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Reaction Time

by Anders Hansson

This is a text to accompany the reaction time test game On Your Marks! The game measures the time it takes for you to press a button after you have heard the sound of a starting gun. For a human it has been estimated that the shortest attainable time is slightly below 0.100 s. During this tenth of a second a complex series of events takes place inside your body.

Overview

You can read about the principle of the nervous system in countless other places. The following overview is brief and focuses on concepts that will make the rest of the text easier to understand and remember.

Electrical impulses along a chain of cells

Sound waves generate a signal at the hair cells inside the ear. The signal then travels as an electrical impulse along a chain of long, thin cells. These are the nerve cells, also called neurons, which are cells specialized for communication. (Neurons bundle to form nerves.) The last link is the muscle cell, also called muscle fiber.

Chemical transmissions

The transfer stations between cells are called synapses and the process is called synaptic transmission. The transfer is of chemical nature. Molecules are released from the end of one cell and then move through random motion (a process called diffusion) until they reach the beginning of the next cell in the chain. This is a rather slow process of at least 0.001 s, but it allows for a transmission that can be controlled (e.g. strengthened or switched).

The route

The launch of the signal

Sound waves are fluctuation of pressure that travels through air. Inside the ear the pressure fluctuation will bend the hair cells and open up channels for electrically charged particles. This will trigger events that ultimately result in a synaptic transmission to nearby neurons.

Into the Central Nervous System

The neuron then takes the signal all the way to the border station (i.e. synapse) at the edge of the Central Nervous System. The Central Nervous System (CNS) consists of the brain and the spinal cord. The brain and the spinal cord are made up of neurons just like the nerves are. It is inside the CNS that much of the controlling and modifying is done. Consequently, the route here is cumbersome with many stops and transfers along the way. When the signal reaches the outer part of the brain – the cortex – we are finally aware of it and decide, obliging to the rules of the test, to press a button. This decision corresponds to further signal traveling and processing and finally results in signals boarding motor neurons at the edge of the CNS.

Away from the CNS towards the muscle fibers

The motor neurons constitute the route away from the CNS. One single neuron covers the whole distance all the way to the muscle fiber. Accordingly, a motor neuron to the calf is much longer than a motor neuron to the neck.

The final destination – the fibrils of the fiber

The final transfer takes place at the middle of the muscle fiber (at the so called neuromuscular junction). From there electrical impulses propagate along the fiber surface to all parts of the fiber. It also propagates into the interior of the muscle by inward folds of the surface. When the signal passes the myofibrils it triggers a chain of event that will ultimately result in force generation.

A net rather than a chain

Note that a there will always be a lot of parallel signals and they don’t act in isolation but communicate with each other. It is more correct to liken the pathway to a network of cells rather than a chain of cells. For example: A single hair cell may transmit to 20 neurons. At the first neuron-to-neuron synapse at the border to the CNS further split-ups occur. Inside the CNS both split-ups and reunions take place. For every neuron entering the CNS there are 10 that leave the CNS. Finally, a motor neuron splits up to exit at several muscle fibers. A fiber, however, only receives input from one neuron (i.e. there are many more fibers than motor neurons).

Factors

We can now try to categorize the factors that decide reaction time:

  1. travel time of sound wave
  2. transfer time between cells
  3. speed along cells
  4. choice of route, which can be divided into
    • number of transfers
    • length of the track
  5. time to produce the muscular force (and eventually movement) of the response.

Factor 1 is not really part of what we would like to measure and should be minimized or standardized.

Factor 5 depends on the kind of response the measuring system can detect. Results of tests that utilize different systems should not be compared.

With these factors in mind let’s examine how they possibly can be influenced.

How to improve reaction time

1. Travel time of sound wave

The speed of sound in air is 343 meter per second. Speakers placed one meter from your ears thus create an unnecessary delay of 0.003 s.

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Use Headphones!

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In sprint competitions there is a speaker attached to each starting block. The distance from the speaker to the ear is about one meter. Individual speakers are not used in minor competitions though, so the athletes have to wait for the sound wave from the actual gun. This puts the athletes that are far away from the gun at an unfair disadvantage.

Ill-considered one-upmanship: Sprinter with headphones
Individual speakers are not used in minor competitions. The starter here stands about 10 m from middle lanes. It takes 0.030 s for the shot sound to travel this distance. Talk to the starter – I am sure he can set something up to help you!

