Investigation of the incidence and type of injuries associated with high-speed treadmill exercise testing
Reasons for performing study: During the past 20 years, treadmill exercise testing has played an important role in both the study of equine exercise physiology and the investigation of poor athletic performance. However, it has been suggested that some trainers and veterinarians may be reluctant to refer horses for treadmill exercise testing because of fears that horses may be at increased risk of musculoskeletal injury during treadmill exercise.
Objective: To investigate the incidence and types of injuries sustained by horses undergoing treadmill exercise.
Methods: Data were collated from 9 centres in the UK, France and Belgium, and the prevalence and types of injury were established.
Results: A total of 2305 records were reviewed, with 2258 horses performing treadmill exercise. There was an overall injury rate of 5.4%. However, the majority of injuries sustained were minor in nature (4.7%). Only 13 horses (0.6%) sustained major injuries in association with treadmill exercise. These included 5 cases of severe exercise-induced myopathy, 4 fractures (of which 1 was catastrophic), 2 tendon injuries, 1 case with undiagnosed severe lameness and 1 with marked exacerbation of a previously diagnosed lameness. Two other major incidents were reported but were not directly associated with treadmill exercise (one had iliac thrombosis and one collapsed and died as a result of a pulmonary embolism).
Conclusions: This study confirms that the majority of horses undergo treadmill exercise without incident. The majority of injuries that did occur were minor in nature and the incidence of major injuries was similar to that reported during competition elsewhere.
Potential relevance: Treadmill exercise is a safe procedure and does not appear to pose an increased risk of injury in comparison with overground exercise.
Treadmills have been used for the scientific study of equine exercise for over 100 years (Erickson 2006). Persson (1967) was the first to use the high-speed treadmill to study exercise physiology in horses and, since that time, research into exercise physiology has increased dramatically with modern treadmills enabling the study of a large range of physiological variables (Erickson 2006; Evans 2007). In addition, during the past 20 years high-speed treadmill exercise (HSTE) testing has played an important role in the investigation of poor athletic performance in equine athletes (Morris and Seeherman 1990, 1991). Clinical exercise testing is important for the investigation of disorders that may limit performance in horses because resting examinations are frequently equivocal or unrewarding (Evans 2007). In particular, high-speed treadmill exercise testing has been invaluable for enabling endoscopic examination of the upper respiratory tract (URT) during exercise. It is now well recognised that resting examinations of the URT are not representative of dynamic events occurring during exercise (Kannegieter and Dore 1995; Tan et al. 2005; Lane et al. 2006). Despite this, the majority of horses undergo treatment for upper airway obstruction on the basis of clinical history and resting endoscopy alone (Franklin 2002). It appears that there is frequently reluctance amongst owners and trainers to refer horses for treadmill investigation. There are a number of proposed reasons for this reluctance to subject horses to a high-speed treadmill exercise test. These include the need to transport horses to a specialist centre, the cost involved and concerns regarding the perceived risk of injury. This has led to a move towards field exercise testing and has recently resulted in the development of overground endoscopy (Franklin et al. 2008; Desmaizieres et al. 2009; Pollock et al. 2009). However recent questions regarding the suitability of some field testing protocols for enabling a diagnosis of conditions such as dorsal displacement of the soft palate suggest that the treadmill remains a useful tool for clinical exercise testing (VanErck-Westergren et al. 2009; Allen and Franklin 2010).
To date, there has been no reported analysis of injury data in horses performing treadmill exercise. The aim of this study was to investigate the incidence and types of injuries sustained during treadmill exercise testing.
Materials and methods
Centres in the UK, Belgium and France that perform treadmill exercise testing were approached and invited to participate in a retrospective study of treadmill safety. All available records from horses undergoing treadmill exercise testing in the past 10 years were reviewed by individuals at each institution and the incidence and type of injuries were recorded. Major injuries were defined as those that were fatal, potentially career limiting or resulted in severe, acute lameness. All other injuries were defined as minor injuries. Injury rates were described as the injury rate per horse rather than the rate per exposure to treadmill exercise, due to incomplete records being available for the number of training sessions that individual horses underwent at some centres.
