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Intensity & Muscle Fiber Recruitment

The single most critical factor in endurance training is intensity. In any structured training program, each workout should have a specific purpose. To achieve the ideal response from the body, the stimulation must be specific to the desired adaptation and must allow quick recovery for the next key workout. Intensity, more than any other variable, determines the body�s response to the training stimulus.

This absolutely does not mean that harder is better. The optimal training schedule for any athlete provides the lowest volume and intensity that will stimulate the desired adaptation, not the highest the athlete can sustain. This is a major paradigm shift for many endurance athletes who grew up hearing, �No pain, No gain.� If hill repeats at 450 watts stimulate increased aerobic capacity, then that is how fast those workouts should be performed even if he can perform the repeats at 470. Being a �tough guy� and always going harder and harder will not yield ideal results.

Tough guys never seem to get any faster, they just get tougher � and it seems like fast guys are always winning the races.

Training efficiently means balancing the cost and benefit of each workout. Every workout has a cost, in terms of recovery. Every workout also increases fitness. Efficient workouts provide training benefits that are worth the recovery cost. Every athlete at any point in time has certain recovery resources. While smart athletes develop habits to maximize these resources, they will always be finite, and need to be budgeted. Anything that is going to expend these resources needs to provide a proportionate benefit.

Recovery resources include, among other things:

Glycogen Storage: Glycogen is the form of carbohydrate, stored in our muscles and in our liver that serves as the primary fuel for endurance exercise. Hard or long workouts expend tremendous amounts of glycogen and demand full fuel tanks at the beginning of the workout. Maintaining optimal intensity makes best use of this limited resource.

Muscular Recovery: Muscles sustain damage during running workouts. Lactic acid accumulation damages the muscles chemically and impact stress damages the muscles mechanically, producing tiny tears called micro-trauma.

Connective Tissue Damage: Every workout, but especially a run workout, stresses and damages connective tissues. These tissues need to heal and rebuild adequately between workouts.

Injury Risk: Injuries are an ever-present risk in any workout, especially run workouts. During high-volume or high-intensity workouts, risk for injuries increases.

Maintaining optimal intensity during every workout is a key aspect of managing these risks. These are all limited resources, and running at the right pace during each workout makes the best use of them. This allows you to benefit maximally from each workout and gets you ready sooner for the next key workout.

Remember that it is the quality of the individual workout that makes an athlete stronger and faster, not just an accumulation of stresses. Many athletes train medium hard all the time. On their hard days, even if they put in 100% effort, they don�t get 100% speed because they are a little tired from yesterday�s workout or from an accumulation of weeks of training without quite enough recovery.

If an athlete should be producing 420 watts on hill repeats, but you keeps hitting 390 because he�s fatigued from yesterday, that is bad training. His legs don�t respond or improve from his 100% effort, they just respond to the 390. He is training himself to climb slowly even though he is working hard. That is bad training.

Muscle Fiber Recruitment

Each muscle in our bodies is composed of thousands of muscle fibers. These muscle fibers come in three basic types. Each of our muscles is composed of some combination of these three types, the percentage of each type depending on individual genetics.

Slow twitch muscle fibers are our endurance fibers. They can keep going all day long, but they are not big, fast, strong, or powerful. Slow twitch muscle fibers are able to burn either fat or carbohydrate for fuel, depending on the intensity.

Fast twitch muscle fibers are our sprint fibers. They are big, fast, strong, and powerful, but they fatigue very quickly. Fast twitch muscle fibers cannot burn fat for fuel.

Our muscles also have an intermediate fiber which produces more power than slow twitch fibers and has greater endurance than fast twitch fibers. These fibers are called F.O.G. fibers (fast oxidative glycolytic).

Every muscle in an athlete�s body is composed of many thousands of muscle fibers. When the muscle contracts, each fiber either contracts with its full force capability, or remains relaxed. When an athlete pedals at 50 watts, very few fibers are required to contract, but those that do contract just as powerfully as when he pedals at 700 watts.

After aerobic plateau, which requires several minutes at the beginning of each workout (and each shift in intensity during a workout), the athlete�s body will recruit muscle fibers according to the power or speed requirement of the activity. The endurance fibers will be recruited first. At low intensity only a few endurance fibers will be recruited. As intensity increases, the speed endurance fibers will be recruited next, and finally the sprint fibers. The more powerful fibers will be recruited in addition to, not instead of, the slow twitch fibers.

