I think it depends on your experience. If you are a novice rider, then you need to do base training before any sprint training. But if you've been riding for 15 years, then you probably have done more then enough base training...
That is not true, things don't work that way. Unless you are a track racer or doing 45min crits you are going to find yourself fading at the end of a race if you neglect some form of base in your training. I actually notice this throughout the season, come July/August my endurance starts to fade a bit and then I repeat a block of base type training for my fall build as I approach a couple key MTB races. Admittedly, I do race with a few guys who have been around for a while that maybe don't need to work quite as hard at building base as I do on younger legs, but these guys are exceptions to the norm. Plus they are former euro-pros/national champions who have been to hell and back 5 times over, their untrained talent is still enough to compete just nicely in our regional Pro/1/2 races, so they don't really count.
Most tour professionals who do this for a living actually still prescribe to the old fashion LSD method, where you ride 30hrs a week at a endurance pace for a couple of months to build a solid base. Those of us who actually have a life outside of a bike don't have that time, so a SST plan is the most practical for us to train IMO. There are lots of "working pros" who go through the off-season with a very steady diet of just 2x20s. Based on my experienced you can't short cut the system, tabattas and stuff like that may make you one of the fast guys at Tuesday night worlds, or may make you a decent Cat 4/5, but if you really want to do things right you have to put in the time. These are just my opinions, but I think that most experienced racers would agree with what I have to say.
That last paragraph can't be more wrong at least judging by the training insight I've gotten from a few WT friends. They might ride 30hrs, but it sure as hell ain't all traditional LSD. Unless motorpacing, threshold work, etc are all part of "LSD".
To the OP- you can work NM year round no problem, but what preference it takes depends on the time of the year. I like to do a small amount of it during the winter as I find that very specific types of work simply make it easier to get back into the race rhythm. Also, for me, a huge weakness is surging and recovering at hard efforts. Improving my FTP drastically helped a bit, but in the end the biggest help was very specific NM work in a somewhat fatigued state (microbursts at the end of hard rides) or adding NM efforts to tempo/SST/FTP efforts. A bit different than all-out sprint work, but again when I actually do sprint workouts it seems I spend a lot less time "getting back to where I was" if that makes sense. Maintraining, rather than training. FWIW, this stuff (for me) comes back a lot faster than any other training level. Moreover, its hard for me to improve sprint capacity during base (in terms of max watts) simply because I'm not fresh enough most of the time so I focus on things such as leg speed, small ring sprints, NM efforts (sprint in huge gear up to 90rpms, HA claims it helps with fiber recruitment), and sprint starts (doing sprints at 90% of max after an effort of some sort, usually tossed on to the end of intervals).
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