Motor Control
In Sprinting
By Joanne Browne, University of South Australia
A basic recapitulation of how the body's system of
motor control operates in sprinting.
INTRODUCTION
The 100-meter sprint involves power, strength, balance, coordination and above all other qualities--speed.
But what makes the difference between a fast runner and a world class athlete? Along with genetics and training, it comes
down to motor control. The various anatomical, physiological, neural and cognitive aspects involved with the execution and
control of sprint movement are vital for understanding and mastering one's success on the track.
THE CNS-WHERE IT ALL HAPPENS
The central
nervous system consists of the spinal cord and brain which are responsible for processing, integrating and coordinating human
movement. The peripheral nervous system consists of the neural tissue outside the CNS, and can be divided into sensory and
motor divisions. When a sprinter hears the call to take his/her mark, the receptors in one's ears hear this stimulus
and
the message is sent to the brain through the sensory division. The motor division is where the message to move into position
is sent from the brain out to the target muscles where the response is seen.
The individual units
responsible for the success of these and other messages are neurons. Sensory neurons deliver the information from the receptor
to the CNS, while motor neurons carry the instructions from the CNS to the target muscle.
To understand
how these messages are sent, the anatomy of the basic neuron needs to be understood. The cell body contains the nucleus, and
around this are dendrites that receive information from other cells. The axon is where the message propagates toward axon
terminals and the message is received by the postsynaptic cell, which is the cell located after the first neuron.
For a message to be sent on to a postsynaptic cell, an action potential must pass along the axon, away from the cell body.
When a stimulus is received by the neuron it causes a depolarizing current which is a short burst of electrical activity that,
when it reaches a threshold, will always fire an action potential. Neurons cannot communicate without the success of action
potentials.
When the message from the nervous system reaches the target muscles, it must pass across
a neuromuscular synapse before the muscle contracts. A single muscle fiber is acted upon by a single motor neuron, and there
is no direct contact between these two compartments. Rather, the signal is electrochemical, and an excitatory neurotransmitter
called acetylcholine triggers the mechanical .response that is a muscular contraction. Without these fundamental neurological
pathways, muscular events such as a sprint race--whereby continual contraction of the athlete's muscles is required--cannot
occur.
There are a number of regions in the brain that control voluntary human movement, and these
areas obviously playa vital role in a sprinter's success. The cerebrum contains four lobes, all of which receive sensory
neurons that are relaying information from outside the brain. Located here are motor neurons that send signals to skeletal
muscles to control rhythm, balance, posture and movement preparation for control over a task such as sprinting.
Also important is the cerebellum, which is responsible for controlling the smooth and accurate motions of a sprinter along
with coordination and learning various movements. The diencephalon acts as a sensory relay station and is important for attention,
memory and pain perception. Finally, the brainstem helps to control a sprinter's balance and breathing while integrating
both sensory and motor information.
Understanding the components of the CNS, the roles of sensory
and motor neurons, action potentials, the events that occur at a neuromuscular synapse and the regions of the brain that contribute
to movement execution are all-important if we want to understand how the human body is able to sprint. Without the cooperation
of these components of motor control people would be unable to perform such tasks as sprinting. The effectiveness of these
systems contributes to the overall success of a track sprinter.
HOW
DOES MOTOR CONTROL IMPACT ON SPRINTING?
There are a range of motor components to be considered in sprinting. Reaction time, acceleration,
muscle coordination and stride rate all playa part in contributing to the success of a sprinter (Bompa 1999). When a race
starts, constant monitoring of muscular output is required so that the sprinter can achieve a fast acceleration and overall
velocity. This requires continual communication between the peripheral and central nervous systems as the feedback from sensory
inputs travels to the brain, is processed, and then the signal to keep working is sent to the active muscles. Proprioceptive
feedback is vital so that the person sprinting is aware of his/her position relative to others in the race and signals
are sent to the muscles to contract simultaneously, to control balance and agility.
An example of
this is when a sprinter does not have to constantly watch the track lines in order to run straight, instead proprioception
allows them to have constant feedback on where the body is in relation to those lines and as a result the athlete can maintain
his balance whilst sprinting.
