The past couple years have yielded much debate about which methods should be employed during training for elite soccer. Most likely, such debate takes place because coaches do not fully understand the exact work demands imposed on soccer players, nor do they understand the required physiological response to training. The sport of soccer is typically termed an “intermittent field sport” in which there is a constant fluctuation of energy system demands placed on soccer players.
How exactly do we better classify soccer and similar field sports? Are they aerobic? Are they anaerobic? If we examine the work performed closely in these sports, we would likely conclude that they could be classified as Alactic-Aerobic.
A recent study by Osgnach et al. (2009) performed a computer-video analysis evaluating the workload of almost 400 players in the Italian Serie A. The research summarizes that the typical workload of an elite level soccer player consists of the following:
* Mean total distance of 10-13km (6.2-8.1 miles) covered
* Soccer players normally spend up to 70% of the duration of the match in purely “aerobic” low intensity activities, with about 30% consisting of 15-20m higher intensity movements.
* “Sprinting” ended up being about 5-10% of the total distance covered in a match, yet only amounted to 1-3% of the total match time, which correlated to a 2-4 second sprint every 90 seconds.
* The most metabolically demanding work was performed when players accelerated up to 3 m/s2 or decelerated at -3 m/s2.
In light of this information, one would need to question which energy system are over- or under-utilized in athletes depending on their training status. In the past, many coaches have felt that the anaerobic system (glycolytic) would be the dominant energy system used to provide energy throughout a game. Viru (2008) discusses the difficulty in determining the dominant mechanisms of ATP production in team sports due the intermittent nature of most of them. Since the mean duration of intense work varies (8 seconds in volleyball, 28 seconds in basketball and 36 seconds in hockey) he concludes that each one will have differing dominant energy sources.
Viru theorized that in such phases of intense activity, volleyball would rely mainly on the breakdown of PCr within the muscle, basketball on PCr and glycolysis, and hockey mainly on glycolysis. If we are to believe the studies examining movement patterns within soccer (2-4 seconds of high-intensity effort followed by 90 seconds of recovery activities on average), then the creatine phosphate (PCr) system is the main source of energy within a match during “non-recovery” activities, followed by aerobic recovery. The PCr system will be stressed during the intense activity, with the aerobic system lagging behind, and providing for recovery of PCr stores.
An analysis of blood lactate levels over the course of an elite level soccer game reveals that most athletes experience average blood lactate levels of 4.5-5.0 mmol/L, which indicates that glycolytic activity is not consistently high during the course of a match. There are moments within the game that players may reach blood lactate levels up to 10.0 mmol/L, but those moments are rare and depend very much on position. In fact, blood lactate levels are normally lower at the conclusion of the game, reconfirming that matches are often decided by the quality and power of the aerobic system to replenish PCr stores.
In addition to these decreased blood lactate levels, there appears to be an increased level of free fatty acids (FFA) in the blood, which will be used as energy throughout the second half. This provides further evidence that a well developed oxidative system aimed at utilizing such fatty acids for aerobic energy are crucial for elite teams in the second half of games. Overall it must be said that mean blood lactate levels in the 4.5-5.0 mmol/L range indicate an overall loading just slightly above LT, not particularly high for a sport which many strength and conditioning coaches want to train in an “anaerobic” way.
Researchers typically emphasize the importance of glycogen, the most important substrate used for energy in elite soccer performance. However, not enough coaches discuss methods in which athletes can improve their ability to spare glycogen over the course of a game. While glycogen is a vital substrate in elite level soccer, that’s all the more reason why athletes should be training to save such a substrate until deep into a match. An improved ability to utilize free fatty acids and quickly replenish PCr stores will help save glycogen stores throughout competition.
Most strength & conditioning coaches are very familiar with the energy production curves of the three main sources over exercise time. Here’s a simplified view of energy production from Joel Jamieson’s book, Ultimate MMA Conditioning (2009):
The alactic system is the most immediate system of energy production in the body; however, it also has the shortest duration of production of energy (6-12 seconds) of the three systems. As you can infer from looking at the above graph, a soccer player will utilize his/her alactic energy stores to meet the constant demand to make short sprints over the course of the game. It is then necessary to have the aerobic power and capacity to help replenish the ATP-PC stores in the alactic system as quickly as possible, so that the athlete is ready to perform the next sprint.
Anaerobic glycolysis could be seen as more of an intermediate energy source that the body will need to “dip” into when the alactic energy stores are fully depleted and the aerobic system is not quite turned on, or powerful enough, to meet energy demands. However, the use of this anaerobic glycolysis comes at a price: lowered pH, and an increase in H+ and lactate build-up, all of which will impede energy production over time.
Traditionally, we have seen many strength and conditioning coaches focus their conditioning programs on what they consider “anaerobic” training methods, with the thought that such methods would both target specific energy systems better than the traditional LSD training, and would be efficient enough to target all systems in a short, economical manner. Is this actually accurate?
Value of “Anaerobic Training”?
While there are no doubt some valuable adaptations made through glycolytic interval training, there is quite a bit of evidence indicating that an over-reliance on high-intensity intervals may not be targeting our energy systems in the most effective way.
According to a review by Tomlin & Wenger (2001), our training goal should be to improve the rate of PCr resynthesis through an improved rate of local oxidative metabolism. In fact, the review indicates that training results should conclude with an ability to “supply more energy through the phosphagen and aerobic systems, thus decreasing the reliance on anaerobic glycolysis, and thereby stemming the rise in H+ during high intensity intermittent work.” In fact, Tomlin & Wenger provide some compelling evidence that there is a close link between aerobic ability and replenishment of PCr stores. This suggests an ability to maintain short-term peak power longer over time, above and beyond what would be possible through anaerobic glycolysis.
The review cites research from Hamilton et al. (1991) in which athletes with higher levels of aerobic power were able to maintain higher 6-second power outputs over the course of 10 repetitions, than less aerobically fit athletes. In fact, Hamilton et al. found that these more “aerobic” athletes were better able to consume oxygen during the 10-sprint protocol, as well as immediately following in the post-exercise period.
There is also evidence that indicates that what has been traditionally referred to as “anaerobic” training, may actually result in a loading and improvement to the aerobic system. In research done by Parolin at al. (1999), three high-intensity bouts of 30 seconds intervals followed by four minutes of rest were examined in terms of energy production. This research reveals that through progressive bouts of 30-second intermittent work, there is actually a progressive shift to oxidation through the aerobic system (increase in oxygen utilization), with a concurrent maintenance of lactate levels and H+ levels following the first interval.
The researchers believe that the increase in H+ above a certain threshold may actually inhibit lactate production and serve to increase rate of pyruvate oxidation, as well as oxidative phosphorylation. It has been suggested that this triggering mechanism due to an increase in H+ within the muscle serves to limit glycogen utilization and inhibit further lactate and H+ accumulation. In other words, an overreliance on glycolysis may actually serve to inhibit glycolysis.
Neville et al. (1994) found similar results when observing the energy production from two 30-second bouts of maximal sprints separated by four minutes of passive rest.
(Above data taken from Neville et al., 1994)
Whether an athlete is performing an alactic (<8 seconds) activity, or a traditionally termed anaerobic activity (20-90 seconds), the examples indicate that each succeeding repetition places a greater reliance on aerobic metabolism, rather than anaerobic metabolism. This begs the question: When we are doing our tabatas or 300 yard shuttles, exactly what physiological adaptation are we looking for?
Stay Tuned for Part II Coming soon…