Biomechanics and the Olympics: Part V

Now that the focus of the Olympics has shifted to track and field, and more specifically the men's and women's 100 meter race, one of the critical factors in winning this race is the reaction time of the sprinters.  Often times, just like in swimming, these races are decided by a few hundredths of a second, and a poor start can be the difference between first place and last place.

Reaction time is the amount of time it takes the body to prepare and initiate a response to a stimulus.  There are three different types of reaction time (RT).

1. Simple RT: there is one stimulus/signal, and only one response to be made to the stimulus.  This is what happens at the start of a race in track and swimming.
2. Choice RT: there are several stimuli/signals, and each one requires a different response.  A traffic light is a good example.  Each color signal requires a different and timely response to avoid an accident, although one could debate what response is to be made to a yellow light.
3. Discrimination RT: there are several stimuli/signals, but the person is only going to respond to one specific signal.  A quarterback calling out the snap count is a good example of this; he is going through several different signals, but the offensive players are only responding to one specific signal and should ignore the rest.

The start of a track race involves simple RT, because the only signal the runners are paying attention to and responding to is the starting gun.  The faster the runners can respond to this signal and begin to move, the greater the chance they have of winning the race.  Reaction time can be broken down into two components, pre-motor time and motor time.

1. Pre-motor time is the amount of time from the onset of the stimulus (the sound of the gun) until electrical activity is detected in the muscle groups used to perform the movement.  This time can be measured using electromyography (EMG), which typically involves placing electrodes over the muscle in order to record the electrical signal associated with the muscle contraction.  What happens during this pre-motor time?  Well, the auditory signal has to be detected by the sensory receptors in the ear, this signal has to travel to the brain for processing, and the brain has to send signals down to the appropriate muscle groups.
2. Motor time is the amount of time from the onset of electrical activity in the muscle until the first movement is detected.  It takes time to develop tension in the muscle and transmit this tension from the muscle to the tendon to the bone for movement to occur.  This time is also known as the electromechanical delay of the muscle.

The take away message is that it takes time for the body to prepare and initiate a response, even to simple signals.  As movement complexity and the number of signals and possible responses increase, reaction time also increases. In all sports, a shorter (faster) reaction time typically leads to a greater chance of success. 

For the elite level sprinters in the Olympics, this entire process takes between 100-200 milliseconds (a millisecond is a thousandth of a second).  This does not seem like a large amount of time, but in a short sprint of 100 or 200 meters (or even 400 and 800 meters), a few milliseconds can make a large difference.  I am not an expert in track, but I know these sprinters spend a lot of practice time working on their starts.