BY JASON R. KARP, PHD, MBA
Dr. Jason Karp is a coach, exercise and sport scientist, and author of 17 books and more than 400 articles. He has given hundreds of lectures across the U.S. and in 15 countries, has been an instructor for USATF’s Level 3 certification, and is the physiology curriculum lead for USATF coaching education. He has won multiple awards for his work, including IDEA National Personal Trainer of the Year and President’s Council on Sports, Fitness & Nutrition Community Leadership award. His run coaching certification was obtained by coaches in 26 countries before being acquired by International Sports Sciences Association. He has lived and coached in Kenya and has served as sports science/coach consultant to the Chinese Athletics Association. His TED Talk, How Running Like an Animal Makes Us Human is a unique look at the body-brain-mind connection. His books are available on Amazon.
One of the “rules” of distance running is that runners must run many miles. Indeed, most runners link their fitness to the number of miles they run, invariably believing that more is better. While mileage is undoubtedly important, runners should train with the whole continuum of speeds, from very slow to very fast.
In an attempt to simplify the prescription of training intensity, several “zones” can be used that divide the intensity continuum based on physiological stress. The most basic, and perhaps most meaningful, zone model is the two-zone model, with the acidosis (lactate) threshold as the dividing line. Zone 1 is below the threshold and zone 2 is above.
The acidosis threshold is an interesting physiological variable, demarcating the transition between running that is almost purely aerobic and running that includes significant oxygen-independent (anaerobic) metabolism. The speed at the acidosis threshold is the fastest speed that can be sustained exclusively by aerobic means. (Since the development of metabolic acidosis begins when the lactate threshold is exceeded, I often refer to the threshold as the “acidosis threshold” to take the emphasis off the innocuous lactate and place it on the fatigue-inducing acidosis, which is the physiological marker of interest.) Thus, the acidosis threshold discerns sustainable, metabolically balanced workloads from non-sustainable, non-metabolically balanced workloads.
Knowing how much training runners do above and below their acidosis threshold gives you a good indication of the stress of training. But it doesn’t tell the whole story. The three-zone model includes the training at the acidosis threshold itself, giving us zone 1 below threshold, zone 2 at (or near) threshold, and zone 3 above threshold.
But that still doesn’t give the whole story, because there are many intensities above the acidosis threshold. So, the 5-zone model elucidates the entire spectrum of intensities, with zone 1 below acidosis threshold, zone 2 at (or near) threshold, zone 3 at (or near) VO2max, zone 4 at anaerobic capacity (speed endurance), and zone 5 at anaerobic power (sprinting; plyometrics).
Zone 1
In all zone models, zone 1 is aerobic training, which may be classified as any speed below the acidosis threshold, which is lower than about 80 to 85 percent max heart rate. In highly trained runners, the acidosis threshold can reach up to 90 percent max heart rate, making anything lower than that intensity aerobic. All easy runs, recovery runs, and long runs fall into zone 1.
Zone 2
Zone 2 is acidosis threshold training, which represents the ceiling of steady-state aerobic running. Raising the threshold enables athletes to run faster before they fatigue because it enables them to run faster before oxygen-independent (anaerobic) metabolism (and, consequently, the development of acidosis) begins to play a significant role. With training, what was once an anaerobic pace becomes high-end aerobic.
The longer the race for which your athletes are training, the more important the acidosis threshold becomes because the more important it is to hold a faster aerobic pace for an extended period. The keys to success for longer races are (1) getting the acidosis threshold pace as fast as possible and (2) being able to run as close to acidosis threshold pace as possible for as long as possible.
Zone 2 threshold pace should feel comfortably hard, a lot faster than the easy pace of zone 1. Threshold runs are one of the most difficult types of workouts for most runners to run correctly—especially those who are young or inexperienced with these workouts—because they require holding back and not pushing the pace. That comfortably hard feeling requires practice to attain and hold. Threshold pace for recreational and intermediate-level runners corresponds to the following:
• About 10-15 seconds per mile slower than 5K race pace
• Equal or very close to 10K race pace (if slower than about 50 minutes for 10K, the pace will be slightly faster than 10K race pace)
• 80-85% max heart rate
Threshold pace for competitive and highly trained runners corresponds to the following:
• About 25-30 seconds per mile slower than 5K race pace
• About 15-20 seconds per mile slower than 10K race pace
• 85-90% max heart rate
Zone 3
Zone 3 is VO2max training, which targets the cardiovascular system’s ability to pump blood and oxygen to the working muscles. VO2max, which is the maximum volume of oxygen the muscles can consume per minute, is best trained with interval workouts, running hard for 3 to 5 minutes at or very near maximum heart rate, with jogging recovery intervals that are slightly less than or equal to the time of the reps.
