THE ROLE OF FATIGUE IN INJURIES

 What is Fatigue?

• Mental fatigue is a subjective experience that can lead to a lack of concentration.
• 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.

• 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 minimized. If the muscular stabilizers 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

• Special sensory nerve endings located in ligaments, tendons and joints 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.

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.
• Years of dedicated practice.
• 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.

Failure of the system:

• If the impulse to take evasive action takes a fraction of a second too long to be executed, it causes an inadequate ‘dynamic stabilization’ 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.

 Robert Pires, French Soccer Player - 2002

France and Arsenal star Robert Pires recently 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 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.” 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.”