(CNN) -- An undercover video shows horses being struck with sticks and subjected to "soring," an illegal process in which chemicals are placed on their lower legs in an effort to induce the signature Tennessee Walking Horse high-stepping gait.
The graphic video shows trainer Jackie L. McConnell of Whitter Stables of Collierville, Tennessee, and others subjecting show horses to practices that were banned 40 years ago. One image shows a writhing horse being subjected to a whip at McConnell's barn. Another receives a shock to the head.
The video, made by a Humane Society of the United States investigator, was first featured Wednesday on the ABC News program "Nightline."
It has raised questions of how pervasive the training techniques are despite recent prosecutions and investigations by the U.S. Department of Agriculture, which enforces the Horse Protection Act.
"There obviously is a huge problem," said Jonathan Lovvorn, senior vice president for litigation and investigations with the Humane Society. "In the competition to get this unnatural gait, trainers are using banned substances to cheat."
Horse show judges value the exaggerated gait, called the "big lick." Shows in Tennessee and elsewhere annually draw thousands of spectators.
PepsiCo on Thursday confirmed it had pulled its sponsorship of this summer's Tennessee Walking Horse National Celebration.
Asked if the sponsorship decision was in response to the 2011 video, PepsiCo spokesperson Vincent Bozek said, "That's all we're saying." The decision was made Wednesday.
The president of a Tennessee horse industry organization, condemning the "disturbing" video, told CNN there is a stringent inspection process at shows.
"I think it's sad that a corporation like Pepsi would go out because of the action of one person and one training barn," said Dr. Stephen Mullins of S.H.O.W.
McConnell and three other men were named in a 52-count federal indictment earlier this year.
According to a defense filing in U.S. District Court for the Eastern District of Tennessee, McConnell will plead guilty next week to a count of conspiracy to violate the Horse Protection Act.
McConnell's attorney, Tom Greenholtz of Chattanooga, confirmed to CNN that his client is wielding a stick on a horse in a portion of the video.
Greenholtz said he could not comment at this time on the specifics of the case or McConnell's view of the allegations.
McConnell faces a maximum five-year prison sentence and expects the government to dismiss the other counts, the attorney said.
According to the indictment, "soring is a cruel and inhumane practice used to accentuate a horse's gait in order to gain a competitive edge in horse shows." Chemicals and other irritants on a horse's ankles and forelegs cause it to lift its front feet and shift its weight unnaturally to the hind legs in order to relieve the pain, the indictment states.
The Humane Society said its investigator documented "stewarding" — training a horse not to react to pain during official show inspections of their legs for soreness -- by striking them in the head when they flinch during mock inspections.
The video was filmed in spring 2011 by an investigator who worked two months as a stablehand at McConnell's barn, according to the Humane Society.
Lovvorn told CNN the society shared the video and results of its investigation with federal prosecutors before the indictment was returned.
In separate cases, a seven-month investigation conducted by the U.S. Department of Agriculture resulted in at least four other Tennessee men being sentenced this year for horse soring violations.
One defendant sentenced to 12 months in prison and a $4,000 fine described how chemical irritants, chains, bolts and other devices were used to bring about the exaggerated gait.
"He stressed the pervasiveness of soring in the gaited horse industry and testified that horses 'have got to be sored to walk,'" according to a press release from the U.S. Attorney for the Eastern District of Tennessee.
S.H.O.W., certified by the U.S. Department of Agriculture (USDA), handles inspections of Tennessee horses at events.
Mullins, a retired equine veterinarian, said self-regulation by the industry has led to three lifetime and 150 one-year suspensions for soring and other violations of the Horse Protection Act.
Trainers shouldn't rely on soring to train horses to hit the "big lick," said Mullins, acknowledging the industry still has problems. "I was given one charge (task). Get rid of the sore horse. I think we are well on our way."
One painless training technique, he said, is to fit pads, about 3 inches tall, below the horse's hoof.
"It can change the way the horse lands on its foot," according to Mullins. "It requires him to exaggerate and keep his foot up for a longer period of time."
S.H.OW.'s oversight, however, does not extend to training facilities and barns.
"Soring is a way to take a horse that is not very good ... to make him look good," said Mullins. "Do I think it goes on around the trainers who show routinely with me? No sir, I do not believe it does."
Lovvorn, of the Humane Society, said soring "has been an open secret in Tennessee for years."
Prosecutions, a stringent update of the Horse Protection Act and recently approved additional resources for the Agriculture Department are key, he said.
A 2010 Office of Inspector General audit within the USDA pointed out the need for more funding of federal inspections and shortcomings and inconsistencies of those done by people associated with horse industry organizations.
The Tennessee Walking Horse Breeders' and Exhibitors' Association, in a statement Thursday, said it reaffirmed its opposition to violations of the Horse Protection Act.
"The walking horse holds an inherent natural gait that has been in existence for nearly 100 years," said group President Marty Irby. "(The association) adopted a zero tolerance policy in regards to soring a number of years ago and has recently challenged every member to adopt a zero tolerance policy themselves."
CNN's Dave Alsup contributed to this report.