036b,d,e−0.312 ± 0.043c
   Tarsus0.052 ± 0.024a,b,c0.066 ± 0.006d,e0.088 ± 0.16a,f0.145 ± 0.051b,d0.146 ± 0.038c,e,f
   MTPJ−0.013 ± 0.007a−0.018 ± 0.003b−0.019 ± 0.006c−0.039 ± 0.016−0.033 ± 0.009a,b,c

MTPJ = Metatarsophalangeal (fetlock) joint.

Within a row, values with the same superscript are significantly (P < 0.05) different.

The shape of the hoof flight arc differed among stimulator types (Figure 3). For the control and strap conditions, the hind limb hoof had a biphasic flight path in which the first peak was higher and was followed by a more prolonged period of hoof elevation in the later part of swing. When the tactile stimulators or limb weights were used, the flight path had a different pattern from the control and strap conditions, with the 2 peaks often merging and the maximal height being reached in the second half of the swing phase. The combination stimulator had the highest flight arc with a single peak in midswing.

Figure 3—
Figure 3—

Flight arc of the right hind hoof without stimulators (control) and with 4 types of stimulators attached around both hind pasterns. Each curve represents 1 trial in 1 horse. Values on the x-axis represent time as a percentage of the swing phase of the horse's stride.

Citation: American Journal of Veterinary Research 72, 11; 10.2460/ajvr.72.11.1489

The main contribution to the increase in hoof height was from increased peak flexions of the stifle and tarsal joints, with metatarsophalangeal (ie, fetlock) joint flexion also contributing when the combination stimulators were used (Table 2). Compared with the control condition, net energy generation increased at the tarsal joint and net energy absorption increased at the stifle joint for the tactile stimulators, limb weights, and combination stimulators, but only the 2 weighted conditions were associated with an increase in energy generation at the hip.

Discussion

This study revealed that a variety of types of stimulators attached around the pastern were associated with significant elevations of the flight arc of the hind limb hoof. The experimental hypotheses were supported by finding significant increases in hoof height and joint flexions with the tactile stimulators and limb weights and by the further significant increases in these variables with the combination stimulator.

The height of the hoof during the swing phase, which represents the summation of the flexions of all the joints of the limb, is more easily assessed by an observer than the angular changes at the individual joints. Thus, hoof height is a useful feature of the gait for a veterinarian or therapist to evaluate in the field. The joints that contribute most to hoof elevation are the stifle and tarsus, which reach their peak flexions at similar times in midswing22 and have their movements synchronized by the reciprocal apparatus.23 Fetlock joint flexion, which peaks earlier than stifle and tarsal joint flexion at around 30% of swing, also affects hoof height, and an increase in fetlock joint flexion contributed to the height increase with the combination stimulators. The hip joint makes less contribution to changes in hoof height but is responsible for protraction and retraction of the entire hind limb. Peak hip flexion did not change significantly with any of the stimulators. Interestingly, other researchers have reported reduced hip protraction-retraction at a trot but not at a walk when horses wore 700-g hoof boots.24

A variety of techniques used in human rehabilitation, including proprioceptive taping,25 bracing,26 and orthotics,27 have their effects through stimulation of proprioceptive awareness rather than physical restriction of movement. The techniques used in the present study appeared to act in a similar manner by enhancing proprioceptive awareness. Perception of the presence of lightweight tactile stimulators is thought to be via stimulation of cutaneous mechanoreceptors,12 such as free nerve endings, that are sensitive to light touch. Limb weights exert deeper pressure because of their greater weight combined with the fact that they must be attached tightly around the pastern to hold them securely in position during locomotion. They stimulate more deeply located mechanoreceptors, such as Pacinian corpuscles, that respond to skin pressure forceful enough to indent the dermis and deform the corpuscle beyond a critical pressure threshold that triggers a volley of action potentials. When a limb is moving, kinesthetic awareness of joint angulations and their rates of change in all planes is provided by proprioceptors in muscle spindles, Golgi tendon organs, and joint proprioceptors that detect and transmit information about position, movement, tension, and force within the body, including awareness of weight and resistance of objects in relation to the body.28 It was particularly interesting that peak hoof height was so much higher for the combination of limb weight plus tactile stimulators than for either the limb weights or tactile stimulators alone, suggesting an additive effect of the tactile and inertial mechanisms of action.