The main reason to jog during the recovery intervals is to increase venous return (the return of blood back to the heart through the venous circulation) via the muscles’ pumping action so the heart can receive and subsequently pump more blood with each beat (stroke volume). Jogging is also important (compared to standing still or walking) to keep oxygen consumption (VO2) elevated throughout the workout. Thus, cardiac performance is enhanced during and because of the recovery intervals, which is why it is called an interval workout.
In comparison to zone 2 threshold training, which is mostly about what’s happening in the athlete’s legs, the site of adaptation of zone 3 VO2max training shifts from the skeletal muscles to the cardiovascular system. Zone 3 VO2max pace should feel hard but manageable, a lot faster than the threshold pace of zone 2. VO2max pace for recreational and intermediate runners corresponds to the following:
• Between 1.5-mile and 2-mile/3K race pace
• About 20-25 seconds per mile faster than 5K race pace
• About 40-45 seconds per mile faster than 10K race pace
• 95-100% max heart rate
VO2max pace for competitive and highly trained runners corresponds to the following:
• Equal or very close to 2-mile/3K race pace
• About 10-15 seconds per mile faster than 5K race pace
• About 25-30 seconds per mile faster than 10K race pace
• 95-100% max heart rate
To determine max heart rate, have your athletes run one mile while wearing a chest-strap heart-rate monitor, starting at a comfortable pace and picking it up each lap of the track until running as fast as they can over the final lap. Have them check the heart-rate monitor a few times over the final lap. The highest number they see is their max heart rate.
Zone 4
Zone 4 is anaerobic capacity (speed endurance) training, which targets anaerobic glycolysis (the breakdown of glucose) to regenerate energy (ATP). When your athletes’ running pace exceeds the rate at which their heart and blood flow can provide oxygen to their muscles, anaerobic metabolism starts to kick into high gear to produce ATP for their muscles. Like zone 3, anaerobic capacity is best trained with interval workouts, running fast reps for 30 seconds to 2 minutes, with jogging recovery intervals that are two to four times as long as the reps.
These workouts increase the enzymes involved in glycolysis, improve the ability to buffer muscle acidosis, and increase running speed by recruiting fast-twitch muscle fibers. When your athletes do anaerobic interval training, they should leave their heart-rate monitors at home, since heart rate does not adequately reflect the workout intensity. Anaerobic training is not about heart rate and oxygen delivery; it’s about the metabolic activity going on inside their skeletal muscles.
Zone 5
Finally, zone 5 is anaerobic power (speed) training, which targets the phosphagen system to regenerate ATP. Like zones 3 and 4, anaerobic power is best trained with interval workouts, running very fast sprints for 5 to 15 seconds, with standing/walking recovery intervals of 3 to 5 minutes. Anaerobic power training can also be done with plyometrics.
The relatively long recovery intervals between sprints enable creatine phosphate to be replenished in the muscles so that it can be used as fuel for the next rep. Although the phosphagen system plays a minor role in a distance runner’s performance, it becomes more important the shorter the race. Even distance runners benefit from training anaerobic power and improving speed, through the recruitment of fast-twitch muscle fibers, the optimization of running technique, and the development of muscle force and power.
Less experienced runners tend to drift between intensity zones, running harder on days reserved for low intensity and not hard enough on days reserved for high intensity. Experienced runners have a slightly different problem—they often run faster on easy days and on hard days than what is appropriate for their performance level. Let’s first take the problem of running too fast on easy days, since that is, perhaps, the biggest mistake runners make.
When I was a kid, I loved watching the TV sitcom, I Love Lucy, starring Lucille Ball. In one episode, Lucy and her friend Ethel work at an assembly line, where they were assigned to wrap pieces of chocolate as they came down the conveyor belt. At first, the job was easy. The chocolate pieces were coming down the belt at a slow enough speed that Lucy and Ethel could easily grab each piece of chocolate and wrap it.