Data were also obtained relating to whether musculoskeletal screening was performed before treadmill testing and details collected relating to the training and exercise test protocols used, the number of personnel involved in HSTE testing as well as details of safety equipment that was used.
Data were collated from all participating centres and descriptive statistics performed using PASW 17. A Chi-squared test was used to compare injury rates between training sessions and the exercise test and to compare injury rates between racehorses and nonracehorses. Statistical significance was set at P<0.05.
Data were provided for 9 centres performing high-speed treadmill exercise testing. A total of 2305 records were examined, including 1880 (82%) racehorses (of which 1593 were Thoroughbreds and 287 were Standardbred trotters) and 425 (18%) were horses involved in other athletic disciplines. The detail of reporting varied between centres: 3 centres had complete records of all injuries incurred. For the remaining centres only the more severe or obviously apparent injuries were noted and minor grazes or temporary exacerbation of pre-existing lameness was not always reported (Table 1).
Table 1. General data
|1 ||2000–2009||749||651 racing TB, 98 other||724||715||61 (8.4%)||4 (0.6%)|| |
|2 ||2000–2009||476||471 racing TB, 5 other||474||471||3 (0.6%)||1 (0.2%)||Limited information on minor injuries|
|3 a & b ||2000–2008||365||137 racing (36 TB, 101 SB), 228 other||93||353||20 (5.6%)||3 (0.8%)||Limited information on minor injuries|
|4 ||1998–2009||291||235 racing TB, 56 other||286||281||26 (9.1%)||3 (1.0%)|| |
|5 a & b ||2004–2008||220||208 racing (22 Tb, 186 SB), 12 other||212||212||5 (2.4%)||3 (1.4%)||Limited information on minor injuries|
|6 ||1999–2001 and 2008–2009||176||158 racing TB, 18 other||175||174||3 (1.7%)||1 (0.6%)||Limited information on minor injuries|
|7 ||2007–2009||28||20 racing TB, 8 other||27||27||4 (14.8%)||0|| |
|Total|| ||2305||1880 racing, 425 other||1991||2233||122 (5.4%)||15 (0.6%)|| |
Of the 2305 horses, 47 (2%) were unable to undergo treadmill training because they were temperamentally unsuitable (n = 18) or because of marked pre-existing lameness that was identified in the clinical examination or during the first training session (n = 29). A further 25 horses underwent initial training but did not perform the high-speed test because of injuries incurred during training (n = 5) or because of difficulties in getting them to gallop or difficulties performing the endoscopic examination (n = 20). A total of 2233 (97%) horses completed the exercise test (Table 1).
A clinical examination to screen for pre-existing lameness was performed at all centres. Palpation of the limbs was always performed. In addition, a trot up in a straight line was performed at 7 centres, whilst at the remaining 2 centres, the gait was assessed during the initial training session. Where more severe lameness (>3–4/10) was identified, trainers and/or owners were routinely contacted to discuss potential risks of lameness being exacerbated during treadmill exercise. However, the majority requested that treadmill testing be continued and only 29 lame horses were excluded as described above.
A safety harness was routinely used during exercise testing at all 9 centres. This involved a strap attached to a support frame above the treadmill that was clipped on to a surcingle worn by the horse. All centres fitted brushing boots or sports medicine boots on fore- and hindlimbs and over-reach boots on the forelimbs. One centre also used knee boots during the first training session.
A minimum of 3 personnel were involved in treadmill testing in addition to the veterinary surgeon performing the examination. This included one or 2 persons holding the horse and one person driving the treadmill. The person driving was positioned at the side of the treadmill behind the horse during training and testing and was responsible for encouraging horses to stay forward on the treadmill. In some centres an additional person was also positioned on the opposite side of the horse to assist in keeping the horse forward.