The equine pastern is a particularly sensitive area,29 which may indicate a high density of sensory receptors in the pastern skin, as in the fingertips of humans. This responsiveness has been exploited in training show horses, by use of weights, chains, or chemical cauterization to the pastern region, to produce more exaggerated joint flexions and greater swing phase elevation of the limbs.30 Compared with the forelimbs, the hind limbs have a more exaggerated response and habituate more slowly to tactile stimulators.31 It has been observed that the boots worn by show jumpers are often tightened or moved further distally over the fetlocks just before the horse enters the arena to compete.32 This may take advantage of the ability to refresh the effect by stimulating a different area of the limb.

Although the kinematic responses to the different types of proprioceptive stimulators are similar, the joint angular changes appear to be mediated via different mechanisms of action. The response to lightweight tactile stimulators is the result of a reflex in which cutaneous afferent stimulation modulates the activity of the tarsal musculature.12 The same stimulus may activate the flexor or extensor muscles, depending on limb position; the muscle of the antagonistic pair that is lengthened is more excitable because of stretching of its muscle spindles.33,34 At the beginning of the swing phase, the tarsus is extended so the flexor musculature is stretched, making it more excitable. Later in the swing phase, the tarsus is flexed and, in this position, the extensor musculature is more excitable. Thus, the same tactile stimulus invokes tarsal flexion in early swing and tarsal extension in late swing. A similar situation pertains in humans; tibial nerve stimulation causes reflex facilitation of the ankle flexors at or just after the stance-swing transition with the response changing to facilitation of the ankle extensors in late swing.35 Electromyographic activity in the anterior tibial muscle is initiated earlier in the stride when humans wear a device on the ankle.36 Furthermore, when the fabric hook-and-loop fastener strapping attaching the ankle device is tightened, the electromyographic changes become more pronounced. This is similar to the response of horses to tactile and weighted stimulators and may explain the early onset of the swing phase and consequent shortening of the stance phase.

The positive net joint energies generated by the hip and tarsal musculature are responsible for driving the swing phase movements of the hind limb.15,16 Protraction requires the hip flexors to exert a force large enough to overcome air resistance and its own weight and inertia.37 Protraction is initiated at the start of swing by active shortening of the hip flexors and is maintained through midswing by passive recoil of the fascia of tensor fascia lata38 assisted by the superficial digital flexor and peroneus tertius, which are components of the reciprocal apparatus that synchronizes movements of the stifle and tarsus.23 When weight is added to the pastern, increased muscular force must be used to overcome inertia when lifting and protracting the limb. The increase in energy generation in the hip musculature in response to the use of limb weights is consistent with previous results,13 and the results of the present study confirmed that only the weighted stimulators were associated with increased energy generation across the hip. A similar effect has been observed in the forelimb in response to applying a shoe weighing 700 g; the elbow joint flexors did more work to pull the weighted limb off the ground, and the elbow joint extensors did more work to overcome the greater momentum of the weighted limb in midswing and initiate retraction in late swing.21 Despite the increased energy generation, the height of the flight arc of the forelimb hoof did not change significantly.

