A recent study would suggest they are.
Metacarpophalangeal joint hyperextension overload is increasingly being recognised by Vets in dressage horses and, like forelimb suspensory ligament injury, tends to be seen in horses with extravagant trot steps. There has however, been limited understanding of the effects of different paces within the trot on forelimb movement from a veterinary perspective thus rendering it difficult for vets to advise rationally on prevention or management of these types of injury.
A study "What Effect Does Medium and Extended Trot Have on the Kinematics of the Forelimb in Dressage Horses?" has been released by the Equine Veterinary Journal with some findings to confirm that medium and extended work really does cause wear and tear on the joints due to increased fetlock extension.
During the study, they took twenty mixed-breed dressage horses (age 9 ± 4 years; height 168 ± 6 cm; weight 600 ± 63 kg; median competition level = advanced medium) were tested at collected and medium/extended trot (age and training level dependent) in a straight line on an artificial surface ridden by their own rider at sitting trot. Four strides of each pace were recorded using high speed motion capture technology (240 Hz). Markers were placed on the horses' forelimbs at predetermined anatomical sites. Fetlock, carpus, elbow and shoulder angles were derived at midstance. Descriptive statistics and mixed effect multilevel regression analyses were performed on the data.
The results showed that fetlock extension angle was significantly increased at medium compared with collected trot (coefficient: 5.70; CI 2.58–8.82; P<0.01) and extended compared with collected trot (coefficient: 8.59; CI 5.16–12.02; P<0.01). Fetlock extension angle was significantly increased when carpus extension angle (coefficient: 0.61; CI = 0.4–0.82; P<0.01) and shoulder flexion angle were increased (coefficient: 0.18; CI 0.01–0.33; P<0.05).
They concluded that fetlock extension increased when the horses performed lengthened trot paces, more in extended than medium trot. The loading of the carpus and shoulder were related to fetlock extension, suggesting that lengthened paces affect the loading of the entire forelimb. Lengthened paces may be contraindicated in horses with fetlock hyperextension or suspensory ligament injury; they may be a potential risk factor for these injuries. Interaction with the surface could also have a role that could be further investigated.
References:
Walker, V.A., Tranquille, C.A., Dyson, S.J., Newton, R. and Murray, R.C. (2015), What Effect Does Medium and Extended Trot Have on the Kinematics of the Forelimb in Dressage Horses?. Equine Veterinary Journal, 47: 12–13. doi: 10.1111/evj.12486_27
Study Author Information: Animal Health Trust, Kentford, Newmarket, Suffolk, UK