Celebrity trainer Tony Horton says people starting his workouts have the most trouble with these 3 exercises

Celebrity trainer Tony Horton is best known for his very successful P90X at-home fitness program.

It was followed by P90X2, P90X3, and a number of smaller programs. Many have found success with the programs over the years, but they are definitely not easy.

We spoke to Tony recently and asked him what exercises people starting his programs have the most trouble with.

Here's what he told us:

"Well, I don't like to say there's a universal truth there, because you know - different athletes, different people, different stages of life ..."

"So, the ones that I her about from different groups - plyo (plyometrics) is a bear. If you haven't jumped off the ground in your life and I'm asking you to get up in the air and then learn how to land softly so you don't hurt your knees, that can be very problematic for some people initially so they have to modify, especially if they have a lot of weight to lose. But if you've been a track athlete or someone who's had explosive leg activities in your life, parkour or whatever, then plyo is hard but it's still do-able."

"Chest and back is another one too, I would say that the pull-up is a conundrum for a lot of people, especially women who have a lot of weight to lose. It just feels like it's impossible. I have a friend, this girl Paige, she couldn't do a pull-up at all. She had to use a pull-up assist band, she had to put all three of the little assist bands on there - she could do like 8 or 9, and now she can do like 4 - doesn't sound like a lot, but when you think about it, she lost a lot of weight and went from using a band to not having to use it at all. And she's a 42-year-old mother of 3, who looks like she's like a professional rock climber. I mean it was a solid 9 months before she went from 'couldn't do it' to 4, but she just didn't miss a day, you know."

"So for some people it's plyo, for some people it's pull-ups, and most certainly it's the yoga. The yoga is a crusher for a lot of men who've never done it before. A lot of big, strong, powerful, fast, athletic dudes - they do that hour and a half yoga class and they feel like absolute neophytes - fitness neophytes. And the smart ones come right back and do it again and again and again."

"I do event after event where people have my programs and know my stuff and have been through several rounds and I'll look out and there'll be 2,000 people out there and half of them are men or more and everyone knows up dog from down dog from chaturunga, from chavasana from a reverse warrior 2 - they know what I'm taking about, they can all do it and 5, 6, 10 years ago they had no idea what I was talking about, couldn't even get close to it. There is a trend, it's changing and more men are getting it. But I would say it's those three: yoga, pull ups, and plyo."