Reins are an important feature of the bridle as they allow for communication between the horse and rider  and the tension the rider creates through pressure on the reins is an important factor to remember when considering the horse’s wellbeing.
Piccolo and Keinapfel  conducted a study in 2019 to compare the rein tension between a ridden and unridden horse when placed in a dressage position, and the possible conflict behaviours that may arise . They found that without a rider, the horses voluntarily maintained a rein tension of 1kg in all gaits, however, when ridden the tension force to maintain the same frame was higher (approximately 3kg) . Therefore, it is possible to maintain a correct dressage frame without much force from the rider. They also found that there was an increase in conflict behaviour from the horses when ridden as compared to without a rider, indicating that too much rein tension could result in pain which could then develop into a conflict response .
Warren-Smith and associates  conducted a similar study, comparing rein tension while riding to that of long-reining . They found that more tension was required in long-reining, though they believe this could be due to the lack of other forms of communication between horse and rider, such as the rider’s seat . They also found that “different horses required different rein tensions to elicit the same responses”, however, overall, they believe that “responses can be achieved with relatively low tensions” and that because of this, “one must question the welfare of horses subjugated to significantly greater tensions” .
Overall, these studies indicate that riders need to be aware of the force used when communicating with their horse as too much tension in the reins could negatively affect the horse’s welfare.
However, a study conducted by Clayton and associates found that “the rider’s perception of tension was very different from the tension data recorded by the strain gage transducers” . They suggest that the rider’s assessment of rein tension is highly subjective, as the perception of tactile sensation is difficult when riders are inundated with stimuli from both the horse’s movements as well as the environment around them . They also claim that there is a contrast in the perception of the left and right hands so that when the rider describes the contact as smooth and consistent, they were unaware of the undulating nature of the rein tension . This indicates that even when riders are attempting to maintain a light, consistent rein tension when communicating with their horse, they may be unaware of how much tension they are creating, leading to riders unknowingly affecting the welfare of their horse.
Some instructors suggest that beginner riders use a martingale or elastic rein inserts to prevent an unsteady hand position from causing discomfort, which could then lead to conflict behaviours . However, Heleski and associates  found that there was minimal conflict behaviour observed in the riding horses, despite there being a higher level of rein tension when a martingale was implemented . It was also found that there was no benefit gained from implementing elasticised rein inserts, due to the low rein tensions used by novice riders . Therefore, neither martingales nor elastic rein inserts help to minimise the amount of rein tension applied to the horse and is not useful in preventing the negative effects of too much rein tension on the welfare of the horse.
 Clayton, H.M, Singleton, W.H, Lanovaz, J.L & Cloud, G.L 2003, ‘Measurement of rein tension during horseback riding using strain gage transducers’, Experimental Techniques, vol. May/June, pp. 34 – 36
 Piccolo, L & Kienapfel, K 2009, ‘Voluntary rein tension in horses when moving unridden in a dressage frame compares with ridden tests of the same horses – a pilot study’, Animals, vol. 9, no. 321, pp. 2 – 10
 Warren-Smith, A, Curtis, R, Greetham, L & McGreevy, P 2007, ‘Rein contact between horse and handler during specific equitation movements’, Applied Animal Behaviour Science, vol. 108, pp. 157 – 169
 Heleski, C.R, McGreevy, P.D, Kaiser, L.J, Lavagnino, M, Tans, E, Bello, N & Clayton, H.M 2009, ‘Effects on behaviour and rein tension on horses ridden with or without martingales and rein inserts’, The Veterinary Journal, vol. 181, pp. 56 – 62