One thing I do not fully grasp is why I have so much more intense Delayed Onset Muscle Soreness ("DOMS") after a bout of weightlifting at the gym during "leg day" versus an equally intense mountain bike ride for example. I'm currently 32, male, weigh 173lbs, and am 5'10".
I have a direct comparison I can make because there are a few trails that I frequent in my area that have EXTREMELY STEEP hills. While I have not measured the grade (I will next time I go out with that phone app that does so), I can estimate that it is probably close to a 35-40% grade - it's so steep that if I try to brake going down, the only way to not go over the handle bars is to be all the way over the rear tire the entire time. Even if I stop going up, if I don't actively try, even standing, I will be pulled downwards and potentially lose the bike.
Anyway, going up those hills on my bike, despite being at ~10% body fat, having lifted weights for 15 years now, and routinely going on 2+ hr rides where I burn 1,000-,1,400 calories, my heart rate will eventually completely max out at around 178-180bpm, and eventually my legs won't physically be able to pedal and I have to hop off and walk it up for a minute or two until my body recovers and I can hop back on for another 1-3 minute stretch.
The crazy part is that I experience almost zero delayed onset muscle soreness (DOMS) from this during the following days. However, if I haven't had a leg weightlifting session for just 1 week, and I go for a workout, doing say 225-295lbs on deadlift for 6 sets of 8 reps, 295lbs for 6 sets of 10 squats, 495lbs on angled leg press for 6 sets of 8-10 reps, I will have extreme DOMS 1-2 days after lifting, and sometimes even start to feel it later the night of lifting. Not just this, but I'll be far more ravenously hungry and physically tired, despite high protein and complex carb consumption (even after, say eating whole grain bagels, oatmeal, whole wheat spaghetti, chicken breast, lean beef, etc...)
I don't understand why this is the case; I thought DOMS was mostly associated with anaerobic exercise and specifically when the muscle was being pushed to near maximal effort, causing micro tears in it. From my (limited) understanding, this would be the case in both the biking situation described and the lifting situation. It's my understanding that sprinting hard up a steep hill and weightlifting would both use fast-twitch muscle fibers and both end (at least in my examples) in total muscle exhaustion. Physically, on the bike, the hill eventually overcomes my leg muscles and causes them to stop, much like weights overcome my muscles in the gym. I don't have a hard time understanding why I don't experience DOMS after a 1-2 hour "stroll" on my bike on flat ground where my heartrate max is only 120-130bpm the whole time though.
Another hypothesis I have is that the muscles used are slightly different, or different aspects of them are used. However, that also doesn't seem to be entirely true, as weightlifting has been reported to improve cycling performance, so there must be at least some overlap there.