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Cycling around on pavement is certainly an option for cardio (you get some of the variety and “going places” benefits described here), but cycling on mountain biking trails blows it completely out of the water, in my opinion. In fact, it seems to me that while there are some other activities that can compare to mountain biking in terms of raw fun/enjoyment/adrenaline (riding motorcycles, carving at speed on electric skateboards, surfing, downhill skiing and snowboarding), it is rather unique in being extremely fun while at the same time providing a good workout. Cross-country skiing also gives an excellent cardio workout, but it is nowhere near as fun as mountain biking in terms of raw enjoyment, as the speeds are slower and you don’t have variation in terrain (just smooth snow).
I suppose I should qualify that this exercise benefit of mountain biking only holds for up-and-down normal trail riding rather than downhill mountain biking (compare cross-country skiing and downhill skiing). Downhill mountain biking proper – where you take lifts up the mountain and ride down – doesn’t offer the same exercise experience. You need to pedal up those hills and on flat sections of the trail to get the good workout.
Electric mountain bikes
You can still get a good workout on electric mountain bikes
Electric mountain bikes do not necessarily rob you of exercise when you mountain bike, but they can if you use so much assistance that it is difficult for you to contribute human power since you are already going too fast for the trail you are riding on.
Rather than thinking of them as an exercise hindrance, I like thinking of electric mountain bikes mostly as a simple speed boost – you as a human can contribute 150 watts of effort (and get a pretty similar workout) with or without the electric assistance, but you’ll just be going much faster with the electric assistance. This is assuming you are using a low-ish overall amount of assistance – so that you aren’t constantly going massively too fast for the kind of trail you are riding on, like you would be if you used a high assistance level. (Technical twisty trails will be more inherently speed-limiting than flowy double track, etc.).
The big difference comes with hills. Without electric assist, you have to crawl up hills in a low gear, but with electric assist, you can kick up the assist level up on hills so that you go fast up them too.
Thus, in getting a similar workout, you will cover a lot more ground and go a lot faster (especially uphill) on an electric mountain bike. Those are pretty large benefits in terms of fun, in my book.
Handling mismatches of greater electric assistance than you need for a given trail: braking to scrub off extra speed lets you still get good exercise
If you take pains to scrub speed with your disc brakes whenever you go into tight speed-locked corners, you can largely go ham on the pedals to your hearts content even on an electric bike. What do I mean by this? I mean that you actually can just about always pedal as hard as you want, even on a high level of electric assist, even on a speed-capped trail with lots of tight corners and switchbacks… you’ll just be constantly braking to scrub off the excess speed generated by a level electric assistance greater than what is truly needed.
The reason why pedaling harder necessarily leads to excess speed is because electric assistance on good electric mountain bikes is a function of pedal effort. That is, it amplifies pedal power, so if you pedal harder, you’ll get more electric assist power too. This is definitely how you want things for intuitive electric mountain biking. The consequence of this, though, is that if you always pedal hard such that you maximize exercise, there may end up being circumstances when the large amount of amplification derived from your high level of pedal power ends up leading to more speed than you can carry through a trail. You then have to brake to get rid of excess speed.
Have you lost anything from this situation? We’ve met our goal of still getting good exercise, and a side-effect is that you’ll always be blasting through trails at just about the fastest speeds you can, which of course is excellent from a fun perspective.
There are three primary downsides: First, you put more wear on your brakes and go through brake pads faster. Second, since you are constantly turning battery power into heat on your disc brakes, you end up paying for some extra electricity (wasted energy). Third, since you are constantly turning battery power into heat on your disc brakes, you get less overall battery life, and therefore a lower overall range.
The first two are essentially disadvantages only in terms of money: they go away if you throw money at them. The third one is a real practical concern, however. If you ride hard in the manner I have described, you do end up turning a good bit of battery power into heat on your brakes, which reduces your range. Buying an extra battery adds a lot of weight, so isn’t a catch-all solution to this issue.
This ultimately comes down to how much battery you have on your bike and how long your rides typically are. If you mountain bike multiple times a week rather than all in one longer ride (better for exercise distribution anyhow, and also spreads the fun/relaxation throughout the week), your rides will probably always stay short enough that this problem will not end up being important. Of course, YMMV.
