I first came across the work of Dr. Øyvind Sandbakk and the Norwegian endurance research community about twenty years ago. At that point I was a young professional alpinist, searching for training models grounded in actual data from elite endurance athletes. I had looked at a lot of different systems, but when I found the research coming out of Norwegian cross-country skiing, something clicked. These were scientists embedded with world-class coaches and athletes, studying what actually worked and trying to tease out the why.
What really changed my relationship to that research was when I started coaching. I went back to Sandbakk’s work with new eyes. I was reading it as someone trying to solve real coaching problems: how to build aerobic base in mountain athletes, how to structure training volume for people who don’t have twenty-five hours a week to train, how to help athletes understand their training intensity zonesheart rate zones and be patient with a process that takes years. His research on aerobic capacity, aerobic threshold, and training intensity distribution gave me a framework for all of that.
Sandbakk is now a professor at the Norwegian School of Sports Sciences and the Director of the School of Elite Sports, both in Oslo. He competed as a cross-country skier at high national and international levels, then coached medal-winning athletes before pursuing his PhD and leading some of the most influential endurance training research of the past two decades. I recently sat down with him for a long conversation about the principles that underpin our approachwhat we teach at Uphill Athlete. What follows are the ideas from that conversation that I think matter most for mountain athletes.
The Norwegian Endurance Training System: Culture, Not a Single Method
One of the first things Sandbakk pushed back on was the idea of a single “Norwegian method.” People ask him about it constantly. Books have been written about it. Presentations are given on it. And the problem, he explained, is that there are Norwegian endurance athletes and coaches succeeding with very different approaches. Some use a threshold-based system. Others follow a more polarized training model. They are successful, but can’t be attributed to any single national prescription. Both are Norwegian. Both produce results.
What Norway actually built is something more fundamental than a specific training prescription. In the 1990s, Norwegian sport developed a shared intensity zone system that everyone uses. Every coach, every athlete, every sport. When a skier writes Zone 2 in a training diary, it means the same thing as when a triathlete writes it, or a speed skater, or a runner. That sounds simple, but it is transformative. It means you can trust the data. You can compare training intensity distribution across individuals, across sports, across decades. Sandbakk pointed out that when heart rate zones get discussed on social media, people are talking about seven different things. In Norway, there is only one understanding.
This shared language created an infrastructure for knowledge exchange that compounded over thirty years. When cross-country skiing had success, the triathletes learned from the skiers and built their own endurance training system on the same foundation. Then they pushed the limits further, and now the skiers are being inspired by what triathletes like Christian Blumenfeld have done. Sandbakk described this cross-sport pollination as the real advantage of Norwegian endurance sports. Not a recipe, but a culture of shared terminology, institutional data, and continuous knowledge exchange.
This matters for mountain athletes because the principle underneath it is the one we follow at Uphill Athlete: analyze the demands of your sport, then apply common endurance training principles to those specific demands. Sandbakk illustrated this vividly with examples from across sports. In swimming, training tends to be more polarized because triathlon swim legs require a fast, aggressive start and the threshold zone is physiologically narrow. In speed skating, on-ice training is inherently high-intensity because the low body position required to reduce drag creates extreme muscular demands regardless of pace, so skaters do most of their aerobic volume on the bike instead. Cyclists can sustain longer sessions at moderate intensity because the mechanical load is lower than running. Road runners are constrained by injury risk and rarely exceed two hundred kilometers per week. In each case, the training design follows directly from what the sport demands, not from a universal prescriptionEach sport trains differently because the constraints are different. There is no universal training intensity distribution. There is only the discipline of understanding what your sport demands and designing training to meet it.
Every Training Session Is a Signal: Why Optimizing Adaptation Beats Following a Plan
A training session, Sandbakk said, is a signal you send to your body that tells it to adapt in a specific direction. That framing has stayed with me.The single idea from this conversation that I keep coming back to is Sandbakk’s framing of what a training session actually is. A training session, he said, is “the signal that you send to the body” that tells it to adapt in a specific direction. The coach’s job is to help the athlete send high-quality signals and then recover well enough to keep sending them.
