The effect of rider weight on Icelandic horses in tölt

Recently, two scientific papers were published in the European journal Animal, authored by staff of the Equine Science Department at Holar University College in cooperation with the Swedish Agricultural University Uppsala (SLU). The articles are in open access on the internet and both are based on the same research which was executed at Holar University College in 2014.
The aim of the research was to measure the effect of rider‘s weight on a) physiological responses e.g. heart rate and plasma lactate, and b) on stride parameters, in tölt in Icelandic horses.  This was done by increasing the body weight ratio (BWR) between rider and horse, in the BWR range common for Icelandic horses, from 20 to 35%.
In the research, eight adult Icelandic riding horses at Holar were used and one experienced rider rode all the horses, but extra weight in the form of bars of lead was added. Each horse performed an incremental exercise test in tölt at a BWR starting at 20%, then increased to 25%, 30% and 35%, and finally 20% was repeated. Each horse performed five phases, 642 m (2 rounds on a 300 m riding track, outer circumference) with each BWR, in tölt at a speed of 5.4 m/s (19 km/h). Between phases, the horses were stopped for approximately 5½ minutes to add lead weights and take blood samples. Heart rate of horses was measured with a heart rate monitor and the kinematics was measured using a high-speed camera (300 frames/s).
The results (paper I) showed that both heart rate and lactate increased with increased BWR which reflects the response of the weight of the rider alone, as the riding style was the same throughout the test. The BWR at the lactate threshold (4 mmol/L) was 23%, with a individual range of 20.0-27.5%, indicating a considerable difference between individual horses.
The results (paper II) showed that increasing BWR from 20% to 35% did not affect beat, symmetry and height of front leg lift in tölt. However, it led to decreased stride length, increased duty factor (stance phase of the legs), and increased bipedal support (two legs on ground), at the expense of decreased unipedal support (one leg on ground). This means that increased rider‘s weight affects some stride parameters of tölt and can possibly negatively affect subjective judgement of tölt, e.g. in competition.
The results of this research showed that adult and properly trained Icelandic riding horses coped well with the exercise test while carrying a rider in a good balance, weighing up to 35% of the horses body weight. The authors emphasise that many factors in addition to rider‘s weight can affect the intensity of an exercise for a horse. This can be factors like body condition of the horse, training status of the horse, gaits and conformation of the horse and not least the balance and riding style of the rider. However, generally it can be advised that the heavier the rider, the more aware he/she should be of how much intensity is put on the horse during riding, e.g. for how long and how fast is ridden.
Paper I.: The effect of rider weight and additional weight in Icelandic horses in tölt: part I. Physiological responses). See:
Paper II.: The effect of rider weight and additional weight in Icelandic horses in tölt: part II. Stride parameters responses). See:
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