Friday, August 8, 2014

Big Announcement Coming Soon!

Hey Readers,

I know I've been radio silent for a little while now - don't worry, I'm still very much invested in this blog, I've just been working on something huge.  If everything goes as planned, I'll be making an announcement very shortly.

I'd also like to take a second to thank you all for reading what I post.  This blog started out generating a couple hits a day, and now it's right around 500 from all around the globe.  Considering that that's entirely content driven, zero advertising, 100% organic reader base, I really could not be happier.  It reaffirms my belief that there ARE people out there looking for the no-nonsense science of fitness.

Stay tuned, and please, continue the support!

Wednesday, March 5, 2014

Conditioning For Athletes: Does Your Program Condition You To Win?

Anaerobic training is far superior to aerobic training for almost every athlete.  There, I said it.

I'll say up front that this article will probably come across as anti-aerobics.  Actually, I kind of hope it does, because I see way too many coaches out there shooting their athletes in the foot by having them waste their time and energy on dedicated aerobic training.  I'll probably end up trashing on conditioning methods I know plenty of coaches still use, and I'm sure I'll hear all about it, but it is what it is. 

I'll also cover myself by saying that this article deals with athletics. I won't discuss the general health implications of aerobics vs anaerobics.

With that, let's get to it.

Some Vocab
The terms "aerobic" and "anaerobic" literally mean with oxygen and without oxygen.  In the context of exercise, these terms are used to describe activities that supposedly either utilize oxygen, or do not.

There's no such thing as a purely anaerobic or aerobic activity (more on that in a bit), so the entire subject is full of misnomers.  We're going to define anaerobic exercise as anything that could not be sustained for more than 2 minutes straight without performance changing.  For example, an all-out sprint cannot be maintained for more than 2 minutes without turning into a run, for reasons we will discuss.  Note that the exercise doesn't have to be done for 2 minutes to be anaerobic, it just has to be intense enough that you could never do it for more than 2 minutes all out.  Anything less intense than this is going to be called aerobic in this article.  Cues I use when I want anaerobic work done are "do it at a sprint pace" or "go balls out".

Some examples of anaerobic conditioning include all types of sprinting, jumping, explosive/athletic movements, gymnastic activities, most interval training, and any other high intensity burst activity.  Conventional resistance training is largely anaerobic, but this article focuses entirely on conditioning.  Some examples of aerobic conditioning include running more than ~800 meters ( a mile, 5k, etc.), biking, jogging, most group exercise classes (zumba, step, most yoga, etc.) cross country skiing, and anything else at low to moderate intensity that can be performed for more than 1-2 minutes without the distinct sensation that your heart is going to explode. 

A Brief Discussion Of Energy Systems
Every activity in life is powered by a particular energy system. This subject has a lot of labels including energy systems, metabolic pathways, bioenergetics, etc. but they all deal with the usage of various molecules for energy production.  This is a quick peek; we're skipping over lots of intermediaries and subsystems. They break down as follows:

ATP Stores
Anaerobic, lasts <2 seconds of intense effort.

Going back to high school bio, adenosine triphosphate is the "energy currency" our bodies use to power activity. Every time you move, ATP is being hydrolyzed to fuel that movement. Unfortunately, ATP is very heavy and we don't store much of it (~250g). Once a high intensity activity starts, ATP stores will be depleted within ~2 seconds.

Phosphocreatine (PCr) System
Anaerobic, lasts < 8-10 seconds of intense effort

As ATP (TRI, 3 phosphate) is broken down, remnant ADP (DI, 2 phosphate) are recycled to form new ATP molecules to power more activity. A single phosphate is cleaved from phosphocreatine and combined with ADP to form new ATP. This system produces no negative byproducts, and therefore does not require any compensatory mechanisms. In other words, the only thing that really governs this system is how long the phosphocreatine stores hold out.  ATP recycling happens constantly, but under intense activity it will be overwhelmed within about ~8-10 seconds depending on training status (creatine supplementation extends this threshold a bit).

When phosphocreatine levels dip, the body immediately begins to resynthesize them. It uses the phosphate released from ATP hydrolysis and combines it with creatine to make new phosphocreatine.  This process takes time.  Full PCr regeneration from a maximal activity can take 3+ minutes of rest.  While we're on the subject, creatine supplementation comes into play here by increasing the intramuscular pool of phosphocreatine to pull phosphates from, thus allowing the PCr system to be used for a longer duration. If creatine interests you, see Everything You Ever Wanted To Know About Creatine.