Training and exercise test protocols
Eight of the 9 centres reported that horses underwent training sessions before performing high-speed exercise tests. This ranged from 1–5 sessions. At one centre, horses did not routinely undergo training. There was some variation in the testing protocols between centres and also depending on the athletic discipline of the horse. For Thoroughbred racehorses, an incremental standardised exercise test to fatigue was performed at 8 centres. This involved a preliminary warm-up followed by increases in speed at 1 min intervals until the point of fatigue or until a diagnosis was made. The remaining centre performed a one step high-speed test to fatigue for racehorses that raced on the flat over short distances (after warm-up, horses were accelerated to a speed of 11–14 m/s, depending on the individual's fitness and remained at that speed until conclusion of the test) or a rapid incremental test (Parente 1996) for horses that were intended to race over longer distances (i.e. >2.0 miles). The exercise tests were routinely performed on an inclined treadmill (6–10%) at all centres. Standardbred racehorses also performed an incremental test but with rest periods at walk or trot between each increment and the incline was lower than for Thoroughbreds (2.5–4%). For horses involved in other athletic disciplines, tests were modified for each individual, depending on the level of fitness and the type of work that the horse performed.
Injuries occurring during treadmill exercise
A total of 122 (5.4%) horses were reported to sustain injuries following treadmill exercise (Table 1). There was no significant difference in the injury rate in racehorses compared with nonracehorses (P = 0.39). There was a significant difference between the proportion of injuries that occurred during training (34%) compared with those occurring during the exercise test (66%) (P = 0.004). However, there was no significant difference in the proportion of major vs. minor injuries occurring during training or exercise testing (P = 0.21).
Major injuries sustained during treadmill exercise
Fifteen (0.7%) horses sustained major injuries or became acutely lame following either treadmill training or high-speed treadmill exercise testing (Table 2). However, 2 of these were not directly related to treadmill exercise per se. One horse collapsed and died as a result of a pulmonary aneurism. The other horse became very distressed and ataxic after the exercise test and was diagnosed with aortic-iliac thrombosis.
Table 2. Details of major injuries
|Severe exercise-induced myopathy||1||4|
|Undiagnosed acute lameness||0||1|
|Marked exacerbation of previous lameness||0||1|
|Fatality due to pulmonary aneurism||1||0|
Four (0.2%) horses (all racehorses) were confirmed to have sustained fractures following treadmill exercise. Of these one was sustained during the preliminary training session when a horse pulled off a shoe, stood on a clench and developed a small marginal fracture of P3. Three other horses were diagnosed with fractures after completing the high-speed exercise test. One horse sustained an ischial fracture after stopping suddenly due to a suspected cardiac arrhythmia and coming off the back of the treadmill. This horse was not wearing a safety harness, even though this was in routine use at the centre. It is unclear why the harness was not fitted on this occasion. One horse became acutely lame within minutes of completing the exercise test and was confirmed to have a complete medial condylar fracture of the third metatarsal bone and was subjected to euthanasia. No lameness was identified in this horse during the preliminary lameness screening, prior to testing. A third horse was observed to be distressed immediately after completion of the exercise test and was shifting weight on his hindlimbs. This horse was reported to have a bilaterally stiff hindlimb action during the initial screening examination. Scintigraphy and radiography was subsequently performed. This revealed a stress fracture of the right distal tibial diaphysis and lateral condylar stress fractures in both hindlimbs. The findings were considered to be consistent with long-standing fractures and were concluded to have been the cause of the poor performance in this horse, rather than occurring as a result of treadmill exercise.
Two (0.1%) horses, both Standardbreds, were identified as having tendon injuries after high-speed treadmill exercise. Neither horse was identified as being lame during the screening examination. One incurred an injury to the superficial digital flexor tendon (SDFT) and one had an injury to the deep digital flexor tendon. Both were core-type lesions rather than strike lesions and were not considered to be career limiting.