This reframing shifts the athlete’s attention from completing the plan to optimizing the adaptation. Sandbakk was explicit about this distinction. The most important thing a coach and athlete do together, he said, is sit down,take time to understand the athlete and the demands of the sport, then and build a good training program around it. The second most important thing is to not follow that program one hundred percent. ABecause athletes are not machines. They bring are flesh and blood, emotions and challenges to every session, sleep well some nights poorly on othersgood sleep and bad sleep, . and their readiness to train shifts accordingly. If the goal is to optimize the adaptation from each signal, you have to respond to those realities rather than pretend they don’t exist.
The distinction he drew is worth sitting with: He drew a line I think is worth underlining: instead of making the goal to perform the training plan, you move the goal to optimizing the adaptation from each session. That shift distinction sounds subtle, but it changes how you approach every single day of endurance training. It is the difference between checking a box and making a decision. For every session, the athlete should be asking: what signal do I want to give my body today? On If it’s an aerobic base day, the signal is low-intensity work that builds slow twitch muscle fibers and fat oxidation capacity. If it’sOn an interval training day, the signal is controlled high-intensity work that pushes drives aerobic capacity higher. Everything else: the pace, the duration, the terrain, is in service to delivering the signal as cleanly as possible. And then the athlete does whatever is necessary to deliver that signal as cleanly as possible.
How Slow Is Slow Enough: Aerobic Threshold, Heart Rate Zones, and Intensity Control
Sandbakk spent a significant part of our conversation on intensity control, and I think this is one of the areas where motivated recreational endurance athletes have the most to learn from what elite coaches and scientists have figured out.
The core insight is straightforward: if your key sessions are too hard, they cost too much. They accumulate a trainingaccumulate training load that compromises the next session and the one after that. The goal is not to bury yourself in any single workout. The goal is to be able to repeat quality sessions frequently enough, and recover well enough between them, to keep the adaptations moving forward.
This is where the aerobicwhere aerobic threshold becomes the anchor for the entire training system. The Aerobic threshold marks the point where lactate production begins to meaningfully outpace clearance, shifting the balance away from fat oxidation toward carbohydrate dependence. —the intensity at which lactate first begins to accumulate above resting levels—defines the upper boundary of truly low-intensity training. Below itaerobic threshold, the body relies predominantly on fat oxidation for fueldominates, the cardiovascular system operates efficiently, and the demands stress on slow twitch muscle fibers drives the structural adaptations that build aerobic base: mitochondrial density, capillarization, and improved movement economy. Above itaerobic threshold, even by a small margin, the metabolic cost rises disproportionately, recovery takes longer, and the training signal shifts away from aerobic development.
This is why we anchor base training to aerobic threshold rather than anaerobic threshold. or lactate threshold. The aAnaerobic threshold, —sometimes called lactate threshold, and, the point where lactate accumulates faster than the body can clear it, —is an important marker for race performance, but is too high to serve as a ceiling for aerobic development. . But for building aerobic base, the relevant boundary is lower. Training below aerobic threshold, session after session, week after week, is what develops the slow twitch fiber adaptations, the fat oxidation capacity, and the cardiovascular efficiency that form the foundation of endurance performance.
Sandbakk described how Norwegian coaches think about this as a balance between sending intensive signals and allowing the body to adapt to those signals. You want to train hard enough that the session produces a meaningful stimulus, but controlled enough that you can come back two or three days later and do it again at the same quality. If you go all out on the last interval, you might feel like you got more from that session, but the recovery cost often means the next key session suffers.
His advice for the motivated non-elite endurance athletes was cleartelling: stop before you have to stop. If you could have done one or two more intervals but you chose not to, you recover much faster. WAnd when the time comes to it’s time to dorepeat that session again, you don’t dread it, you don’t wait too long, and the quality stays high. He described the feeling you should have at the end of each training week: recovered and looking forward to the next week. Not at your on the limit. Not barely hanging on. That is what makes the process sustainable, and sustainability is what produces results over years.
Elite endurance athletes are extremely disciplined about staying below their aerobic threshold heart rate during low-intensity sessions. If the best in the world respect the intensity boundary, recreational athletes should too. The most common mistake we see at Uphill Athlete is athletes training in a gray zone—above aerobic threshold but below anaerobic threshold—where the work feels productive but actually compromises both recovery and the quality of the aerobic base signal. When athletes with an underdeveloped aerobic system symptoms of aerobic deficiency come to us, showing —flat performance despite hard training, chronic fatigue, inability to sustain pace on long efforts, —the root cause is almost always too much time spent in this gray zone and not enough truly easy volume below aerobic threshold.
