Horses carrying 30% body weight showed a significant increase in muscle soreness and muscle tightness scores. The changes were less marked when they carried 25% body weight.
Dr Debra Powell and colleagues at the Ohio State University Agricultural Technical Institute, Wooster, conducted a study in which horses, carrying up to 30% of body weight, were monitored performing a standardised ridden exercise test in an indoor school arena. After five minutes active walk to warm up, the horses were ridden at a trot (3m/s) for 4.8km, followed by 1.6km at a canter (5m/s). This exercise schedule was chosen to simulate a 45-minute work period of work typical of an intermediate-level riding school horse.
The researchers measured heart rate, plasma lactate concentration and creatine kinase. Lactate is produced in the muscles during exercise. At low levels of work the body can metabolise it and so levels in the plasma remain low. As the work level increases the rate of lactate production exceeds the body's ability to remove it and so concentrations rise. Creatine kinase (CK), an enzyme present in the muscles, is released into the blood as a result of some types of muscle damage.
An animal massage therapist assessed muscle soreness and muscle tightness before and after exercise.
The findings seemed to support the view that horses can carry up to 20% of their body weight without difficulty. There was little difference between all the measures when horses carried either 15% or 20% of body weight. However, when the weight carried increased further, the scientists started to detect differences.
When horses carried 25% or 30% of their body weight their heart rate remained elevated for longer after exercise. The serum CK level was higher immediately after exercise, and 24 and 48 hours later, in horses carrying 30% body weight compared with those carrying 25% or less. There was no change in CK when horses carried 15 and 20%. Plasma lactate levels were higher immediately after exercise and 10 minutes after end of exercise, in horses that carried 30% of their body weight.
The study also investigated whether the horse's conformation affected its weight-carrying capacity. The scientists looked at the horse's height, circumference of the cannon midway between knee and fetlock, and width of the back (loin) behind the saddle - between the last rib and pelvis.
They found that horses with wider loins showed less muscle soreness and tightness when carrying 25% and 30% body weight.
This was a small study involving only 8 horses. The scientists suggest that further investigations into the value of loin width as an indication of weight carrying ability would be worthwhile.
Evaluation of indicators of weight-carrying ability of light riding horses.
DM Powell, K Bennett-Wimbush, A Peeples, M Duthie.
J Eq Vet Sci (2008) 28. 28 - 33.