Other causes of severe lameness
One horse developed acute hindlimb lameness immediately after the exercise test. This was suspected to be a recurrence of a previous tibial stress fracture although no abnormalities were detected on radiography and further investigation including scintigraphy was declined by the owner. Another horse with a previously diagnosed femoro-tibial arthropathy had a marked increase in lameness following treadmill exercise.
Five (0.2%) horses (3 Thoroughbred racehorses, one Arab endurance horse and one Quarter Horse) developed severe post exercise myopathy that required i.v. fluid therapy. Two of these 5 cases occurred after submaximal exercise only: one occurred in a racehorse following a training session and the other occurred in the endurance horse following an exercise test at submaximal speeds. Both the Arab and Quarter Horse had a previous history of recurrent episodes of exercise-induced myopathy and the Quarter Horse was subsequently diagnosed with polysaccharide storage myopathy. For the remaining 3 horses there was either no previous history of exercise-induced myopathy or that information was not available.
A total of 107 (4.7%) horses were reported to sustain minor injuries (Table 3). There was some variation between the reported prevalence in the different centres (Table 1), which may reflect under-reporting of what were considered to be insignificant lesions in 4 centres that provided limited information on minor injuries. The most common injuries reported were minor grazes, wounds or abrasions (n = 34). Of these, 7 occurred as a result of the horse rubbing its shoulders against the bars of the treadmill during exercise, 4 were associated with the horse coming off the back of the treadmill or attempting to jump the bar during the initial training session (of which one sustained a cut lip that required suturing), 3 were associated with the horse losing a shoe, one occurred when the horse reared and fell, one occurred when the horse collapsed as a result of a cardiac arrhythmia and the cause of the remainder were unspecified. Nineteen horses sustained small overreach injuries to the heel bulbs and a further 9 horses were reported to be foot sore following treadmill exercise. One horse also sustained a solar puncture after pulling off a shoe and standing on a nail. A number of horses were also reported with increased heat and/or soft tissue swellings (associated with tendon sheath or joint effusions) that were not associated with any lameness (n = 17). A total of 11 horses were reported to have mild or moderate lameness for which the cause was not diagnosed and 11 horses had some exacerbation of pre-existing low grade lameness. Five cases were diagnosed with mild exercise-induced myopathy. These horses had elevated levels of serum creatine kinase (CK) and aspartate aminotransferase (AST) and signs of mild muscle stiffness following strenuous exercise, which resolved within 48 h.
Table 3. Details of minor injuries
|Heat/soft tissue swelling not associated with lameness||5||12|
|Mild exacerbation of pre-existing lameness||5||6|
|Mild/moderate undiagnosed lameness||4||7|
|Muscle stiffness, mild exercise-induced myopathy||0||5|
This is the first paper to attempt to determine injury rates in horses undergoing high-speed treadmill exercise testing. The majority of horses underwent treadmill training and exercise testing without incident. Overall, 5.4% of horses sustained some form of injury in association with treadmill exercise. Most injuries (4.7%) that were incurred were minor in nature. Major injuries were sustained in 0.6% of horses, with only one (0.04%) sustaining a catastrophic injury that required euthanasia. A previous study comparing conditioning of racehorses on a treadmill vs. the track also reported that horses learned to gallop on the treadmill with minimal problems and that exercise training on a treadmill was considered to be a safe procedure (Kobluk et al. 1996). That study examined 107 horses exercised on the treadmill for at least 60 days prior to racing. Although accurate records of injury rates were not kept, no major accidents were reported and there was a perception that the treadmill-trained horses incurred fewer injuries than those undergoing conventional training.
