Using a power meter to train can be the difference between a good season, and a bad one. Fundamentally, a pm simply tells you what you’re doing, regardless of how you feel you’re doing. It’s the hard facts of the pressure you’re putting on the pedals. But why do you need power on a mtb? Isn’t the power so variant that it is unusable?
Precise Efforts. In any given ride, you have specific goals. Let’s say I’m recovering, and want to push less than 200w average for a ride. If I’m feeling particularly perky, I might push the pace a bit unknowingly, and end up not achieving my goal! On the flip side of that, sometimes I’m out doing intervals (long or short) and want to hold a particular effort. Looking at power keeps me honest when I start to get tired, and that 300w feels so much harder than it did when I started! Your speed and heartrate can all fluctuate and are thus unreliable variables.
Pushing Limits. Ever been on a climb and you are on your limit trying to hold the wheel in front of you? When you glance down and see the power you are doing is well within what you did in training, that can provide you the confidence you need to hang on a bit longer! Sometimes it’s just a mental aid to tell you that what you’re doing is possible for you.
Efficiency. Mountain biking is about efficiency and momentum. The most efficient riders always win races as they know how to ride smoothly and evenly. Picking a good line and maneuvering around obstacles is just half the battle to being efficient. A good rider won’t spike their power output every time they hit a climb, or drop it when they are on a flat. Consistency is key, and you can only see how you’re doing when you have power to look and analyze that. Your average power shows you exactly what you averaged, while your normalized power eliminates the coasting and shows you what you could have done had you been even. The difference between those two values is how efficient you are!
Improvement. Like many, I use Strava to track my training data. Strava becomes a powerful tool when coupled with GPS, heart rate, and power; especially for a mountain bike ride. Take this scenario. I go on a training ride and hit a particular stretch of trail as hard as I can. When I get home, I look at all of my past efforts on that same trail and analyze the differences between my speed, heart rate and power. Your power then shows you if you're getting smoother as a rider or if you are wasting power. There have been plenty of times when I have gone slower, using more power on the trails, and that is always something to work on.
Pacing. There are very few mtb trails, courses and races that are 100% technical riding where steady efforts are impossible. Most have climbs, flats, or some smooth spots where you can hold a steady power for at least a minute or two. Being able to gauge your output in these sections is critical to finishing strong, and not burning out. Whether you’re out for a ride with friends or racing, staying within yourself at all times is the key to success.
Analysis. After a given ride or race, it’s helpful to look at what happened out there to see where you have weaknesses. Let’s say you were racing an STXC race and held onto the front group until the last lap, where you popped on one short roller. Going back to look at your power output very each previous lap would show you exactly what it took to hang on before, and from there you can approximate how much power you would need to hang on in the future. For instance, if you hit this roller 6 times during the race, and each time did 500w for 30 seconds, but on the last lap you only did 450w, then the problem becomes clear. Your training should then be focused to repeated 500w efforts.
This year I raced with power for every race and it was a whole different experience. I knew precisely how hard I had gone, what the training load of the race was on my body, and where I needed to improve in the future. During an XC race I did early in the season, I made sure to limit all of my climbing efforts to 400w and under. When I saw numbers over that, I immediately dialed it back so that I wouldn’t tire before the 90 minute mark. I also did a 100 miler a few weeks later. There, I used the pm similarly, but relied on it heavily as I raced all day. I set a goal for myself to climb at around 230-260w all day. Each climb I did I settled into a rhythm that achieved that target zone, never straying outside of it. My hope was that at the end of the day, I would feel good enough to be able to push harder than that for the final miles; I did.
Power meters are just as useful on a mountain bike as a road bike. It simply takes a bit more on the bike analytics and short snapshots of data processing to make it worthwhile.