Whips, spurs, nosebands, bits and bridles are used by riders around the world, but the line dividing horse abuse from an acceptable form of training and control can seem elusive.

Charlotte Dujardin pulled out of the Olympics – and then was banned from her sport for six months – after she was filmed whipping a horse in training. “What happened was completely out of character and does not reflect how I train my horses or coach my pupils, however there is no excuse,” she said. “I am deeply ashamed and should have set a better example in that moment.”

Related: Dujardin’s career in tatters after horse whipping costs her damehood and funding

At one end of the spectrum, horse trainers will say that they understand what horses need, from working with them day in and day out, in a way that a layperson watching from the side will not. They will also argue that the horse, pretty quickly, will tell you what it wants, and that it’s a foolish trainer who tries to dominate rather than work with one of these animals.

“We have been riding and training horses for at least 4,000 years,” said Sarah Arnot Mulhern, an amateur eventer who lives in South Africa. “I believe one of the earliest descriptions of training horses comes from Xenophon [The Art of Horsemanship] in 355 BC.

“You could not get an animal that weighs 500 to 600 kilos to do something it doesn’t want to. That’s why we don’t ride zebras. They don’t want to be trained. With horses, we develop this incredible bond.”

But at the far end of the debate are concerns about practices such as hyperflexion, where the horse’s neck is pulled into a contorted, unnatural angle. Also known as Rollkur, this technique has been controversial for years, and is banned by the Fédération Equestre Internationale (FEI), but is still used by some trainers.

Horses can also show signs of blue tongue, which results from too little oxygenated blood flowing to the tissue, while over-tightened nosebands can create stress.

Other abusive techniques include overuse of spurs, underfeeding, denying water and rapping, a practice used by showjumpers that involves raising a jump bar as a horse passes over to ensure it clears the obstacle.

It is in the middle that things become particularly complex, with the use of crops and whips. The use of a whip is tightly regulated during races, and can be used a maximum “of six times in a flat race or seven times in a jump race”. Any more than this will prompt the stewards to review the ride.

Prof Paul McGreevy, veterinary ethologist at the University of Sydney’s school of veterinary science, said: “Dressage as a sport is in trouble and there is a raft of welfare issues that mean the dressage world needs to get its house in order. Given that the standard you walk past is the standard you accept, horse people around the world are appreciating they can no longer look away.”

But dressage whips are rarely used during sporting events, according to Madeleine Hall, a former Horse and Hound journalist. She told Radio 4’s Today programme that “generally in dressage, the whip is used to encourage the horse to use more impulsion from behind or perhaps to lift its legs a little bit higher. But you can’t use the whip too harshly because horses are very sensitive and they’ll react badly, they’ll overreact.” In dressage, she said, it is mostly used as a training aid and quite cautiously.

But behind closed doors, the boundary is harder to define. The Spanish dressage trainer Patricia Sánchez argues that those who work with horses need to understand where the line is. “I don’t see any problem in training horses as long as it is done with respect, knowledge and, above all, seeking to create a co‑worker with whom we can play, grow and have fun together, a friend who you should care for and love.

“The line that should not be crossed is when you stop respecting the animal, when it is used as a machine and not as a partner that you should love and protect above your priorities, when there is submission instead of conviction and when for the horse it is no longer fun to be with you, but a sacrifice.”

Dr Eva Van Avermaet – a rider, vet and founder of Collective pour les Chevaux, a group that fights for horse welfare – agrees, saying that perhaps the border should lie between what creates pain and fear, and what does not. “If you use a whip, which is not indispensable to train a horse, it should only be used as an extension of your arm, to indicate; not to hurt nor frighten.”

At elite levels, problems with welfare can be compounded, said Julie Taylor, a horse welfare activist and author of a book that argues for removing equestrian sport from the Olympics called I Can’t Watch Anymore. “In elite equestrian sport horses are more or less constantly exposed to painful stimuli in order to coerce them to perform.

“This doesn’t just happen behind closed doors or in the warm-up arena at shows. It happens right in front of the judges and is televised and admired by millions of people the world over who are unable to recognise the signs of pain and fear displayed by the horses,” she said.

Some groups even question whether horses should be ridden at all. “This question of what is acceptable or not, does not take into account the horse being, him or her,” said José De Giorgio-Schoorl of Learning Animals and author of Equus Lost?, a book on horse-human relations. The Learning Animals association embraces an anti-speciesist ethology and argues every animal wants to live on their own terms.

“It [riding horses] raises the same questions as circuses and farming or any animal use. We should not be riding horses or training them at all. Because every time you train a horse, you peel away a bit of their agency.”