"I hate doing this exercise, I'm so bad at it"
"Sprint? No way, I'm awful at it!"
"You want me to jump rope? I can't! I'm terrible at it!"
People, we all suck at something. For instance, I suck at rowing (on the ergometer) and swimming. These are things that, when performed, leave me absolutely smoked. Why, you may ask, would I want to do these things then? Why not do something that I'm good at, like sprinting?
Well, think of it from the perspective of your car. When it's brand new and runs perfectly, it is super efficient and burns a minimal amount of gas in order to get you around town. When your car is a 12-year old hoopty, the engine is beat up. It doesn't run smoothly, and as a result you burn through gas like nothing.
In real life, your body is the car, your muscles are the engine, and your energy systems (ATP/CP, Anaerobic and Aerobic) are the fuel. When you're good at something, let's say running, you can do it in a very efficient manner and burn the minimal amount of "gas" necessary. When you're bad at something (rowing) you flail around like a monkey humping a doorknob, and as a result burn huge amounts of energy. The good thing about all of this, is the caloric expenditure that you get. When I do a workout that involves rowing, I'll always be ridiculously hungry that night. Just not enough food to put into my body. Why? I burned a million calories during the workout, and then my EPOC (Exercise Post Oxygen Consumption) was through the roof for the rest of the day. Boom, metabolism: stoked.
This is why you will often (hopefully more often than not) hear your trainer say that steady-state cardio is NOT the ideal way to lose weight. Your body gets into a groove with steady-state cardio that allows it to conserve energy. It's like going 65 mph on the highway; you're just cruising. Now, go get in your car and jam the gas pedal to the floor and get up to 40 mph, then hit the brakes. Once you're down to 15 mph, jam the gas again. Repeat multiple times. What happened to your gas tank? It's probably empty, because you just did interval training with your car. Speeding up and slowing down repeatedly is an extremely inefficient way to get anywhere, whether you're running or driving. You wanna burn a shit-ton of calories, while making sure to boost your metabolism for the next few hours? Do some sprint intervals.
You don't necessarily have to be "poor" at a movement for it to be inefficient; some movements are intrinsically inefficient. Burpees, sled pushing, lunges (ugh), and some loaded carries (overhead carries, offset, odd objects) are all movements that require a huge muscular effort from your whole body, as well as requiring quite a bit of input from your cardiovascular system. Regardless of how "good" you get at these movements, they are going to be tough. They are just exercises that aren't very efficient.
So, the next time your trainer to tells you to do something that you're not good at, consider not complaining it. There's probably a good reason for it, and it probably benefits you.
Go lift something heavy!