Glycolytic Systems - Anaerobic Glycolysis/Lactic Acid and Aerobic Glycolysis
Anaerobic glycolysis lasts < 2 minutes of intense effort, aerobic glycolysis system lasts until glycogen runs out.

At this point, muscle glycogen is broken down through glycolysis, producing pyruvate-->lactate to form new ATP, hence it often being called the lactic acid cycle.  Anaerobic glycolysis occurs when ATP must be made rapidly, IE during intense activity.  It is less efficient, yielding much less ATP, but requires no oxygen and provides fuel much more rapidly.  This system is often referred to as fast glycolysis.

Aerobic glycolysis starts in the same way, but grabs those pyruvate molecules before they turn into lactate, runs them through the Krebs cycle, and finishes with oxidative phosphorylation.  These extra steps yield a lot more ATP, but make the process slow enough that it cannot meet the fuel demands of intense activity.  The term "aerobic glycolysis" is actually inherently wrong since glycolysis is always anaerobic - the process doesn't become aerobic until oxygen enters the equation during subsequent cycles, but oh well.  This system is often referred to as slow glycolysis. 

The obvious question is, "Why don't we just use anaerobic glycolysis indefinitely until glycogen runs out?".  The limiting factor is that glycolysis generates H+ ions, which gradually acidify the blood.  The formation of lactate from pyruvate buffers some of these ions, but intense activity eventually overwhelms buffering capacity.  As [H+] increases, muscle burn and fatigue also increase .  This is why no one, no matter how trained, can sustain a dead sprint for long - waste products eventually overwhelm the system, forcing intensity to decrease.

Similar to how creatine supplementation improves the PCr system, beta alanine is a supplement with the purported ability to improve fast glycolysis.  Beta alanine has been fairly well confirmed to increase intramuscular concentrations of carnosine, which is a powerful buffering agent.  This, in turn, delays blood acidosis and allows for more time in the fast glycolysis system.  Beta alanine is fairly well studied, but if it's something you've never heard of, check out Effects of Beta-Alanine on Muscle Carnosine and Exercise Performance:A Review of the Current Literature.


Fat Oxidation System
Aerobic, takes place at all times, lasts indefinitely.

Activity past 2 minutes of high intensity will be regulated by slow glycolysis and fat oxidation, and the longer the activity goes, the more energy production will shift towards fat oxidation.
For very long events where glycogen will eventually deplete, (that nasty point in a marathon where heavy fatigue sets in, known as "bonking out") activity is fueled almost entirely by fat and some protein degradation.  It's beyond the scope of this article, but I should mention that this does not implicitly make steady state aerobics better for fat loss.

To recap:
  •  Energy systems
    • Anaerobic
      • ATP-PCr - Very high power, very short duration
      • Fast glycolytic - high power, short duration
    • Aerobic
      • Slow glycolytic - moderate power, long duration
      • Fat oxidation - low power, very long duration

The Bioenergetic Continuum
The bioenergetic continuum is just a fancy way of saying that energy systems do not work independently, and this is where most people get it all wrong.

The PCr system is a rocket ship - massive power, burns out quick, needs a ton of time to refuel.  Anaerobic glycolysis is an F1 car - tons of power, terrible fuel efficiency, needs frequent pit stops to keep working.  Aerobic glycolysis is a 4-cylinder hatchback - meh to good power, great fuel efficiency, super reliable.  Fat oxidation is a Geo Metro running on vegetable oil (get it?!  They both run on fat ahahahaha!!!) - garbage power, goes forever, tons of fuel just laying around all over the place. 

The amazing thing is that our bodies have this incredible metabolism which can seamlessly use all of the above at the same time, however much or little as needed, to best fit the demands of activity.  Need 100% power for a short period?  You're a rocket ship.  Need 95% power for 6 seconds? ?  You're half rocket ship and half V12 sports car.  Need 80% power for ~1 minute?  You're mostly a rocket ship and a V12, some Ford Focus, and even a little bit Geo Metro.  Need 25% power for 10 minutes?  You're some V12, mostly hatchback, and a good bit of Metro.  Distance runner?  You're pretty much all 4-cylinder and grease car.

It's vital to understand that though we generally use the term "anaerobic" to describe intense activity lasting less than 2 minutes, and "aerobic" to describe anything over that, the fact that every energy system is at work to some extent at all times. This is especially true of most game conditions where athletic demands are constantly shifting - you've got periods of all out activity where ATP/creatine is being depleted, high intensity periods where glycogen is the primary fuel source, intermittent relative rest periods that allow for creatine stores to at least partially replenish, and all the while, aerobic systems are contributing to a varying extent.