Training Intensity Zones

Seven zones for training intensity are described below. Each intensity zone stimulates a specific desired adaptation that will enable you to race more effectively. Make sure that every workout has a specific purpose and maintain the appropriate intensity to achieve that end.

The most efficient training intensities stimulate the muscle fibers in groups. Since each category of muscle fiber type is a relatively homogenous (though there is gradation even within the categories), they adapt to stimulation similarly. In addition, fibers within each category will have similar recovery qualities.

Zone 1: Active Recovery: Power / speed requirement demands recruitment of just a few slow twitch fibers. The goal is to maintain an intensity that is high enough to stimulate increased circulation to deliver nutrients to the muscles and to remove toxins, and hard enough to stimulate a growth hormone release (which speeds recovery), but not high enough to demand more recovery.

Zone 2: Basic Endurance Training: Aerobic threshold is the intensity at which almost all of the ST fibers are being used, but none of the F.O.G. (IIa) fibers or FT (IIb) fibers are required to be recruited to meet the power demand. Basic endurance training is best accomplished at or slightly below aerobic threshold intensity. Most athletes don�t do enough of this type of training and go too hard when they try to.

Zone 3: Tempo Training: In Zone 3, the body is recruiting all of the ST fibers and some if the F.O.G. fibers, but not nearly all. Most athletes spend too much time training at this medium-hard intensity and would do better to slow down for most of their training time. The ST fibers can withstand enormous volume; the F.O.G. fibers can withstand moderate volume. Doing basic endurance training too hard is the primary cause of overtraining. Training in zone 3 instead of zone 2 reduces the threshold for overtraining significantly.

If adequate oxygen is available in proportion to the workload, ST fibers use fat as a significant fuel source. Recruitment of the FOG fibers reduces the availability of oxygen to the ST fibers. Since it requires more oxygen to release one calorie of energy from fat than from carbohydrate, fuel use shifts dramatically from a relatively even blend of fat and glucose burning to burning mostly glucose.

At this intensity the athlete isn�t going hard enough to really make them faster, but they are going hard enough to deplete themselves for tomorrow�s workout which is designed to make them faster.

This intensity zone can be effective for maximizing glycogen storage in the muscles and for preparing the body for the demands of higher intensity training. Zone 3 training should only be used early in base training, when there are no high-intensity workouts to interfere with, or during the build and peak periods when training for races which will take place at this intensity (race pace training). Zone 3 training has its place, but most athletes spend too much time at this intensity.

Zone 4: Lactate Threshold Training: Whenever an athlete exercises at any intensity, even walking, lactic acid (lactate) is constantly being produced. Fortunately, our bodies also constantly recycle lactate, actually burning it up and using it for fuel. As intensity increases, lactate production also increases. Lactate threshold is the highest intensity at which an athlete recycles lactate as quickly as it is produced, so that lactate does not accumulate. Muscle and blood levels of lactate are moderately high at lactate threshold intensity, but do not increase over time. Increasing pace just slightly will cause lactate to accumulate, increasing discomfort, damaging the muscles, and delaying recovery for tomorrow�s workout.

At lactate threshold intensity, the body recruits all of the ST muscle fibers and all of the FOG fibers, but does not recruit the FT (IIb) fibers. Just as the ST fibers can sustain much higher training volumes than the FOG fibers, the FOG fibers can withstand much, much greater training volume than the FT (IIb) fibers.

Lactate threshold training, in the right doses at the right time of season, is important for almost every cyclist and triathlete. For most athletes lactate threshold training has the best cost to benefit ratio of any type of training. This intensity is high enough to stimulate adaptations which dramatically increase speed-endurance, but because lactate is not accumulating, damage to the muscles and blood vessels is minimal and the recovery cost of the workout, if conducted properly, is modest.

At LT intensity, the FOG fibers create a lot of lactic acid, but only at a rate at which the ST fibers can burn it up and use it for fuel. Sustaining this intensity trains the FOG fibers to work more aerobically so that they produce less acid and trains the ST fibers to burn more acid, both of which push the threshold to a higher speed.

LT training is the only effective endurance training for the FOG. At lower intensities they are not recruited. At higher intensities, FT (IIb) fibers are recruited causing lactate to accumulate. This reduces the duration that the athlete can sustain the intensity and dramatically delays recovery.

