This is an excerpt from Complete Conditioning for Rugby by Paul Pook.
Physical and Physiological Demands of Rugby
Rugby is one of the most intriguing sports to analyse from a physiological perspective because it has such a variety of demands and each match is unique. Historically, rugby was regarded as an aerobic sport, and as such players performed a high volume of aerobic training including long-distance running, particularly during the pre-season phase. More recently, with the help of research and match analysis, this is no longer an accepted principle. The start-stop nature of the game and the high volume of collisions and grappling activities indicate that the predominant energy source is anaerobic. Duthie and colleagues (2003) showed that rugby players' intense efforts place considerable stress on anaerobic energy sources, whereas the aerobic system provides energy during repeated efforts and recovery.
Players rely heavily on both aerobic and anaerobic sources to repeat movement patterns and recover. Movements such as tackling and rucking are performed too rapidly for the aerobic system to supply enough energy to the muscles, so anaerobic fuel powers these movements. The aerobic system promotes recovery between dynamic movements and fuels less intense activities such as walking and jogging.
Strength and Power
Tackling, being tackled, rucking and mauling all involve maximal efforts that challenge players' strength and power. These are full-body movements that require strength in various planes of movement, so players require not only high levels of general and specific strength but also stability and mobility. Expressing strength quickly produces power, and this is required to break through tackles, accelerate at a high speed to make tackles and jump to catch a ball, for example.
The high-intensity and contact nature of the game clearly demonstrates the need for muscles to provide stability to support the joints, including the knees, pelvis, shoulders and neck, during impacts. Particular emphasis needs to be placed on the key stabilisers of the trunk, which brace the spine and support the efforts of the arms and legs during all rugby movements.
Speed and Agility
The multi-directional nature of rugby highlights the need for speed and agility. This often involves fast changes in direction when reacting to the position of a defender, decelerating sharply to hit a ruck or accelerating to make or break a tackle. Analysis of the game pinpoints acceleration as the major requirement as opposed to top end speed, although outside backs are more likely to reach top speed when they are afforded sufficient space. Running also includes backward and lateral movements, such as retreating to avoid the offside line, shadowing an attacker and evading opponents during a lineout.
Rugby relies heavily on acceleration (i.e., the ability to rapidly reach a high speed from various starting positions) supported by agility, which is the ability to change direction and decelerate quickly.