Then, the speed of the conveyor belt quickened, and Lucy and Ethel had their hands full. Literally. They couldn’t wrap each piece of chocolate in time before the next piece was already passing them, so they grabbed handfuls of chocolate and shoved them in their pockets and in their mouths. It was hilarious, and that episode became a famous part of TV sitcom history.
Little did the director of that scene know that he revealed the secret to how to become a better distance runner.
Clearly, increasing the speed of the conveyor belt didn’t work. Lucy and Ethel couldn’t keep up with the pace of the belt. If the company that Lucy and Ethel worked for wanted to produce more wrapped chocolates in less time, they should have had more factories with more assembly lines and more workers, like Lucy and Ethel, wrapping chocolates traveling down the multiple conveyor belts.
Deep inside your athletes’ muscle fibers, those factories are the mitochondria, and those workers—the Lucys and Ethels—are the enzymes that catalyze the chemical reactions involved in aerobic metabolism. The more mitochondria their muscles have, the greater their muscles’ capacity to use oxygen and the faster pace they’ll be able to sustain. The most efficient way to make more mitochondria—more factories, more assembly lines, and more workers—is to run more. And to run more, athletes must slow down their runs, because there is an inverse relationship between training intensity and duration.
The number and size of mitochondria in muscle fibers is sensitive to the volume of work performed. When the factories are stressed because of greater demand, more and larger factories will be built to increase their supply to match the demand. If those pieces of chocolate kept traveling down the conveyor belt long after the 30-minute I
Love Lucy episode was over, more conveyor belts, and more and larger factories to hold those conveyor belts, would have been built to keep up with the demand for chocolate.
By running faster on easy days, your athletes add unnecessary stress to their legs without any extra benefit and it will prevent them from adequately recovering in time for their harder days. Since many of the cellular adaptations associated with aerobic training are volume-dependent, not intensity-dependent, the speed of easy runs is not as important as their duration. Slowing down their easy runs has at least three benefits: (1) it decreases the chance of injury because of less physical stress, (2) it enables athletes to get more out of their harder days because they’ll have less residual fatigue, and (3) it enables athletes to increase their overall training volume. Training is about optimization—obtaining the most benefit with the least amount of stress.
The other problem is running too slow on hard days, which, while not as detrimental as running too fast on easy days, is also not optimal because your athletes end up falling short of the intended purpose of the (high-intensity) workout. There are great physical and psychological benefits that come from running fast and holding those faster speeds for longer periods of time. While lower-intensity runs comprise a much larger percentage of your athletes’ training, when it’s time to run fast, they better be ready for it.
In slower runners, the reason for not running fast enough on hard days may be due to the inexperience of workouts that are physically uncomfortable. It’s tough for beginner (e.g., high school freshmen) and many recreational runners to push themselves; they feel like their lungs are going to explode and their legs are going to fall off. For faster, more competitive runners, the reason for not running fast enough on hard days is likely due to the accumulated fatigue from the problem of running too fast on easy days.
The other problem, which is mostly specific to headstrong runners, is running too fast on hard days, which happens when runners either don’t fully understand and internalize the purpose of the workout, they lack self-control, or they think that to run faster in races, they need to run faster workouts.
For example, say one of your athletes does an interval workout of 800-meter reps at VO2max pace at 6:00 per mile, or 3:00 for each 800-meter rep. If running each rep in 3:00 causes her to reach her VO2max during the rep (which is the goal of the workout), running each rep in 2:55 (5:50 pace) will certainly also cause her to reach VO2max. But why run each rep in 2:55 when she can run it in 3:00 and still get the same benefit? To improve VO2max, running faster than VO2max pace is not better than running at VO2max pace. All running faster does is add more stress and more fatigue to her legs without any extra benefit. For a distance runner, it’s better to run more volume at the correct pace than to run less volume at faster than the correct pace. The optimal way to do the workout is to run at the correct pace and either run more reps, increase the duration (distance) of each rep, or reduce the time of the recovery intervals between reps.
How much training should your athletes do in each intensity zone? That’s a tough question to answer and depends on many factors. Several research studies have tried to approach an answer to this question by documenting what elite distance runners typically do. Those studies have found that, using measurements of blood lactate levels, heart rate, and perceived exertion, elite distance runners run about 75 to 80 percent of their volume in zone 1 below acidosis threshold (low intensity) and about 20 to 25 percent at and above acidosis threshold (high intensity) (Enoksen et al., 2011; Seiler & Kjerland, 2006; Esteve-Lanao et al., 2005; Tønnessen et al., 2014; Seiler, 2010; Seiler, 2012).