During and before a sprint race, there are a number of sensory inputs
being processed by the CNS. Examples of these include visual inputs such as recognizing the distance between yourself and
the finish line. Acknowledging the roar of the crowd and hearing the call for your marks to be taken are auditory inputs processed
by the CNS. Internal information is also processed, so the sprinter is aware of how nervous he feels, whether the heart is
racing or if he is feeling ill. Every sensory input is processed to give the sprinter accurate feedback so that he'll
know how he is performing.
The most critical stimulus to be considered is the sound of the starter's
gun. When the gun is fired, it will take a certain amount of time to reach each sprinter, depending on how far from the source
they are. The time it takes from when the gun sounds to the presence of potential for muscle action in runners is called pre-motor
time, because muscle action has not yet begun.
When the muscles are fired and the response is seen to contract and push
the sprinter out of the blocks, this is motor time (Botwinick et al. 1966). When the sound of the gun reaches the receptors
in the sprinter's ears the message is sent via the afferent neural division to the brain, where the signal to start the race
is processed and the response is initiated by the forward portion of the frontal lobe.
The area of
the brain that interprets the auditory input is the temporal lobe of the cerebrum (see Figure 2). Here the sound of the gun
is processed as the sprinter realizes that he must start sprinting. This information is sent via the efferent division of
the nervous system to the target muscles, such as the quadriceps and calf muscles, to contract and push them out of the blocks.
As the instructions leave the CNS and enter the peripheral nervous system, the message travels through
the motor neurons as action potentials are generated. When the signal reaches the neuromuscular synapse, the excitatory neurotransmitter
acetylcholine leaves the axon terminal and binds to receptors on the working muscles. Reaction time concludes when the message
reaches the muscles, and without the success of this system, the sprint race cannot be initiated.
REACTION TIME IN SPRINTING
According
to Collet (1999), simple reaction time can be defined as "a measurement of the time from the arrival of a suddenly presented
and unanticipated signal to the beginning of the response to it." For a sprinter, this signal is the starter's gun
(not unanticipated, however) and the various aspects of motor control that lead to a response support this statement.
The International Association of Athletics Federations' approved minimum simple auditory reaction time
is 100/1000ths of a second (IAAF 2009), a reaction time usually achieved only by elite sprinters. An ongoing debate into whether
this time is acceptable exists, as a number of studies claim to either support or dispute this standard. Understanding motor
control can give a better idea as to how the approved time has been reached. Years of evidence conclude that the time it takes
for the stimulus to arrive at the sensory organ, its conversion into a nervous impulse, its interpretation and processing
within the brain and the onset of muscle activation are all taken into account to reach an agreeable time to react.
A study by Pain et al. (2007) suggests that the neuromuscular component of reaction time to a gun in a sprint race can be
as low as 85/1000ths of a second. Differences in an individual's physiology may impact on the fairness of the accepted
standard, and furthermore the validity of this time frame is questioned when sprinters guess the arrival of the stimulus rather
than wait. To account for certain neurological factors the IAAF has agreed that 100/1000ths is the minimum time frame in which
a 100m sprinter can legally react to the gun. Motor control therefore contributes greatly to both the theory and practical
application of sprinting.
CONCLUSION
Motor control plays a vital role in sprinting, and can greatly affect the outcome of a race when
it comes down to sprinters' differing reaction times to the starter's gun. The processes within the CNS, including the regions
of the brain that contribute to smooth, accurate and continual movement of limbs are all responsible for the responses seen
within a sprinter. Sensory inputs, which may be auditory, visual or proprioceptive, are interpreted by the brain and the effectiveness
of the motor neurons in relaying these messages to the sprinter's muscles during the race are fundamental to their success.
The nervous system also determines a sprinter's feedback throughout the race, enabling the sprinter to constantly assess
his position and performance.
Perhaps the most important aspect of motor control with regards to
the 100m sprint is reaction time--the phases of which (premotor and motor time) are affected hugely by the sprinter's
neuromuscular conditioning. Speed and resistance training, along with practicing reaction time drills can all help to improve
a sprinter's reaction time. Motor control and the relative anatomical, neurological and physiological processes that occur
in the human body are therefore essential for both elite and aspiring athletes when it comes to a race such as the 100m sprint.
FROM TRACK COACH 191
