In 2002, France and Arsenal star Robert Pires damaged the anterior cruciate ligament (ACL) in his right knee. Pires and the French national team coach Roger Lemerre have both blamed fatigue as a contributory factor in the injury. Lemerre has been deprived of the services of one of the world’s top midfield players for the World Cup in Japan and South Korea and the manager was critical of the high number of games his English based players have to play in a season. Speaking from the French Football Federation HQ at Clarefontaine he said, “We know that the players who are playing loads of games during the season become very fragile and we think that Pires’ injury is due to the accumulation of games.”
Lemerre seemed most concerned about the welfare of his players based at Arsenal, who have been involved in the Champions League, FA Cup and Premiership. The French coach added, “Pires and (Thierry) Henry are the two players who have played the most this season. They have played around 45 or 47 games. After a certain limit, it becomes very dangerous. They play at a very high level. The competition and the pressures are very high, and it can have serious consequences on their health.”
Pires damaged his knee when landing awkwardly trying to avoid a tackle from Newcastle United’s Nicos Dabizas. The player’s right foot was planted in the turf as his knee twisted under the players’ momentum – a common method of damaging the cruciate ligaments without the need for a collision with an opponent. After the diagnosis had been confirmed, by Professor Jean-Henri Jaeger in Strasbourg, Pires himself reflected on his circumstances, admitting, “I am very tired and the succession of games was probably too high, even if I like playing every three to four days. My injury does not make me change my opinion on that. In all jobs there are risks. Maybe I have not been careful enough, when I see the video it was a lack of concentration.”
There is scientific evidence to support Lemerre’s theory that playing too many games is the major reason he will not have the services of Pires in Japan and South Korea. Although we all know what we mean when we refer to fatigue, it is a difficult concept to define from a scientific viewpoint. Many definitions exist, but the simplest is ‘the inability to maintain work output’. This can manifest itself in several ways. Mental fatigue is a subjective experience that can lead to a lack of concentration. Alternatively, physiological fatigue can be defined objectively as a decline in work output when a task is measured or timed and results from changes in the nervous system, or in the muscle tissue, which adversely affect the athletes’ performance and ability to avoid injury.
In order to understand how physiological fatigue can contribute to knee ligament injuries, it is necessary to understand the way the nervous system and muscles interact to contribute to joint stability.
A player is about to land from a jump and his foot is about to make contact with the turf. In advance of the landing, the nervous system sends messages to activate the musculature around a joint (e.g. the knee), so that the impact of landing can be coordinated smoothly, and shearing forces to the actual joint can be minimised. If the muscular stabilisers of a joint are not activated in time for the landing then all the forces of landing are taken through the passive restraints of a joint – i.e. the ligaments. Crucial for this advanced preparation is the ability of the nervous system to detect the body’s, or specific limb’s, position as it is about to land and to initiate the correct muscle activation at the precise moment. This ability to sense ones body position in space is known as ‘proprioception’ and the system that coordinates this proprioceptive input with the body’s motor output is known as the ‘sensorimotor system’.
Proprioception is achieved by special sensory nerve endings located in ligaments, tendons and joints, which send continuous signals to the brain providing information on the position of each muscle and joint. The cerebellum in the brain collates and interprets this information and formulates a plan on what the next muscle action should be to achieve the desired task (e.g. jumping over an opposing player). The cerebellum then sends this plan to the motor cortex of the brain, which in turn sends the appropriate commands through the nervous system to the involved muscles. This all occurs in a fraction of a millisecond, and the whole time the cerebellum is receiving more proprioceptive feedback and continually adjusting its motor plan to fine tune the movements being taken. The high level athlete does not have to consciously think about this process – practice means it is as automatic to them as picking up a cup of coffee is to us.
A player of Robert Pires’s calibre has extremely advanced proprioceptive skills, as demonstrated by the poise of his movements and his ability to skip past similarly finely tuned elite players. His goal against Aston Villa two weeks ago provided ample evidence of this. His ability to sprint after Fredi Ljungberg’s pass then decelerate, and simultaneously control the ball while changing direction, left opponent George Boateng on the floor. Pires then composed himself before executing a perfect lob over Peter Schmeichel. To football fans this may be goal of the season, but from a physiological perspective it represents a sensorimotor system that has been developed with 20 years of dedicated practice. However, the success of the sensorimotor system is largely dependent upon the speed of the nerve signals to and from the brain, as well as the muscles ability to rapidly develop sufficient power to take evasive action.
Nerve impulses are dependent upon the propagation of an electrical current, which is dependent on sodium and potassium ions. Once the impulse reaches the target muscle calcium ions are released causing excitation of muscle fibres and a muscle contraction. Muscle contraction itself has been shown to be affected by deficiencies in glycogen (the fuel for muscle contraction) as well as other muscle substrates. This can be classified as fatigue and, together with deficiencies in electrolytes such as calcium, potassium or sodium, can decrease the efficiency of the sensorimotor system.
The impulse to take evasive action takes a fraction of a second too long to be executed, causing inadequate ‘dynamic stabilisation’ of a joint. Once this system has failed to provide restraint, all the force of the uncoordinated movement is opposed only by ligaments which provide ‘passive stability’ to a joint. If the force is excessive the ligament will fail and become damaged. Which ligament or ligaments are affected is dependent on the direction to which the joint is unrestrained, with the degree of damage directly dependent on the level of force placed on the ligament.