Training Volume, Training Intensity Distribution, and the Minimum Effective Dose
At Uphill Athlete, Tthe most common question we get after “how slow is slow enough?” is “how much training volume do I actually need?” Sandbakk’s research gives a nuanced answer.
Whatever the specific approach, the common foundation acrossThe foundation of the Norwegian endurance sports training system is a high volume of low-intensity training. On top of that sit two to three key sessions per week where the athlete works at higher intensities, —typically interval training at or above anaerobic threshold. The rest of the week is built around recovering from those sessions and accumulating aerobic volume. Sandbakk confirmed that this training intensity distribution—roughly eighty to ninety percent of total volume at low intensity, with the remainder divided between threshold and high-intensity interval training—holds across virtually every endurance sport Norway has studied.
World-class cross-country skiers train between eight hundred and nine hundred hours per year, with the vast majority of that training volume spent below aerobic threshold. But the critical insight for our athletes, who rarely have more than eight to twelve hours per week, is that the ratio of easy-to-hard matters more than total training volume. Getting the training intensity distribution right first—making sure that most of your hours are genuine aerobic base work below aerobic threshold—produces better outcomes than simply adding more hours at moderate intensity.
Sandbakk also addressed the polarized training versus pyramidal training debate. Both approaches can produce aerobic capacity gains, and both are used successfully in Norway. What matters more than which model you choose is that the training intensity distribution is deliberate, that the low-intensity work is truly low, and that the high-intensity sessions are high enough to drive adaptation. The middle ground—too-hard easy days that are too hard and too-easy hard days that are too easy—is what undermines development.
The periodization shifts through the year as well. Athletes move from building broad capacities, —emphasizing aerobic base, low-intensity training volume, and strength training, —to becoming more competition-specific as target events approach. In that specific phase, interval training increases, the aerobic base sessions become more sport-specific, and the overall training load is managed more tightly. But the aerobic base never goes away. It’s the permanent foundation, not a phase toyou complete and move past.
The Long Arc of Aerobic Capacity: Why Building Aerobic Base Takes Years
Every new athlete we onboard at Uphill Athlete asks some version of the same question: how long will it take? How long until I’m ready for this event, this climb, this race? The answer is almost always longer than they want to hear. Sandbakk’s research gives that answer scientific weight.
He has seen data on endurance athletes who peaked their VO2max at thirty to thirty-five years of ageold. Others peak at twenty and then continue developing other performance factors. But his working hypothesis, grounded in decades of data, is that most athletes can continue developing parts of their aerobic system until they are thirty. His perspective: most endurance athletes are not on top before they are thirty.
I know this from That resonated with my own experience as an athlete. I had been training in a serious, structured way forAfter about a year and a half of serious, structured training as a professional climber, I felt like I was operating on and I felt like I was on a totally new completely different level. Then I learned that I might be eight and a half years away from my actual aerobic potential. That was eye-opening. And that long-term athlete development perspective is something I try to instill in every athlete I work with.
Sandbakk explained the mechanism behind this patience. Short-term training studies typically show that you can optimize a given factor for a few weeks and then see a plateau. But in the real world, athletes are not optimizing one factor in isolation. They emphasize low-intensity training volume for a period, building aerobic base through slow twitch fiber development and fat oxidation improvements. Then they shift toward more threshold-based work, developing the ability to sustain effort near anaerobic threshold. Then they bring in higher-intensity interval training, pushing VO2max and anaerobic capacity. They develop different capacities in sequence, maintaining each one while building the next. Done well, this process can produce progression for a very long time.
He also described what happens when development stalls after years of the same approach. Sometimes the answer is counterintuitive: drop the high-intensity work significantly and just push training volume for a period. Or restructure the weekly pattern so that instead of training the same way every week, you alternate between an easy week and a peak-load week to allow harder sessions followed by genuine recovery. And sSometimes , for athletes who have been training at their limit for too long —showing signs of overtraining or an underdeveloped aerobic base. The answer for them may be aerobic deficiency—the answer is simply to train less. By Rreduceing training volume by 20% and twenty percent, recovering properly, and let the quality of each session improves. That, Sandbakk said, is the art of coaching.