Some research has narrowed in on the 20 percent at and above threshold, and has found that the three-zone training intensity distribution is about 75-80/5-10/15 percent for below, at, and above acidosis threshold (Seiler & Kjerland, 2006; Laursen, 2010). It seems that elite endurance athletes have settled on a training pattern that involves a large volume of work at low intensities (zone 1; less than 80% VO2max) combined with a small amount of work at intensities higher than threshold (zone 3; greater than 90% VO2max), with not much time spent in the middle at threshold (zone 2; 85-90% VO2max). (This apparent lack of training at acidosis threshold is contrasted by elite Kenyan marathon runners, who do a considerable amount of training at or near acidosis threshold pace.)
While most research has quantified the intensity distribution based on time or distance in each zone, some research has quantified it based on the primary work portion of the session (instead of counting the low-intensity warm-up and cool-down before and after interval workouts and the jogging recovery intervals between reps, which would inflate the amount of low-intensity work). That research has shown that different methods of training intensity quantification impact the distribution quantification. For example, in elite endurance athletes, a 90/10 low/high intensity distribution based on the amount of time spent in each zone typically corresponds to an 80/20 distribution based on the type of workout, which equates to two to three workouts per week at acidosis threshold (zone 2) or at higher intensities (zones 3, 4, 5) for athletes training 10 to 14 times per week (Seiler, 2010).
When calculating the intensity distribution as a percentage of distance or time run per week, the percentage of low-intensity training is always going to be much greater than the percentage of high-intensity training. Runners can run a lot more volume of easy than of hard. That’s because of the inverse relationship between volume and intensity. The higher the intensity, the less work (time or distance) runners can do. Thus, 80/20, which is based on elite athletes who train 10 to 14 times, or 10 to 25 hours, per week, really is an artifact of the inverse volume-intensity relationship rather than a true reflection of how runners train.
One of the reasons for skewing training to low intensity may be optimization. Low-intensity training may be the most optimal intensity at which to adapt because it comes with the least stressful work. Your athletes could get equal or even greater adaptation with higher-intensity work, but that comes at the price of a lot of fatigue and physiological stress, with large hormonal disturbances and downregulation (reduced activity) of the sympathetic nervous system. When the amount of training in zone 3 is increased, for example, to a larger percentage of total training time, runners find it too demanding after just two to three weeks and show signs of overtraining (Esteve-Lanao et al., 2007).
Given the importance of both high-volume and high-intensity training, the art of distance running training, and of coaching, is the precise manipulation of the different training zones throughout the year and balancing them with adequate recovery. A big piece of the training puzzle is learning how and when to manipulate the volume and intensity through different training periods of the year.
References
Enoksen, E., Tjelta, A.R., and Tjelta, L.I. Distribution of training volume and intensity of elite male and female track and marathon runners. International Journal of Sports Science & Coaching, 6(2):273-293, 2011.
Seiler, K.S. and Kjerland, G.Ø. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution? Scandinavian Journal of Medicine & Science in Sports, 16(1):49-56, 2006.
Esteve-Lanao, J., San Juan, A.F., Earnest, C.P., Foster, C., and Lucia, A. How do endurance runners actually train? Relationship with competition performance. Medicine and Science in Sports and Exercise, 37(3):496-504, 2005.
Tønnessen, E., Sylta, Ø., Haugen, T.A., Hem, E., Svendsen, I.S., and Seiler, S. The road to gold: training and peaking characteristics in the year prior to a gold medal endurance performance. PLoS ONE, 9(7):e101796, 2014.
Seiler, S. What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 5:276-291, 2010.
Seiler, S. Training Intensity Distribution. In Mujika, I. (Ed). Endurance Training: Science and Practice. Basque Country, Spain: Vitoria-Gasteiz, 31-39, 2012.
Laursen, P.B. Training for intense exercise performance: high-intensity or high-volume training? Scandinavian Journal of Medicine & Science in Sports, 20(Suppl. 2):1-10, 2010.
Esteve-Lanao, J., Foster, C., Seiler, S., and Lucia, A. Impact of training intensity distribution on performance in endurance athletes. Journal of Strength and Conditioning Research, 21(3):943-949, 2007.