Duffield et al, 2004 directly measured the metabolic contributions for the 100m and 200m sprint. They found that it was right around 75%/25% and 70%-30% anaerobic to aerobic respectively, with a slightly higher ratio of anaerobic to aerobic for males. Gastin, 2001 indicates that aerobic metabolism contributes significantly even at durations/intensity classically considered purely anaerobic, and that anaerobic and aerobic energy systems reach a 50/50 contribution split between 1-2 minutes, likely at ~70 seconds. Losnegard et al, 2012, did a similar study on elite cross-country skiers skiing at maximal effort for ~3 minutes. They found that overall contribution was 25%/75% anaerobic to aerobic.  Case in point, aerobic contribution starts a lot earlier than we previously realized.

Furthermore, it seems aerobic metabolism can up it's game when anaerobic fuel is depleted. Bogdanis et al, 1996 indicates that in repeat-bout sprinting activity, anaerobic fuel contribution is lower on subsequent sprints (this is always going to be true unless enough rest for full PCr resynthesis occurs, very unlikely in a game unless a player is benched). Once this happens, aerobic systems kick in to help cover the gap. The study cited a 41% decrease in anaerobic fuel in the repeat bouts, but only an 18% performance drop due to aerobic systems sharing the load.

It is clear that aerobic contributions occur much earlier than previously assumed.  The bottom line is that the aerobic systems do contribute a significant amount of energy to what we always considered "anaerobics".  Doesn't this imply that we should be spending a significant amount of effort improving these systems?  Well, maybe not so much.

This graph summarizes energy systems nicely (note that PCr=ATP-PC, fast glycolytic=lactic acid, and aerobic system=slow glycolysis and fat oxidation):





What About Game Time?
Every major sport has been analyzed for it's metabolic demands. Various methods have reported slightly different contributions, but the general trends have long been established. Really, they're only estimates anyway since even within a sport, every athlete and every game is different.


  Figure 5.1 Essentials of strength training and conditioning (3rd ed., p. 95)

Some sports make it very easy to approximate energy demands - a short distance track athlete, for instance, is going to sprint a set distance for a more or less fixed duration, and we can predict with relatively high accuracy what energy systems are needed for that activity. Similarly, football and baseball/softball are fairly simple as well; short bursts of all out activity, high ratios of activity:rest, no real active recovery - all rocket ship and formula 1.  The tricky part is doing it with a field athlete in constant play, or a combat athlete.

A review by Spencer et al, 2005 looked at average sprint distance, number of sprints, and time to rest between sprints for elite level field sports such as soccer, lax, field hockey, etc. The review indicated that the average sprint time for these sports is a very consistent 2-3 seconds or 10-20 meters across many trials. Similarly, the average maximal sprint (the longest an athlete would have to sprint during a game) is 4.1 +/- 2.1 seconds. The average rest time between sprints was 20-60 seconds. The studies reviewed all resoundingly stated that while the player was in the game, 95% of rest was active, meaning that "rest" between sprints equates to moving around the field at a jog pace.  

The same review reports that for these sprinting activities, the contributions are 10% stored ATP, 55% PCr, 32% anerobic glycolytic, and 3% aerobic. This makes complete sense given the demands of a 3 second sprint, but what about when an athlete is forced to repeat this over and over as a game progresses?

Gaitanos et al, 1993 tested for energy contributions on a sprint protocol consisting of 6 10 second sprints followed by 30 seconds of rest. They found that after the first sprint, PCr stores had fallen to 57% of baseline, and that by the 10th sprint, PCr stores had fallen to only 16% of baseline. Given a full 3 minutes to recover, PCr had climbed back to 84% of baseline. It should be obvious that full PCr resynthesis will never take place in a game unless an athlete is benched.

It is well established that glycogen stores fall with activity. Somewhat surprisingly, using the same protocol, Gaitanos et al found that rate of glycogenolysis (how fast new sugar can be made) fell 11 fold, and rate of glycolysis fell 8 fold. Take home point - not only do glycogen stores fall, an athlete's ability to even utilize and make new glycogen falls as well.  I don't know why this occurs, but my guess would be that it's a self-limiting control to prevent acidosis.  The same review by Spencer et al reported that a single soccer game played at an elite level burned through 80-90% of glycogen stores. It also stated that those with lower starting glycogen sprinted less, walked more, had reduced running speed, and covered an average of 1800m less distance over the game.