Slow Twitch Fibers, Fat Oxidation, and the Aerobic Adaptations You Cannot See
One of the most important ideas from this conversation was Sandbakk’s explanation of why athletes often feel like they have plateaued when they actually haven’t.
The aerobic adaptations that matter most,—the ones that build lasting aerobic capacity,—operate on a longer timescale than most athletes expect. Slow twitch muscle fiber development, the growth of mitochondrial density within those fibers, increased capillarization around working muscle, and improved fat oxidation: —these changes accumulate gradually over months and years, not weeks. An athlete might not run a faster time trial for six months while all of these adaptations are quietly progressing.
This is what we call the “invisible gains” problem. Improved fat oxidation means the athlete can sustain effort longer before running out of glycogen, but that doesn’t show up in a short test. Greater slow twitch fiber recruitment means better muscular endurance on long climbs, but the athlete might not notice until they attempt a full-day objective. Better movement economy under fatigue means the athlete holds form longer, which saves energy, but only becomes visible in competition or on peak efforts.
Sandbakk’s advice was to watch for trends, not snapshots. The adaptations are is happening,. The slow twitch fibers are developing. The fat oxidation capacity is improving. The aerobic base is deepening. Bbut these changes require patience and consistent low-intensity endurance training volume to accumulate. When athletes experience with symptoms of aerobic deficiency—flat performance despite high training load, chronic fatigue, and poor recovery—come to us, the issue is almost always that they prioritized high intensity training over the patient low-intensity volume that drives chased visible results through too much interval training at the expense of these deeper, slower aerobic adaptations.
Muscular Endurance, Strength Training, and the Gap Analysis for Mountain Athletes
I asked Sandbakk about muscular endurance because it is one of the areas where mountain sports share the most with cross-country skiing. In skiing, double poling demands enormous muscular endurance from the upper body. Local muscular fatigue becomes the performance limiter before the cardiovascular system runs out of capacity. Our athletes face the same problem on steep uphill terrain, whether they are running or climbing. The local muscular demand becomes the limiter well before the heart and lungs run out of aerobic capacity.
Sandbakk’s approach to this is characteristically analytical. He starts with what he calls a gap analysis: look at where the athlete is losing time in competition compared to what is optimal, then cross-reference that with their physiological profile. If the athlete is losing time on longer uphills and their VO2max is below target, that becomes the priority. But you have to be careful not to sacrifice existing strengths while developing weaknesses. If the athlete’s advantage is in VO2 kinetics and the ability to handle repeated anaerobic surges, you cannot abandon that training entirely while you work on VO2max. You maintain what is strong and develop what is weak, evaluating systematically along the way.
When it came to muscular endurance specifically, he described it as one tool in the toolbox. You use it when the gap analysis tells you it is needed for a specific athlete, and you make the training as specific to the demand as possible. Low-cadence cycling, weighted vest work on uphills, hill sprints for neuromuscular development and fast twitch muscle fiber recruitment—these are tools, not programs. You deploy them based on what the individual athlete needs, monitor the cost, and adjust.
The broader point about strength training for endurance athletes came up naturally in this context. Sandbakk sees strength training as serving multiple purposes: injury prevention, improved movement economy, and development of both slow twitch and fast twitch muscle fibers in ways that complement aerobic training. The key is that strength training supports the aerobic base rather than competing with it. The training load from strength work needs to be accounted for in the overall plan so it doesn’t compromise recovery from key endurance sessions.
This is how we approach it at Uphill Athlete as well: muscular endurance training is earned, it is specific, and it is finite. Strength training is part of the plan from the start, but it is structured to build the athlete up rather than break them down. You add sSport-specific muscular endurance work is added when the aerobic foundation supports it, and is adjustedyou pull back to ensure before the cost does not exceeds the benefit.
How to Measure Your Aerobic Base: Heart Rate Drift, Lactate Testing, and Field Tests
One of the phrases Sandbakk used that stuck with me was “calibrated gut feeling.” He was describing how coaches and endurance athletes actually make training decisions, and the phrase captures something important about the relationship between data and intuition.
