Effective communication between driver and horse through the reins is essential in harness racing to promote safety and optimise performance. Yet, the magnitudes of rein tension applied to driven horses, particularly Standardbred trotters (ST) are currently poorly understood. This is surprising given the number of reports that speak of mouth lesions after competition and equipment use that give raise to horse welfare concerns. Combining rein tension measurements with behavioural parameters has the potential to characterize “driveability” (as compared to “rideability”, the equivalent industry term for riding horses). Thus, the aims of the current study were: (i) identify how drivers perceive ST’s behavioural reactions in response to rein signals when driven on a racetrack, (ii) investigate whether drivers’ subjective appraisals of horses’ behavioural responses align with measured rein tensions (RT), (iii) relate these appraisals to the horses’ perceived driveability (score 1 = poor, 10 = excellent), and (iv) assess whether drivers differ in their scoring of horses’ driveability. Nine ST (5 geldings, 7.8 mean ± 2.1 SE years; 4 mares, 6.8 ± 0.5) were driven by 11 drivers (7 females; 4 males) all of whom were experienced in driving ST. Nine drivers tested three different horses each, and two drivers drove two of the horses. This resulted in 31 test drives involving several segments, each comprising a series of changes of gait and direction of travel. Rein tension meters were used throughout. After each test, drivers were asked to report on their experience of each horse they had just driven and to estimate RT (continuous rating scale from 0 to 50 kg) while driving that horse. Overall, segment had a significant effect on median RT (P < 0.001), with RT rising significantly in racing trot (average 59 N; Trot Left/Trot Right) than trotting on a circle (23 N; Circle Left/Circle Right) and walking (8 N), and was higher in counter-clockwise than in clockwise direction in racing trot (P = 0.058). Furthermore, there was an alignment between recorded RT and drivers’ estimates of perceived RT (P < 0.001), and driveability scores increased when estimated RT increased. The current study has confirmed that rein tensiometry may have a place in providing an evidence-base for consistent rein use, especially when horses are driven by multiple drivers. Whether or not soft tissue damage after competitive racing is correlated to higher peak RT, caused by the use of harsher equipment, or a combination of both, merits further investigation.