Results of the present study confirmed that different types of pastern stimulators have different applications in rehabilitation. In the early stages of a rehabilitation program, especially after a joint has been immobilized, the application of a lightweight strap to the hind pastern may be sufficient to initiate increased flexion of the stifle, tarsal, and metatarsophalangeal joints. Even though the increases in hoof height and joint flexions were not significant for the strap alone, there was a small but consistent effect of the strap that may be sufficient to initiate treatment, especially after a period of immobilization. The effect is greatly increased by the addition of lightweight tactile stimulators, and a variety of lightweight devices that brush gently against the skin are likely to produce a similar effect. The fact that individual horses differed in their responses suggests that it may be necessary to try different lightweight devices to get the desired effect, and it may be necessary to change the device to refresh its effect. A further progression in the therapeutic program involves use of the inertial effect of a weighted stimulator to activate the hip musculature, which does not appear to respond to tactile stimulation alone. In the human rehabilitation field, training with weights on the peripheral limbs is recognized as a useful therapeutic technique. For example, ankle weights yield benefits in rehabilitation of a variety of conditions, including incomplete spinal cord injury39 and stroke.40 The use of limb weights is also effective for improving strength of quadriceps to alleviate the crouched hip and back posture in children with cerebral palsy41 by use of limb weights that are considerably heavier than those used in the study reported here. Apparently, the therapeutic benefits outweigh any risks associated with instability of dysfunctional muscles. In horses, weight can be added to the distal portion of the limb in the form of hoof boots, weighted bell boots, pastern weights, or weighted splint boots. The further distally the weight is applied, the larger its effect on limb inertia and energetics.41 In a therapeutic situation, it has been recommended to start with a light weight (200 g) and progress gradually to a heavier weight (up to 1,100 g) as the horse's musculature adapts over a period of several weeks.42 However, the effect of weights > 700 g has not been investigated from a research perspective. It is beneficial to perform strengthening exercises with limb weights before a horse undergoing rehabilitation is reshod. After shoeing, the weight of the stimulators should be reduced by an amount approximately equal to that of the shoes, which typically weigh approximately 700 g. Further support for the recommendation to start with light weights and increase gradually comes from a study32 in which weighted (700-g) metatarsal boots affected motion of the lumbar vertebrae. During walking, there was a small (< 1°) but significant increase in lumbar flexion and extension without any change in lateral bending, whereas during trotting, weighted boots caused a small (< 1°) but significant decrease in lateral bending at the thoracolumbar junction with no change in flexion and extension. Thus, a trotting horse increases vertebral stabilization as part of the response to limb weights and this opens up the possibility of strengthening the back muscles through the use of limb weights.

The first stage of rehabilitation is to restore the range of motion, and this is followed by the strengthening phase.26 Both of these are important steps toward restoration of athletic ability and prevention of reinjury.11 To accomplish these aims in an equine rehabilitation program, a logical progression is to start with a strap and progress to lightweight tactile stimulators, heavier inertial stimulators, and finally to combination stimulators. The more exaggerated response to the combination stimulator suggests that it recruits a larger percentage of muscle fibers, which has a strengthening effect. Therefore, combination stimulators are likely to be most useful in the later stages of rehabilitation.

Horses habituate to the effects of tactile31 and inertial13 stimulators within a single treatment session, but significant elevation of the hind hooves persists over a fairly long distance (> 300 m for tactile stimulators12; 150 m for limb weights13). It is possible that even after the elevation is no longer significantly increased, the exercise may continue to have a beneficial effect in terms of muscle stimulation and endurance training. Even after exercising horses on a treadmill with hoof weights for 30 min/d, 5 d/wk for 1 month, hind limb joint flexions were still increased when horses wore hoof weights, compared with the nonweighted condition.43

It is recommended that the treatment sessions should initially be short in duration to avoid fatigue or habituation, although multiple sessions may be performed in 1 day to take advantage of refreshing the effect upon reapplication of the stimulators. Over time, progressive loading is achieved by increasing the duration of the exercise sessions, increasing the speed or intensity of exercise, or increasing the weight of the stimulator, although only one of these variables should be increased at a time. Caution is advised when increasing exercise intensity or weight of the stimulators to avoid overloading, especially in the early stages of rehabilitation.

Results of the present study suggest that the use of different types of tactile, inertial, and combination stimulators attached around the hind pastern can be used in a rehabilitation program to increase joint flexions and muscle recruitment. Lightweight stimulators are indicated to treat toe dragging. Limb weights are more appropriate in horses with persistent short strides associated with reduced protraction or retraction. Light limb weights may also be used in restoring function in horses with neurologic deficits. Tactile stimulation facilitates the tarsal musculature, whereas inertial and combination stimulators facilitate the tarsal and hip musculature. A progression of stimulator types and weights can be used in combination with increases in exercise intensity or duration to increase dynamic mobility of the joints and to selectively improve muscle strength or endurance.

a.

Motion Analysis Corp, Santa Rosa, Calif.

b.

Cortex 1.1.4.368 software, Motion Analysis Corp, Santa Rosa, Calif.

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Contributor Notes

Supported by the McPhail Endowment.

The authors thank Dr. Sandra Nauwelaerts for assistance with data analysis.

Address correspondence to Dr. Clayton (claytonh@cvm.msu.edu).