ORLANDO, Fla. -- Service dogs are becoming more and more common in daily life, but many of the ways in which they assist their masters still go largely unnoticed.

These animals don't just help people who have visual or hearing impairments or mobility issues. They also aid those who experience seizures or deal with conditions such as diabetes, post-traumatic stress disorder and depression. A number of competitors at the 2016 Invictus Games rely on service dogs.

Here are a few of their stories:

Brett Parks and Freedom

Brett Parks may have originally balked at the idea of having a standard poodle as a service dog, but now he's the breed's biggest fan. Bill Frakes for ESPN

Parks is a retired U.S. Navy aircrewman who lost his lower right leg as a result of being shot while attempting to break up a robbery in Jacksonville, Florida. A therapist advised him to get a service dog to help deal with his post-traumatic stress disorder.

Parks had thought the animals were only for people who had been in combat situations, but he learned that service dogs can assist anyone who has experienced a traumatic event. He still wasn't sold, so the therapist explained that a dog could retrieve items such as a phone or keys and could help him get up if he fell.

Finally convinced, Parks put his name on a two-year waiting list to acquire a dog. After six months, he got a phone call. The conversation went something like this:

Hey, we've got a dog for you. We have an open spot. Are you interested?

Oh yeah, I'll take it.

Fast forward a few minutes:

By the way, what kind of dog is it?

Oh, it's a poodle.

It's a what?!

It's a poodle.

Waaaiiit a second. I have a reputation to uphold. I don't want to be walking around with a poodle.

Brett, he's a standard poodle. I want you to Google standard poodles and do some research on them.

Parks learned that standard poodles were originally bred as hunting dogs. They typically grow to weigh about 50 pounds, and they shed less than most breeds.

"I've been with him ever since, and he's just been great," says Parks, who readily admits that the dog, named Freedom, has improved his quality of life. "Now I'm spoiled. I don't want any dog other than poodles."

Freedom is a male, but he spent the first two years of his life at a women's prison. Parks later met two of the inmates who trained him, and they told him the experience made a difference in their lives. "It was very special to hear," Parks said. "Freedom is a life-changer."

Anthony Rios and Bugsy

Bugsy wasn't always intended to be a service dog, but when Anthony Rios needed help, Bugsy got the training he needed to fill the role. Bill Frakes for ESPN

Rios is a U.S. Marine Corps gunnery sergeant who can no longer make full use of his left leg after he was hit with a rocket-propelled grenade and later injured the leg further in a car accident. When a counselor first suggested that Rios consider using a service dog, he resisted the idea. He didn't want to answer questions about why he relied on an animal or be perceived as weak.

"I've worked very hard to not show that I have to walk with an assisted device or walk with a limp or show any kind of frailty," Rios says. "Call it what you want, being a Marine or just being a man, having pride in who you are and trying to be upright and get back to that status I was before I was injured."

Besides, even if he decided a service dog could help him, he already had Bugsy, an American Staffordshire terrier, who was nearly a year old at that point. He didn't think he was in any position to get another dog. But the issue that he thought made the whole idea a nonstarter turned out to not be a problem after all. Bugsy could be trained as a service dog, and Rios eventually agreed to the process.

Now he says that training Bugsy was one of the best decisions of his life. In addition to helping with difficulties caused by Rios' leg injury, Bugsy soothes Rios when he's stressed or angry. And Rios wants others to know about the benefits that service dogs can provide. Specifically, Rios points to the alarming suicide rate among veterans.

"I've never met someone who had a service dog who committed suicide," Rios says. "It could be coincidental, but it gives me an every-day purpose. I'm not thinking about the past all the time. I'm not considering what the future could be like."

Indeed, when you have an animal who depends on you, it keeps you focused on the present. Is the dog hungry? Is she going to bite that other dog? Does he need to go outside? "While we're taking care of them, they're taking care of you," Rios says. "I'm not a real mushy guy, but it's powerful."

August O'Niell and Kai

Bill Frakes for ESPN

O'Niell is a U.S. Air Force staff sergeant who was shot in the left knee during a pararescue mission over Afghanistan. While undergoing a long series of surgeries on his lower left leg, which would later be amputated, he battled depression and found out that a service dog could help. He was fortunate to find a training facility that didn't have a long wait list, and he acquired a German shepherd named Kai.

In addition to companionship, Kai provided much-needed blocking protection and weight support for O'Neill when he still had his injured leg. Even though the injured portion of the leg is now gone, Kai still does those things, and with a bit of prodding, he will reluctantly retrieve O'Niell's phone and keys.

"The mental aspect is huge because Kai notices when I'm really depressed or very angry. He comes up to me, and he'll come and lift my hand and put his head on my lap or put his head on my chest. It kinda gets me to switch my mode to be like, 'All right, I need to calm down; it's all good.'

"Kai has just been there for me for everything."

Jesse Graham and Stanley

Stanley goes above and beyond for Jesse Graham: He opens and closes doors and can even turn lights off and on. Bill Frakes for ESPN

Graham is a U.S. Air Force veteran crew chief, and a 2014 snowboarding accident left him partially paralyzed.

While undergoing outpatient rehabilitation at the Walter Reed National Military Medical Center in Bethesda, Maryland, he noticed a number of service dogs being used there and became involved in training them. He saw the amazing things the animals can do and the many ways in which they could assist him. Eleven months after applying for a service dog, Graham received Stanley, an English Labrador.

Stanley retrieves items. He can open and close doors. He can turn lights on and off. He can brace Graham's body to make it easier to balance. He most definitely helps Graham get around.

"He can pull me. He just gets beside me, I grab onto the vest -- and we go for a ride," Graham says. "There are times he goes really fast. The amount of energy I give him is the amount of energy he gives me. If I get him amped up, we go fast. We fly through airports. It's great."

Stanley has his own Facebook page, where you can follow his adventures as he plays tug-of-war with Invictus competitors, mingles with Disney characters and visits airplane cockpits. But he isn't without at least one guilty pleasure. When he gets close to water, it's time to swim. If Graham isn't vigilant around a pool, Stanley will swiftly revert from service animal to regular dog. Plunge. Splash. There goes Stanley -- leash, service vest and all -- ambling over lane lines and getting into swimmers' faces to greet them.

"Lastly, he's a companion," Graham said. "A lot of these sports [competitions], I have to go on my own. My family is left behind, and it's nice to have a buddy with me when I go places."