This was not a randomised controlled study and therefore it is not possible to make direct comparisons with injury rates in the field. Additionally, when comparing with previous studies, the true incidence rate of injury associated with exposure to treadmill exercise is in fact likely to be significantly lower than our injury rates suggest, because we report the injury rate per horse and not the injury rate per treadmill exercise session. Most horses underwent more than one training session, plus the exercise test (i.e. a minimum of 2 separate exposures to treadmill exercise). Nevertheless, a number of studies have been performed to assess injury rates during training, racing and competition and hence it may be useful to compare the injury rates reported in this study with those reported elsewhere.
Musculoskeletal injury is common in athletic horses and several studies have examined the incidence incurred during racing. Recently, Pinchbeck et al. (2004) reported that musculoskeletal injuries occurred in 1.9% of 2879 National Hunt (jump) race starts in the UK. However, injury rate varied between racecourses (1.6–3.5%). Previously, Williams et al. (2000) reported an overall incidence of 0.8% of 222,993 starts on UK racecourses between 1996 and 1998. Both of these studies only included injuries that were reported to the official veterinary surgeon or were detected by the veterinary officer at the end of the race. Therefore, it is likely that minor injuries such as small wounds and grazes went unreported.
Musculoskeletal injury and lameness also occurs frequently in horses involved in other athletic disciplines and horses competing in different sports may be predisposed to different injuries (Murray et al. 2006). A study investigating injuries in event horses reported an overall injury rate of 0.45% in horses during the cross country phase. However, the injury rate varied according to the level of the competition and was found to be substantially higher in CCI (3-day event) competitions, where 7.7% of cross country starters were reported as injured to a veterinarian (Singer et al. 2008). The increased incidence in this population was proposed to be because of the higher level of exertion at CCI level events and also due to increased veterinary supervision at these events.
The incidence of major injuries in this study was comparable with those reported in the literature. The fracture rate during treadmill exercise was 0.2% compared with 0.4% during racing (Pinchbeck et al. 2004) and 0.02% in event horses (Singer et al. 2008). Three of the 4 fractures in the current series were identified following high-speed exercise and all were in Thoroughbred racehorses. In 2 cases, the fractures were related to specific traumatic incidents and the other 2 confirmed as stress fractures. Another horse was also suspected to have a recurrence of a tibial stress fracture, although this was not confirmed. Stress fractures are associated with pre-existing pathology (periosteal and endosteal new bone formation) (Riggs 2002) and these injuries are as likely to have occurred in these horses during training or racing as during treadmill exercise. Indeed, in one case, the fractures were considered to be long-standing and were the likely cause of the poor performance, rather than being incurred as a result of HSTE testing.
Stress fractures are considered to be the most important cause of lameness in racehorses in training, resulting in the most days lost to training (Dyson et al. 2008). Recent studies have reported fracture rates of 1.1 and 1.15/100 horse months during training (Verheyen and Wood 2004; Ely et al. 2009). Stress fractures and other overuse injuries are also common in human athletes (Fredricson et al. 2006). Tibial stress fractures are the most common fracture type in overground runners (Brunet et al. 1990). There are no comparable epidemiological data for treadmill runners in the literature. However, a recent study found significantly higher tension and compression strains and strain rates in the tibia during overground running, compared with treadmill running (Milgram et al. 2003), suggesting that treadmill runners are in fact at a reduced risk of tibial stress fracture than overground runners.
Tendon and ligament injuries are also common in racehorses (Williams et al. 2001; Pinchbeck et al. 2004; Wilsher et al. 2006;Ely et al. 2009), eventers (Singer et al. 2008) and showjumpers (Dyson 2002) during both training and competition. In racehorses, tendon and ligament injuries are reported to be the most common injury incurred during racing, with studies reporting an overall incidence rate of 0.4–0.7% (Williams et al. 2001, Pinchbeck et al. 2004). Tendon injuries may occur as sequelae to muscular fatigue (Butcher et al. 2007) and are also more common in horses that jump (showjumpers, eventers and National Hunt racehorses) (Murray et al. 2006). Singer et al. (2008) reported an overall incidence of 0.05% tendon or ligament injuries during horse trials in Britain in 2002. However, 3-day event horses were at significantly increased risk (1.2% starters) in comparison with one-day eventers (0.02%).