In that same Gaitanos et al protocol, sprint performance from first to last only dropped by 27%. That's very significant over 6 repeat sprints (and would obviously continue to degrade past that point), but it should be much lower given the PCr and glycolytic degradations. The explanation? The aerobic systems pick up the slack, but don't do anywhere near as good a job.  The aerobic systems are a lot like that industrial-size can of Folgers you keep around for when the good coffee runs out.  It may not be the greatest, but it's there when you need it, it gets the job done, and it seems to never run out.


Energy Demands and Conditioning Strategies

I've spent the better part of this article illustrating that many sports have a large aerobic component. I've also used a good deal of it showing that what we always considered to be "anaerobic" work, actually has a very large aerobic component. You might be asking yourself at this point why I would ever say that aerobic training is a waste of time. Many coaches in the past have taken the standpoint that if sports have an aerobic component, then we need to train "aerobically".

Understanding that anaerobic conditioning actually involves a huge contribution from the aerobic system essentially invalidates this notion. Remember that over a SINGLE bout of maximal intensity sprinting, aerobic and anaerobic contributions match each other at ~70 seconds. Aerobic contribution only increases over repeat bouts, depending largely on rest periods used. This means that practically every form of known anaerobic training with any kind of reasonable rest period gives all the aerobic training most athletes ever need, while also maximally training the anaerobic systems. This is especially true if active rest is used.


Then again, why not just throw in more aerobic work, just to be safe and cover our bases? Put simply, excess aerobic training detracts from sports performance. Aerobic and anaerobic training have wildly different, diametrically opposed physiological adaptations; this is well documented and understood, and it's why distance runners can't sprint or jump to save their lives. An athlete simply cannot be highly conditioned in all systems. In this regard, I take the standpoint of "do as much aerobic work as necessary, and not a single bit more." Given the confirmed aerobic crossover of traditionally anaerobic work, for many athletes this means zero dedicated aerobic training. This is especially important when dealing with athletes that have precious little time to devote to S&C.


In a nut shell, aerobic performance is crucial, yet usually requires no dedicated improvement if anaerobic training is performed.


Plenty More Than Energy Systems
Beyond considering which energy system each sport utilizes, we also need to examine other adaptations that occur. *note: I'm not going to bother with references in the following section because frankly, it's all common knowledge that can be found in any textbook or reliable website and I'm too lazy to regurgitate.

Skill Practice
Ask any coach, and they'll tell you that sprinting/jumping/explosive movements are among the most difficult to teach. One major benefit of incorporating high intensity anaerobic training is that it allows for a lot of practice at the actual skill of being fast/jumping high/performing explosive movements. This is absolutely critical because of the huge skill component involved in sprinting quickly and efficiently.

A major mistake many coaches make is assuming that if athletes spend a lot of time running, then they'll become good at sprinting. In reality, sprinting mechanics and jogging mechanics are worlds apart. Sprinting for conditioning isn't going to confer the same benefits as dedicated speed work (done fresh, never to fatigue, with mental effort going towards optimal, smooth form), but it does give athletes a chance to get a whole lot of repetitions in under the physical demands of a game situation.

Substrate Storage
Athletes need glycogen, and plenty of it. As indicated by reviewed studies, athletes who carry submaximal loads of glycogen take a major hit to their game performance. Luckily, there's a way to essentially over-stuff muscle tissue with extra glycogen by utilizing a principle known as glycogen supercompensation.

The glycogen supercompensation window opens in response to an activity which depletes a large portion of stored glycogen. We can create this window by conditioning with activities that heavily tax the glycolytic system. It can be done with aerobic training as well, but it takes quite a long time to deplete glycogen enough to open a supercompensation window with low intensity activity.


I won't go too far into glycogen loading here - I will say that lot of people get it wrong by trying to do it a few hours before a game (there's nothing wrong with getting carbs in before a game, but that isn't supercompensation). The window is open after the taxing event, so get the carbs in then.

Substrate storage improvement isn't limited to glycogen.  Creatine phosphate, enzyme concentration, and even ATP stores are all subject to supercompensation induced by anaerobic training.

Power Generation and Muscle Composition
I probably don't need to tell you that athletes require the ability to generate power. Practically every athletic movement a player makes is the direct result of their ability to develop force rapidly and efficiently over a particular motor pattern.