I am perfectly willing to accept the proposition that adding electric assistance to mountain bikes will, in practice, probably decrease the amount of exercise you get by some amount. No matter how much you take pains to always pedal hard and just scrub speed where necessary (as above), you will probably not be perfect in this at absolutely all times.
However, it is my opinion that the differences here are quite small overall. The fact that it ends up being easier in practice to zone out and fail to get good exercise does not mean that you must necessarily get a worse workout; it is not inherent, but a self-inflicted problem (due to “user error,” as it were) that is soluble in large part. To put all this differently, you just have to be much more intentional about getting exercise on electric mountain bikes than normal mountain bikes.
The main takeaway though, in my eyes, is that whatever minor decreases in overall exercise potential exist are more than made up for by the large increases in overall fun.
Here’s some specific benefits of mountain biking:
Mountain biking yields more time in nature
Mountain biking gets you out in nature more than any other form of cardio I can think of. (Maybe vigorous snorkeling is on the same level? Kayaking fast on a lake at sunset?).
While there have been some studies suggesting health benefits to spending time in nature, the observational nature (heh…) of most of the research makes determining causality difficult. Put simply, nature could be more or less a red-herring: for example, the people in the studies might be healthier not because they spent time in green spaces, but because when spending time in green spaces, they exercised more and got more social interaction, both things causally linked to positive health outcomes (such as lower cortisol levels).
Nonetheless, the romantic in me is on board with the idea that taking in lots of fresh air and natural beauty is healthy for mind and body, and it certainly can’t be construed as a con.
Mountain biking involves superior novelty, staving off boredom
Mountain biking trails have constant turns, lots of obstacles (like rocks and roots), frequent (and sometimes rapid) changes in elevation, and so on. Contrast all this novelty with riding on an open, paved road. Which environment do you think is likely to keep you interested? For me and my easily-bored brain, it’s not even close.
Now, to be fair, riding around objects on the street (like crowds and urban features) is possible with some forms of cardio, but mostly too dangerous. Compare urban freeride in inline skating circles (as in here). The world is your playground, but unlike on restricted-use trails, you share it with many more people, and have a lot more sources of potential risk.
Mountain biking makes you live in the moment
To be able to effectively deal with all that novelty that we just discussed, mountain biking actively demands one’s attention – all of it, more or less. Riding on a smooth street is not even close to the same in terms of forcing “being in the moment.” Being in the moment is hugely beneficial, both in terms of increasing enjoyment, and helping you de-stress by temporarily forgetting all your problems and worries, with your mind instead laser-focused on the present.
Mountain biking does not carry problematically high levels of risk
While I would love to own a motorcycle, I have never been able to get over the high amounts of otherwise-preventable risk that motorcycles entail. To put things simply, I refuse to introduce the risk of serious injury or death in any activities I do, unless I absolutely can’t avoid such.
Fortunately, mountain biking of a non-downhill variety involves very little overall risk of serious injury (as long as you wear a helmet). While exercising on appropriate roads vastly reduces the threat of being harmed by cars/the irresponsibility of others, exercising on trails completely eliminates the threat. (This isn’t unique to mountain biking, by the way – trail running also shares this advantage, for example).
However, it is true that there are in general many more objects to collide with (notably, trees) on trails, while if you wipe out on pavement, it’s mostly just abrasion and fall impact forces you have to deal with. It seems probable to me that the overall risk on tame to moderate single-track is comparable to moving fast on pavement, while proper downhill mountain biking is likely more dangerous to a substantial degree. So as long as you avoid the most dangerous forms of mountain biking, your risk of serious injury is quite low.
Mountain biking scratches the adrenaline itch
I have more to say here, but that will come on its own page eventually. For now, suffice it to say that mountain biking is an excellent activity for getting one’s adrenaline fix.
In a single sentence:
I think mountain biking is a great activity since it lets you safely get good exercise while interfacing with nature and having lots of fun, mediated through novelty (turns, rocks/roots, changes in elevation, etc.) and adrenaline from traveling at speed in close proximity to obstacles.
I have personally decided to bike to work to get my cardio instead of mountain biking (see here), but that in no way diminishes my appreciation for the activity.