Rein tension was the subject of a pilot study performed at the University of Helsinki (Finland) and summarized last fall in a news release from its Faculty of Veterinary Medicine. In this study, high rein tension was found to be associated with trotters opening their mouths, indicating pain or discomfort in the mouth.

Rein tension denotes the force employed by the rider or driver through the reins. It can be measured with a sensor attached between the bit and the rein.

Behavior and injury

“Our group has previously investigated mouth injuries in trotters, and we found that moderate and severe injuries were associated with certain bit types,” says researcher and veterinarian Kati Tuomolafrom of the university’s Faculty of Veterinary Medicine. “It is possible that drivers ended up using those bit types due to problems experienced with lighter rein cues. This is why we wanted to explore rein tension.”

Eight horses and their drivers were recruited for the study.

With the drivers driving their horses on a racetrack at walk and a trot, the researchers measured rein tension. They also recorded video of the horses’ behavior from a moving vehicle.

Each horse was equipped with a regular single-jointed bit. Subsequently, one researcher coded the horse’s behavior from the videos in accordance with a predetermined catalogue of behaviors. The coder was unaware of any findings associated with rein tension and mouth injuries among these horses.

None of the horses had mouth injuries before the driving. After the drive, three horses had moderate bruises in their mouths.

Tension measurement

Their median rein tension was numerically higher (approximately 3.5 kg per rein) than that in horses without injuries (approximately 2 kg per rein), and they showed longer periods of a rushed walk.

The median rein tension for a single rein varied between 0.5 kg and 3.7 kg, with the highest tension varying between 11 kg and 24 kg. According to the researchers, these rein tensions can be considered rather high. This is because prior studies have shown that horses avoid tension exceeding 0.6-1 kg. To investigate behavioral differences during different rein tensions, five 30-second periods were visually selected from the rein tension graphs of all horses. These represented samples of tension ranging from low to high.

During low rein tension, the horses mainly walked and mostly kept their mouths closed.

During periods of higher rein tension, the horses mainly trotted either slowly or quickly, keeping their mouths open for longer periods of time.

“Horse trainers should monitor the horse’s mouth behavior, arousal state and ability to walk calmly, and adjust the training accordingly. The horse keeping its mouth widely or repeatedly open may indicate evasive behavior, meaning discomfort or pain in the mouth.

In addition, rushed walking may indicate high arousal, which in turn can increase the risk of mouth injuries,” says Tuomola, the article’s lead author.

Story Source:

Materials provided by University of Helsinki. Note: Content may be edited for style and length.

Journal Reference:

1. Kati Tuomola, Nina Mäki-Kihniä, Riikka Sirviö, Anna Valros. Is rein tension associated with horse behaviour and mouth injuries in harness racing trotters? – Pilot study. Applied Animal Behaviour Science, 2024; 277: 106356 DOI: 10.1016/J.APPLANIM.2024.106356

University of Helsinki. (2024, September 3). Rein tension may affect horse behavior. ScienceDaily. Retrieved April 15, 2025 from www.sciencedaily.com/releases/2024/09/240903144416.htm