Photo: Getty Images
In the same way that a car needs fuel, muscles require energy to work. This energy is provided by a chemical called Adenosine Tri Phosphate (ATP). A small amount of ATP is present naturally in the muscles and provides enough energy for the first few seconds of activity. After that, the body has to produce more ATP.
Initial production comes about using the phosphagen system, which uses the muscles’ stores of creatine phosphate to generate ATP. This process lasts only a few seconds and is the predominant fuel source used by 100-metre sprinters, field athletes and weight-lifters. It does not require oxygen to work, doesn’t produce lactic acid and is therefore described as an alactic anaerobic process.
After this initial time, the contribution of different energy systems depends upon the intensity of the work being done and the level of fitness of the player. If the exercise intensity is low, the muscle is able to obtain ATP from the breakdown of food substrates via a series of chemical reactions dependent upon the presence of oxygen; this is called the “aerobic” energy system.
The Anaerobic System
If the intensity of the exercise is high (for the individual player), the aerobic energy system cannot produce ATP quickly enough. The ATP is then produced by the anaerobic (literally “without oxygen”) system, and specifically fast glycolysis. Although the anaerobic system can produce ATP much more quickly, it cannot produce it for nearly as long as the aerobic system. It also causes the build-up of lactic acid. As a result, exercise that is intense enough to require the anaerobic system cannot normally be maintained for more than a few minutes.
The highest intensity of exercise where a person can work aerobically and deal with any waste products caused by anaerobic metabolism is known as the anaerobic threshold. A person with a higher anaerobic threshold will be able to work much longer at the same intensity than a person with a lower anaerobic threshold.
Cardiovascular fitness is important for performance in rugby and may help to prevent injury. Work capacity (also known as stamina) refers to a player’s ability to maintain his work rate throughout the game. This is not the same stamina that is needed by a distance runner: this involves the ability to walk, jog and run the required distances (up to 9km in the average game), but also the ability to compete in the contact areas.
Position Specific Stamina
Photo: Getty Images
Unlike running events, distance in a rugby game is covered in a number of short, multi-directional movements. Players will sprint during games and these movements are position-specific in terms of time and distance. Generally outside backs cover the greatest distances in each individual sprint they make. International Test match data shows that wings and full backs will perform more than 40 high-speed runs within the 40 minutes that the ball is in play during a match.
With 15-20 instances of contact per game also inducing fatigue, the importance of good speed endurance for rugby players cannot be over-emphasised. At the other end of the spectrum, some forwards will experience 100 collisions during a game which in themselves cause fatigue.
Studies have shown that a large percentage of the total time and distance covered during a match is done so at low intensities, which means that the aerobic system makes the most significant contribution to energy delivery. It is therefore important for a rugby player to have a sufficiently developed aerobic system.