The Norwegian system tracks a lotathletes and coaches track a lot of data. Athletes use GPS watches and heart rate monitors every session to stay in the right heart ratetraining zones. They keep detailed training diaries and Sandbakk described these as having that serve three purposes. The first is : a daily debrief: after each session , where the athlete records writes down what they did, how it felt, and the quality of the effort. The second is session; a communication, platform so giving the coach can follow the athletea close and continuous view of the athlete’s training without requiring a conversation after every session. The third is closely; and a learning tool, because memory is unreliable and it is easy to forget what you actually did, and a diary creates a record that lets you learn both athlete and coach draw lessons from both success and failure over time. Sandbakk emphasized that this debrief should take no more than a minute or two, but the habit compounds over time.
Beyond the diary, athletes do lab testing roughly every two months, profiling VO2max, lactate threshold markers, and work economy. Lactate testing remains the gold standard for precisely determining both aerobic threshold and anaerobic threshold precisely. But the more powerful ongoing tool, in Sandbakk’s view, is the standardized training session. He described how he uses the same treadmill, the same terrain, the same session structure on a weekly basis to track progression without waiting months for a lab result. You can see the tendencies week by week in the data from these repeatable sessions.
Heart rate drift during steady-state efforts is one of the most accessible field measures of aerobic base fitness. When an athlete holds a constant pace below aerobic threshold, a well-developed aerobic base will keep the heart rate relatively stable throughout the session. Excessive heart rate drift (—heart rate climbing progressively at the same pace)—indicates that the aerobic system is fatiguing and the body is recruiting additional fast twitch muscle fibers to compensate. Over weeks and months, watching heart rate drift diminish at the same pace is one of the clearest signs that aerobic capacity is improving.
Heart rate variability offers another window into aerobic adaptation and training load. Tracking resting heart rate and heart rate variability over time can reveal patterns in recovery and readiness that help athletes and coaches make better decisions about when to push and when to rest. These measures don’t replace lactate testing or lab work, but they provide daily feedback that supports the ongoing monitoring of aerobic development.
For athletes who want to determine their training estimate their heart rate zones without a lab, there are field-based approaches. Calculating zones from maximum heart rate and resting heart rate provides a starting framework, though determining your actual aerobic threshold through a structured field test —like the ones we offer at Uphill Athlete—gives a much more accurate anchor for your training. Once you determineknow your aerobic threshold heart rate, you have a concrete number that defines your aerobic trainingheart rate zones and tells you exactly how easy “easy” needs to be.
Sandbakk practices this himself. He runs regularly and tests himself at least twice a week on a treadmill in his basement using standardized sessions. Simple, he said, but very systematic. The same principles as a top endurance athlete, just without the lab. The principles scale.
This is directly relevant to what we are building at Uphill Athlete. Most oOur athletes Most of our Uphill Athletes do not have convenient access to labs. They are working professionals with families. We have shared So we built tools that analyze their training data and estimate their aerobic threshold on a continuous basis, giving them a running measure of aerobic development without requiring a lab visit. When those simple measures are combined with a coach’s observation and the athlete’s own perception, you have what Sandbakk calls informed decision-making. Most decisions still come from gut feeling. But it is a very well-calibrated gut feeling, because all those measures—heart rate zones, heart rate drift, resting heart rate trends, standardized session data—are feeding it.
Why Aerobic Base Matters: The Same Principles at Every Level
Talking with Sandbakk reinforced something I have believed for a long time. The principles that produce world-class endurance athletes are the same principles that produce the best outcomes for a forty-five-year-old trail runner training eight hours a week. The specifics change. The training volume is different. The events look nothing alike. But the underlying physiology does not care whether you are racing a World Cup 50K or trying to finish your first mountain ultra.
Aerobic capacity is built through patient, consistent, mostly low-intensity endurance training below aerobic threshold. The aerobic base adaptations—slow twitch muscle fiber development, improved fat oxidation, greater mitochondrial density, better movement economy—accumulate over years, not weeks. Key interval training sessions should be high enough quality to drive adaptation but controlled enough to be repeatable. Training intensity distribution matters more than total training volume. And the whole process operates on a timeline measured in years, not weeks.
This is the foundation we built Uphill Athlete on. We offer coaching, training groups, and aerobic threshold testing tools because we believe that when endurance athletes understand the science behind their training —when they know their heart rate zones, when they can see their aerobic threshold tracking upward over months, when they understand why their slow twitch fibers and fat oxidation capacity need years of patient work—they make better decisions and stay in the game long enough for the adaptations to arrive. Sandbakk’s research helped us build that framework, and this conversation reminded me why it works.