There's mountains of studies indicating the negative relationship between aerobics and force production. This is largely due to neurological adaptations, over training, and hypertrophy inhibition (from the catabolic nature of aerobics). Its been shown conclusively that combining aerobic training with strength training and/or anaerobic conditioning reduces strength and power output. Furthermore, this relationship is unidirectional; aerobic training impairs anaerobic performance, but anaerobic training does not impair aerobic performance.

Injury Resistance
Bone and connective tissue adapt to stress just as muscle tissue does. High intensity training results in the growth and strengthening of bone and connective tissue, to better withstand the forces the body is subjected to during intense activity. This style of training also causes tendons to stiffen, making them more efficient at transferring muscular force.

These adaptations makes the body more resistant to injury and more efficient at force production. They occur as a response to high intensity anaerobics, but do not occur with low intensity aerobics.

Trade Offs and Risks
Is there anything to be lost by ditching aerobic training? Purely from a performance standpoint, and assuming the athlete is focused on only one sport or activity, then no. There are, however, other factors to consider.

First off, aerobic training increases VO2 max more than anaerobic training. This is well known and well documented. Whether or not this matters is up to the individual coach/athlete. There are still many coaches who strongly associate VO2 max and overall athleticism or fitness. I'm of the opinion that an athlete really needs no higher a VO2 max than what their sport demands, but it is what it is. Aerobics will definitely improve VO2 max more than anaerobics.

Secondly, the risk of over-training is considerably higher with anaerobic training. Burning an athlete out on anaerobic conditioning, especially when it's combined with resistance training, is a very real concern that isn't present with aerobic training. This concern ultimately falls on the shoulders of coaches. Implementing anaerobic conditioning into a complete training protocol requires much more skill, observation, and attention to detail on the coaches' part. Same goes for whoever handles the nutrition (which is usually the coach at most levels). Athletes, especially younger ones, must be strongly encouraged to consume enough calories/carbs and get enough rest, or they will burn out on a resistance exercise/anaerobic conditioning program.

Similarly, there's risk of injury to consider. It isn't necessarily higher with anaerobic conditioning, as aerobic conditioning has a whole set of chronic injuries and conditions that go along with it. Risk of acute injury is, however, certainly higher. Again, this is on the coach to make sure kids are adequately warmed up, neurologically primed, have no serious flexibility/mobility issues, and that his/her athletes have a decent of strength (debatable, but if your athletes can't squat/dead their body weight, they have no business sprinting).

One of the worst things a coach can do is dump a bunch of anaerobic work on a group of novices. The best way to handle anaerobic sessions is to treat the same way you'd treat strength training sessions - with attention to technique, fatigue, and recovery. 

Summary and Implications
The primary objective of any conditioning program is to prolong time to fatigue from game conditions. Baseline ATP storage is relatively immutable, so don't expect much improvement there. Some studies suggest that a well trained individual might carry more ATP, but considering the very short supply of local ATP stores (~2 seconds), it's unlikely that this would offer much actual performance increase.

The PCr system, and by extension fast ATP recycling, is limited by intramuscular creatine pools. The system itself can't really be significantly improved - once the fuel is gone, the system is done providing ATP until phosphocreatine is resynthesized. Some studies suggest that well trained athletes do resynthesize phosphocreatine more rapidly, but there's very little data to be found. That being said, there's piles of data to be found on oral creatine supplementation dramatically improving time to fatigue of the PCr system and resynthesis of phosphocreatine. In this regard, one of the best ways to improve the PCr system's conditioning is simply to supplement with creatine.

The fast glycolytic system is a different story. Because glycolysis creates negative byproducts, conditioning in the 10 second - 2 minute area depends largely on how well H+ ions can be buffered, and how much a person can tolerate the drop in blood pH (and the pain that comes with it). Conditioning above the lactate threshold has been shown to result in a 16-38% increase in blood buffering, as well as a higher tolerance to the physical discomfort of being above the lactate threshold.  In the real world, this means more time in fast glycolysis, AKA more time in a high energy system before fatigue.

Nutrition, and to a lesser extent supplementation, are paramount for high intensity endurance.  Creatine supplementation can offer a lot of benefit to the PCr system, and beta alanine to the slow glycolytic system (perhaps to a lesser extent) but nothing will ruin on field performance like starting a game with depleted glycogen.  We've all heard the adage "Strength is built at the gym, size is built at the dinner table. " - you could easily add in "conditioning is built at both."
The take home points:  though there is an aerobic component to plenty of sports, it is the anaerobic activity that wins or loses games.  Furthermore, classically "anaerobic" conditioning generally covers these aerobic needs, as there's actually plenty of aerobic work being done - especially when anaerobic work is combined with active rest.  Lastly, glycogen availability, and to a lesser extent supplementation, are huge factors in sports conditioning.  

