Working equid biomechanics

Working equids play a key role in supporting the livelihoods of many people in low and middle income countries (LMICs). They are typically used for work involving load carrying or draft power for transporting either people or goods, and their welfare is inextricably linked to that of their owners (Tadich, 2020). Historically, their economic importance has been overlooked as it is less easy to quantify than the contribution of production animals, but this is beginning to change. The loads that they carry and pull are usually far more than what is expected of competition or leisure horses, and often they will transport these loads for a prolonged period of time, and under challenging environmental conditions. For example, while there is regional variation, a quarter of donkeys in Pakistan have been reported to carry loads equal to 90% or more of their own bodyweight, with some donkeys estimated by their owners to be carrying more than 150% of their bodyweight (Bukhari et al., 2022). This is far from unusual, and has been reported in multiple locations.

Given the obvious welfare issues faced by working equids, such as poor body condition, musculoskeletal problems, parasitism, behavioral issues and poor access to veterinary care, biomechanics may seem an odd topic to cover. However, biomechanics are simply how the animal (and its load) moves, and this has a key impact on musculoskeletal conditions, which are a leading cause of poor welfare in working equids. An understanding of how this works is helpful when considering problems and developing interventions, although most work on biomechanics has been conducted on sports horses and Thoroughbred racehorses, and may not be directly transferable to donkeys or mules.

In terms of locomotor abnormalities when load carrying, increasing loads can lead to a reduction in gait symmetry and gait stability, as well as alterations to stance time (the time each hoof is in contact with the ground) and the horse’s head and neck position (Bukhari et al., 2021). These changes may occur at a relatively low load, and could provide a useful tool for professionals to assess load appropriateness in conjunction with behavioral and physiological indicators. The biomechanics of load pulling have been less well-studied than load carrying. The impact of pulling a load is different to carrying one, and depends on the type of vehicle or load (e.g., two or four wheeled cart, sled, log, etc.). Draught force can be defined as the force required to pull a load in the same direction of travel as the animal, and this and the harness have a large impact on the forces experienced by the animal (Bukhari and Parkes, 2023). For example, when donkeys are used in logging, a greater force is required to pull a sled compared to a wheeled vehicle, and more force still is required to pull a log alone (Rodrigues et al, 2023). In addition, full collars, shaped or adjusted to take into account the different shape of a donkey’s neck in comparison to a horse’s, spread force more effectively and are likely to result in improved welfare (Garrett et al., 2021). This knowledge, alongside practical considerations such as availability and cost of materials, may impact on an owner’s choice of draught equipment.

When considering interventions for working equids, it is essential to keep in mind the function of the animal, the environment in which they work, and what resources are available locally. For example, it is more helpful to an owner to help them redesign their donkey’s collar using locally available material, so that they can undertake future modifications and repair themselves, than it is to give them an imported item of equipment. Novel approaches to problems (e.g., looking at biomechanics research in horses used for sports) can help in answering some questions and provide solutions. Future work should focus on using an understanding of working equid biomechanics to identify areas for practical improvements, help those working in the field, and inform policy.