I think for everyone its difficult to make the arms bigger. It takes a bit of long time to have bigger biceps if you don't take steroids.
Deadlifts in terms of complexity and level of exertion necessary for high volume. In terms of muscle group: many of my clients struggle heavily with lower back and calves.
Front squats wreck me everytime. And power cleans take a close second.
Honestly it depends on individual, but would I say most people woueadliftld find conventional deadlifts to be the hardest and they take more to recover from. Personally I find them the hardest for me.
Hardest to grow? Neck muscles probably. Arms arent hard to grow you need to typically put on 10lbs for every inch you want them to be bigger and most people don't do enough tricep work compared to bicep, and tricep is like 2/3 of the upper arms.
Weighted pull ups I find it the most difficult exercise to do. Hardest muscle to grow for me is my legs and calves.
Vendetta (10-28-2018)
Legs won't grow somehow. I can bench more than I can squat at the moment.
I hate muscle ups :/ super hard but once you do your first one it feels amazing
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everyday is chest day
Legs, if done right, can be so exhausting just due to the leg-mass:body-mass ratio. I walk out of the gym feeling like I will pass out any minute.
Pull ups are the bane of my existence