 Here's how it breaks down in the real world:
  • Field sports
    • dominated by those who have well conditioned and prepared fast glycolytic system combined with an adequately conditioned aerobic system
      • anaerobic training will drastically improve buffering ability and performance
      • muscle glycogen should be as close to full as possible or performance will be severely impacted
      • creatine supplementation can benefit conditioning
      • beta alanine likely also beneficial
    • have a significant slow glycolytic component
      • anaerobic training with active rest covers this aerobic demand, no more aerobic training is needed
  • Football/short track/comp lifters
    • played almost entirely anaerobically
      • rest periods long enough/complete enough to stay anaerobic
      • should be conditioned purely with anaerobic training 
      • well conditioned and prepared fast glycolytic system is make or break
      • creatine supplementation can be hugely beneficial
  • Combat sports 
    • harder to predict given the chaotic nature of each match 
    • energy requirements largely dependent on pace of match
    • shorter play time, but much longer average bouts of maximal intensity
    • glycogen stores key, creatine and beta alanine beneficial
      • harder to manage due to water retention and weight gain 
    • K1/Thai Boxing
      • 3 minute round times push demands more towards aerobic conditioning
        • aerobic conditioning will become a larger factor in later rounds
    • MMA
      • 5 minute round times require a greater emphasis on aerobic conditioning, especially in the later rounds
      • high intensity nature + long duration necessitates more aerobic conditioning
      • carb loading should begin as soon as the weigh in is over
    • Wrestling
      • 2-3 minute round times and fewer rounds mean greater emphasis on anaerobic conditioning 
        • conditioning can be done at maximal intensity with little to no need for active rest
    • Boxing
      • potential for high intensity over large amount of rounds
        • aerobic conditioning necessary for late rounds, especially if early rounds have been very active
References
Energy system interaction and relative contribution during maximal exercise. Gastin, P. Sports Medicine. 2001, 31(10):725-741, 2001. Retrieved from http://ovidsp.tx.ovid.com/sp-3.11.0a/ovidweb.cgi?T=JS&PAGE=fulltext&D=ovft&AN=00007256-200131100-00003&NEWS=N&CSC=Y&CHANNEL=PubMed.

Human muscle metabolism during intermittent maximal exercise.

Gaitanos GC, Williams C, Boobis LH, Brooks S. J Appl Physiol (1985). 1993 Aug;75(2):712-9. Retrieved from http://jap.physiology.org/content/75/2/712.long.

Baechle, T. R., & Earle, R. W. (2008). Adaptations To Anaerobic Training Programs. Essentials of strength training and conditioning (3rd ed., p. 95). Champaign, IL: Human Kinetics.


Energy system contribution to 100-m and 200-m track running events.
Duffield R, Dawson B, Goodman C. J Sci Med Sport. 2004 Sep;7(3):302-13. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15518295.

Metabolic Limitations of Performance and Fatigue in Football. Abdullah F. Alghannam. Asian J Sports Med. Jun 2012; 3(2): 65–73. PMCID: PMC3426724. Retrieved from http://europepmc.org/articles/PMC3426724.

Anaerobic capacity as a determinant of performance in sprint skiing. Losnegard T, Myklebust H, Hallén J.2012 Apr;44(4):673-81. Med Sci Sports Exerc. doi: 10.1249/MSS.0b013e3182388684. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/21952633.

Contribution of phosphocreatine and aerobic metabolism to energy supply during repeated sprint exercise. J Appl Physiol (1985). 1996 Mar;80(3):876-84. Bogdanis GC, Nevill ME, Boobis LH, Lakomy HK. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/8964751.

Physiological and metabolic responses of repeated-sprint activities:specific to field-based team sports. Spencer M, Bishop D, Dawson B, Goodman C. Sports Med. 2005;35(12):1025-44. Retrieved from http://www.google.com/url?q=http://www.fmh.utl.pt/agon/cpfmh/docs/documentos/recursos/112/GlaisterSprintRepetidos.pdf&sa=U&ei=6OQVU6upB4Pt2QWttoDYDQ&ved=0CCcQFjAD&sig2=GCcAPu4w2jlWCRsd3r91Cg&usg=AFQjCNHdM3AXUZ-a2rx_jEXMaGunzybojQ.