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Part 2

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.

< Part 1 - Robert Pires

Related articles:

Anterior Cruciate Ligament (ACL) injuries explained >

Proprioception explained >

References:

Lephart, S.M and Fu, F. (1995) The role of proprioception in the treatment of sports injuries. Sports Exerc Inj Vol 1, pp 96 – 102.

Zhou, S et al (1996) Effect of fatigue and sprint training on electromechanical delay of knee extensor muscles. European Journal of Applied Physiology and Occupational Physiology Vol 72, No 5-6, pp 410 – 416.

Wojtys, E.W et al (1996) The effects of muscle fatigue on neuromuscular function and anterior tibial translation in healthy knees. American Journal of Sports Medicine Vol 24, No 5, pp615 – 621.

Skinner, H.B et al (1986) Effect of fatigue on joint position sense of the knee. Orthopaedic Research Vol 4, pp 112 – 118.

McComas, A.J et al (1993) The role of the sodium / potassium pump in delaying muscle fatigue. In Neuromuscular fatigue. Sargeant, A.J and Kernell, D. (eds) Amsterdam, North: Holland pp 35 – 43.

Woods, J.S. et al (1987) Evidence for a fatigue-induced reflex inhibition of motor neuron firing rates. Neuro Physiology Vol 58, No 1, pp 125 – 137.

Clark, F.J et al, (1985) Role of intramuscular receptors in the awareness of limb position. Neuro Physiology Vol 54, pp 1529 – 1540.

Johansson, H. et al (1991) A sensory role for the cruciate ligaments. Clin Orthopaedics Vol 268, pp 161 – 178.

Wyke, B. (1981) The neurology of joints: A review of general principles. Clin Rheum Dis Vol 7, pp 223 – 239.

Zimny, M.L (1988) Mechanoreceptors in articular tissues. American Journal of Anatomy Vol 182, pp 16 – 32.

Gao, J.H et al (1996) Cerebellum implicated in sensory acquisition and discrimination rather than motor control. Science Vol 272, pp 545 – 547.

Barrack, R. et al (1994) Knee joint proprioception revisited. Journal of Sports Rehab Vol 3, pp 18 – 42.

Johansson, H. (1982) Nerve supply of the human knee and it’s functional importance. American Journal of Sports Medicine Vol 10, pp 329 – 335.

Leanderson, J. et al (1996) Proprioception in classical ballet dancers. American Journal of Sports Medicine Vol 24, pp 370 – 374.





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