2. Transfer time between cells

The louder the sound, the more molecules

The louder the sound, the more the hair cell will bend, and the larger its release of molecules. Therefore it will take less time to accumulate enough molecules at the adjoining neuron for the neuron to generate an electric signal. The electric signal itself is always of the same size, but there will be more frequent signals if the release of molecules is large. More frequent signals will in turn lead to a larger release of molecules at the next synapse and the aforementioned process continues. That is, the loudness of the sound will induce a chain reaction of quick transfers.

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Turn up the volume, place your ear against the speakers, use headphones, and so forth.

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When a loud gun is used, athletes far away from the gun will not only receive the sound later, but also at a considerably weaker intensity. (The intensity is inversely proportional to the square of the distance.)

In the Olympics a loud gun is used in addition to the individual speakers. In the 2004 Olympics the mean reaction time of lane 1 was 0.015 s less than the mean time of lanes 2–8.

In an experiment with sprinters a 120 dB shot sound, compared to an 80 dB shot sound, improved reaction time with 0.018 s. Interestingly, the rate of force development (but not peak force) was also improved. That is, the chain reaction lasted all the way to the force generating process beyond the end of the electric signal.

Artificial addition of molecules

The molecules that are released at a synapse are called neurotransmitters. There are also other molecules involved in the transfer process, but neurotransmitters are the principal ones. The kind of neurotransmitter together with the kind of binding site determine if the outcome will be excitatory (help to generate an electric signal at the neuron that hold the binding site) or inhibitory. The neurotransmitters themselves may be facilitated or hindered by drugs in many different ways. Drug may also mimic the neurotransmitters.

Have in mind that the effect of drugs is not directed specifically at the signal path that determines reaction time. That is, the amount of noise will increase and this may have detrimental effect.

Nicotine mimics a neurotransmitter (acetylcholine) that produces an excitatory effect and will improve transfer time in the CNS. However, at the exit station to the muscles, the sites are slightly different and will not bind nicotine.

Caffeine mimics a neurotransmitter (adenosine) that produces an inhibitory effect: but it does not mimic well enough to actually trigger the inhibitory action. It will therefore block out the real neurotransmitter and its inhibition. The net effect of caffeine is therefore excitatory.

There have been lots of studies on the effect of caffeine and nicotine on complex reaction tests where you have to process the signal and make a choice before responding to it. Studies on simple reactions test – like this one, with no choices involved – also indicate a small but significant improvement due to the drugs.

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Test your time before and after a cup of coffee to see if it really makes any difference. Perhaps you need several cups? Also, note that the effects of caffeine peak about one hour after consumption.

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Caffeine is, since 2004, no longer on the World Anti Doping Agency’s list of prohibited substances. Before that, only very high concentration was deemed illegal. It should be noted that caffeine does more than merely act as a CNS stimulant.

Depraved sprint diva resorts to drugs to boost reaction and healthy complexion
The headphone cable solution did not turn out so well after all. Desperate for a competitive edge our athlete turned to drugs.

In most part of the brain ethanol (alcohol) inhibits the most important excitatory neurotransmitter (glutamate) of the CNS. It also facilitates an inhibitory neurotransmitter (GABA) of the CNS. In one test blood alcohol concentration of 0.05 % was found to slow down reaction time with more than 0.020 s.

Temperature – faster moving molecules

If temperature is raised, energy is added and molecules move faster: everything chemical should speed up. Not only do the neurotransmitters move across the synapse in less time; collisions between certain molecules will occur more often and with more energy – this will increase the rate of chemical reactions.

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Accordingly, reaction time should generally be faster in the afternoon than in the morning as body temperature is at its lowest in the morning. Physical exercise should also improve reaction time. That is, you should do a warm-up just like you would before a physical test.

Again, conduct some scientific tests on body temperature and reaction time on yourself. Try to exclude other factors, but be aware that circadian rhythm (which regulates daily change in body temperature) and physical exercise will influence a lot of things in your body.

3. Speed along cells

As stated, the signal travels as an electrical impulse along the long and thin neuron. It should nevertheless not be likened to the conduction along a piece of insulated wire. The plus and minus pools of the wire are at its separate ends. In the neuron they are instead at the outside and inside of the neuron’s surface, i.e. its cell membrane. Consequently, charges move along small local currents across the surface. The electrical field of such a current opens up channels through neighboring surface. These channels will allow charges to form new currents. That is, the conduction along the neuron is the result of a chain reaction of currents!