Anecdotally, it has been suggested that horses may be at increased risk of tendon injury when exercising on a treadmill. However, in this study there were only 2 reports of tendon injury following treadmill exercise (0.09%). A study by Takahashi et al. (2002) found that the peak forces in the SDFT were lower during inclined running compared to the flat and hence uphill exercise (as is usually performed on a treadmill) may reduce the risk of SDFT injury.
A total of 10 (0.4%) horses were found to have exercise-induced myopathy following treadmill exercise. This is lower than that reported by Singer et al. (2008) in CCI level eventers (1.2% of starters) but higher than in one-day event horses (0.02%). Neither study of racing injuries reported the incidence of exercise-induced myopathy. However, others have reported a prevalence of approximately 7% in Thoroughbred racehorses, with 74% experiencing recurrent episodes (McGowan et al. 2002; Upjohn et al. 2005). The prevalence of recurrent exertional rhabdomyolysis (RER) is reported to be higher in Thoroughbred racehorses compared with the general population of riding horses (Harris 1991; Cole et al. 2004). This is probably due to a combination of factors. Thoroughbred horses may have an underlying abnormality in muscle calcium regulation that predisposes them to this condition (Hodgson 1993; Lentz et al. 1999; Mlekoday et al. 2001). Also, racehorses are exposed to higher exercise intensities than horses undergoing other athletic disciplines and are hence more likely to be over-exerted. Eight out of the 10 cases that developed exercise-induced myopathy following treadmill exercise were Thoroughbred racehorses. It was unclear if they had a previous history of RER. However, it is possible that at least in some of these horses, this may have been the cause of their poor performance. The 2 remaining horses that developed severe myopathy following treadmill exercise were an Arab and a Quarter Horse, both of which had previous histories of recurrent episodes.
Most injuries incurred during treadmill exercise were considered to be minor in nature. Wounds and abrasions were the most common finding (in 1.5% horses). Overreach injuries were also common in this study despite all centres reporting the routine use of overreach boots. Few comparative data exist for horses undertaking overground exercise, although skin wounds were the most common injuries (0.26%) sustained by eventers in the study by Singer et al. (2008) and lacerations and wounds were also reported by Pinchbeck et al. (2004) to be the second most common injury in NH racehorses (0.6%). The lower figures in these studies may be because only injuries requiring veterinary attention were included and hence minor wounds and grazes are unlikely to have been reported. However, although the study by Kobluk et al. (1996) did not provide evidence of such, it is possible horses may be at increased risk of incurring minor injuries during treadmill exercise due to incoordination, rubbing against the bars of the treadmill or through loose/lost shoes. Further, prospective studies are recommended in order to more thoroughly address potential risk factors.
Only one horse was reported to sustain a major injury as a result of failure to wear a safety harness, which meant that it came off the back of the treadmill at speed, and 2 others sustained minor grazes when coming off the back of the treadmill during the first training session when the treadmill was started. Although records were not kept, it is likely that the use of a safety harness prevented a number of other horses from coming off the back of the treadmill, for example if they stumbled or stopped suddenly during testing. This was illustrated in one horse that collapsed due to a cardiac arrhythmia but sustained only minor grazes. Horses may also be at risk of coming off the back of the treadmill when fatigued at the end of the test. The use of a safety harness is therefore strongly recommended, particularly when performing exercise at speed. Furthermore, it is recommended to have someone positioned parallel with the rear of the horse in order to encourage them to keep forwards, especially when the belt is moving at speed.
In conclusion, all horses undergoing strenuous exercise are at some risk of incurring musculoskeletal injury. However, this study confirms that treadmill exercise is a relatively safe procedure. The majority of horses performed treadmill exercise without injury and the prevalence of major injuries was found to be similar to those reported during racing and eventing.
Conflict of interest
The authors have not declared any potential conflicts.