Most athletes and coaches overestimate LT intensity and, when sustaining the correct pace, feel like they are not going hard enough. This creates a major problem, either limiting potential training volume or inducing overtraining. Training one percent over lactate threshold, at an intensity at which lactate accumulates slowly, causes much greater damage and requires much greater recovery time than the same duration at lactate threshold. Reducing training volume or suffering overtraining are poor tradeoffs for the slight benefits from the 1% increase in intensity. For a twenty to thirty minute segment, it does not feel that much harder. Lactate accumulates slowly, but continually, and does damage in the muscles that the athlete cannot necessarily feel. Several minutes into a segment at 101% of lactate threshold, lactate levels in the muscles and in the blood will be only slightly higher, but later in a long set they may be dramatically higher. Muscle damage and recovery time may increase enormously. A well trained athlete can sustain lactate threshold intensity for seventy five minutes or more with 100% effort, such as in a race.

Every athlete likes to think that they are more motivated than the rest. They will train harder, be more consistent and more disciplined. Coaches like to think the same about their clients. Remember though, no matter how passionate and motivated the client is, this is a finite resource. Budget it wisely. Correct use of LT training, generally a little on the conservative side, plays a big role in sustaining motivation.

Zone 5a Super-Threshold Training: In zone 5, the body recruits all of the slow twitch muscle fibers, all of the F.O.G. fibers, and begins recruiting the fast twitch (IIb) fibers. Intensity is slightly above lactate threshold, so acid is accumulating in the muscles and in the blood. At this intensity, acid accumulates gradually so that this intensity can be sustained for a long period of time. Well conditioned athletes race in zone 5a for 40K time trials or 10K runs.

Many athletes mistakenly train in zone 5a when they intend to be in zone 4 doing lactate threshold training. This is a costly mistake because the recovery time from zone 5a training much greater than from zone 4 training. Since acid accumulates very slowly in zone 5a, the difference between 4 and 5a is negligible when sustained for short periods of time. However, when sustained for moderate to long durations, the acid accumulation does significant damage to the muscle cells and requires significantly more recovery time.

Zone 5a training is excellent for improving acid tolerance. The muscles are trained to continue to produce speed efficiently despite the buildup of acid. This is a benefit, but it comes at a great cost in terms of the volume and intensity of other workouts.

Zone 5b Aerobic Capacity Training: Maxing the VO2: Maximal aerobic capacity or VO2 Max, the amount of oxygen consumed in one minute of maximal aerobic exercise, is widely considered the standard test for aerobic conditioning. Improving VO2 Max is a crucial step in maximizing endurance performance in any event lasting four minutes or longer. The higher an athlete�s VO2 Max, the greater the contribution of the aerobic system to energy production. This translates into greater endurance at any intensity.

Each muscle fiber type has both aerobic and anaerobic capabilities. Endurance fibers are mostly aerobic but do have some anaerobic metabolism. Speed-endurance fibers are more balanced. Sprint fibers are mostly anaerobic, but their aerobic abilities can still be important in racing. Aerobic capacity workouts improve the aerobic capabilities of the sprint muscle fibers.

Stimulating these adaptations requires maintaining an intensity that is high enough to demand recruitment of the sprint fibers, but low enough to enable the athlete to sustain the intensity for a duration that will stimulate aerobic adaptations in those fibers, instead of only anaerobic adaptations.

Zone 5c Anaerobic Endurance Training: Anaerobic endurance training involves near-maximal exertion of all of the muscle fibers. This type of training increases fuel storage for the ATP-PC energy system. This helps improve a rider�s sprint endurance � the distance over which he can sustain full sprint wattage.

Training at this intensity also improves the synchronicity of muscle fiber contractions. Humans are imperfect machines. Even the most skilled athletes can get nowhere near 100% of their muscle fibers firing simultaneously at just the right moment. Improving synchronicity increases peak power.

Training at this intensity also improves economy at every intensity. An athlete who time trials at 390 watts will expend less energy to produce 390 watts in a race by including 20-second efforts at 550 watts in his training.


A primary objective of intensity zones is training each category of muscle fiber type as a group. Fibers within each category are relatively homogeneous for many qualities, including response to the training stimulus (improvement) and rate of recovery following a workout. Training them together optimizes both stimulation and recovery.

Ken Mierke is Head Coach of Fitness Concepts, Director of Training for Joe Friel's Ultrafit, and author of The Triathlete's Guide to Run Training and Training for Time Trials (due out 2006). Ken is a two time I.T.U. World Champion. His clients include 13 National Champions in 4 sports and 28 Team USA members.

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Posted: January 11, 2006

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