Myelin

Therefore nerve conduction speed is much slower than wire conduction speed (which equals the speed of light). Speed varies considerably among neurons and the fastest ones are so because they imitate wire conduction to some degree.

The surface of a neuron is often covered by other smaller cells that are rich in myelin, a fatty substance that blocks currents across the surface and instead force it along the axis (called axon) of the neuron. The myelin cells do not cover the axon completely but leave spaces between each other. At these spaces (called nodes of Ranvier) a new current is born as described above. The process of current generation slows down the conduction speed, which at 10–100 m/s is still a far cry from the speed of light. Unmyelinated neurons may, however, barely manage a speed of 1 m/s. (Please, note that myelination does not only promote speed, but also precision, as it hinders interference between axons.)

The spaces occur at intervals of 0.3–2.0 mm. The maximum functional distance is limited by the insulating capacity of the myelin. The thicker the myelin sheath, the better its insulating capacity. As longer intervals means fewer slowdowns, thicker myelin sheath thus make faster conduction speed possible. Myelin is built up during early age, and full speed of the neurons that form peripheral nerves are reached already at about five years of age. Myelin build-up inside the CNS, however, may continue well into the twenties and may also be stimulated by practice and repetition as increased traffic is sensed by the myelin building cells. In adults this mechanism may be at work as well but to a much lower extent.

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So, if you are young, practice diligently at our reaction test game every day (and click a few ads while you’re at it smiley) and one day, several years from now, you may find yourself at the top of the Best Ranked Player list ...

At about the age of forty a slow decline of myelin begins. Also, nerve injuries (and diseases like Multiple Sclerosis and Cerebral Palsy) can reduce myelin.

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If you are old, staying healthy and young in mind may be your only options.

Cerebral Palsy is known to reduce myelin. Fortunately track starters are both tough and fair.
Diseases can reduce myelin. If you are seriously afflicted (like this textbook case), plead nicely. Though a hardy bunch, track starters are also known to be fair and may set you up with a handicap.

Temperature again

Nerve conduction speed is, just like synaptic transmission time, improved by higher temperature. Chemical processes are responsible for the start of each local current in the chain and these processes are sped up at higher temperatures. (Note, however, that the speed of the electric impulse itself is independent of the speed of the electrical charges (ions) of the current.)

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The speed increase has been found to be about 2.5 m/s for every 1°C increase in body temperature. This means that, for example, a speed of 52.5 m/s will be 0.010 s faster over a distance of 1 meter compared to a speed of 50 m/s. Note that this is without considering speed-up of transmissions.

Additionally it may be interesting to know that these chemical processes are dependent of energy from the same molecule that the muscles need for their force generating process – namely the ATP molecule. Phosphocreatine assures a steady supply of ATP and creatine supplementation has been shown to improve intelligence significantly (if there was an initial deficit of phosphocreatine).

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Repeated reaction time test trials could very well benefit from creatine supplementation.

4. Route

The up-going route (ascending pathway) of the signal up to the auditory cortex (where awareness of the sound happens) is well known. It involves only four synapses.

The down-going route (descending pathway) from the sensorimotor cortex (where movement plans are initiated) to the muscles, is also well known. In its most simple form it involves only one synapse, but usually many more for feedback and necessary adjustments to the original plan.

Plan ahead

The route from the auditory cortex to the sensorimotor cortex, however, is not well known. Actually “route” is not the right word as it is a complex process that usually involves much of the brain. This process equals the “wish to do something”.

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While waiting for the sound signal, don’t listen for the sound – just focus on the movement, i.e. pressing a button. That way you give the above process a head start.

The startle reflex

Not all movements need to involve the cortex. As illustrated by a running headless chicken, some movements do not need to involve the brain at all.

A sudden and loud sound may trigger the so-called acoustic startle reflex. The path of this reflex takes a considerable short cut and never engages the cortex. Its final destination is the strong muscles of the legs and upper arms, which can act to move the body away from the sound.

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To utilize this, first of all you must crank up the volume. The higher the volume, the better the chance for the reflex to kick in. 120 dB may do the trick. Also, you need to find a way to trigger the button by extending your arms or legs. (Mad Catz, please take notice. There may be a huge demand for starting blocks replica controllers smiley.)

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The sound of the starter’s gun may be loud enough to trigger the startle reflex – at least for those lucky athletes close to the gun. The study, mentioned above, on the effect of loudness on reaction time, concluded that the startle reflex could reduce reaction time a further 0.018 s. This improvement was not accompanied with a faster development of force; illustrating its short cut nature.

The arms lead, the legs follow

The final part of the route is away from the CNS along a single long neuron all the way to the muscle. Neurons to the leg muscles travel along the spinal cord before leaving the CNS. All together this means that the path to the calf muscles is about 1.5 m longer than the path to the shoulder muscles. Allowing for a maximum nerve conducting speed of 100 m/s, this equals a time difference of 0.015 s.

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The muscles of the fingers are located in the upper part of the forearm. You can thus only gain a little by finding a way to press the button with the upper arm muscles as they are located only a little closer to the brain (but also consider the possibility of the startle reflex).

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The starting block sensors detect the extension movements of the legs. They are, however, preceded by movements initiated by the shoulder muscles. The first visible observation of a start is therefore the release of the hands from the track.

5. Time to produce the muscular response

A reaction time below 0.100 s in sprint competitions is considered a consequence of anticipating the gun and is not allowed. Studies have shown that electrical activity in the calf muscle can be detected already after 0.060 s. How then can the 0.100 s limit be justified?

Impulse propagation

The detected electrical activity corresponds to the arrival of the electric signal at the neuromuscular junction at the middle of the muscle fiber, but remember that this is not the end of the journey for the electric signal. The spread of the signal from its position on the middle of the fiber to the end of the fiber may take a substantial time. Muscle fibers do not have myelin to speed up the propagation.

Contraction through chemical reactions

When a myofibril finally is reached there is another delay before the myofibril actually produces tension. Several chemical reactions are involved in this process, which involve calcium ions, contractile proteins, and ATP.

Slack

Even when tension inside the myofibril is present, it may not immediately register at the larger scale because of slack in structures that are serially attached to the contractile proteins. Movement is ultimately caused by force transmission through the tendon to the bone and the tendon itself may account for some of the slack.

These three factors constitute the total delay from the first electric impulse on the fiber surface to the first detection of force at the tendon. This delay is called the electromechanical delay and wildly different values from less than 0.020 s to 0.120 s have been reported even for the same muscle. The discrepancy is likely caused chiefly be different measuring methods.

Summation of tension

However, reaction time tests often require a larger force or an actual movement to register a response. The bigger the required force, the larger the additional delay. A single electrical impulse only takes the force of a fiber so far. Several impulses at a high frequency are needed to maximize the fiber’s force potential. Also, while a single motor neuron splits up and transmits to several fibers (a so called motor unit), the response may require a force that can only be produced by the synchronized efforts of several motor neurons.

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The force required to press the keyboard or the mouse is minimal, but compared to a mouse button a keyboard key typically has to be pushed a longer distance to achieve switch closure. The mouse button should therefore be quicker.

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There are several different manufacturers of starting equipment for use in sprint competitions and they all use different mechanisms to detect the response of the runner. One system uses force sensors in the blocks and registers a start response when a 25 kg threshold is crossed. A successor to this system instead uses the steepest rise of the force curve to determine the start response. Another system uses a switch closure.

Those systems will detect the same start at different times. But they all use the same lower limit of 0.100 s to judge a start as illegal!

A recent study on sprint starts sheds light on the problem. It should be noted that a shot sound volume of 100 dB was used in the experiment and it was therefore unlikely to evoke the startle reflex. Electrical activity in most of the muscles of the rear leg was detected after about 0.065 s. Custom software analysis together with visual observation of force sensor data were used to detect the start response. One of the participants then had a mean reaction time of 0.087 s. However, when a less sensitive threshold detection method was used, reaction times increased on average with 0.025 s. When a high-speed (500 Hz) camera was used to detect the first movement, reaction time increased with 0.060 s. (Note that this is as long as it takes for the signal to travel all the way from the ear to the muscle!)

On Your Marks ScreenshotOn Your Marks!

Time to test yourself on our auditory (but visually pleasing!) reaction test. It is hands down the most accurate one you’ll find on the web – not just a random number generator bolstered with assorted Wikipedia wisdom like most of the other ones.


Athletic Design, 7 October 2009

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