How did a false start rule of 0.10s or 100 ms determined?

Disqualification often occurs in sprint events from a false start. Example, read here.  

For a false start offence, the IAAF has set a limit of 0.100s or 100 ms reaction. Meaning that, when an athlete register a time of 0.999s (99 ms) or less, he or she will be convicted of a false start offence, and disqualified.

How did they come up with such a rule?

Since 1991, the 100 ms limit was included in the IAAF rule book; decided during the IAAF Congress in 1989 at Barcelona.

There is a mechanism of human reaction time, or how the 100 ms becomes a threshold.

• When the starter pull the trigger, the time will start. Actually it takes approximately 5 ms to reach the ear.
• Then, the signal is processed within the cerebral cortex in the brain, and this stage alone takes approximately 50 ms.
• Subsequently, the signal is sent to the muscles via spinal cord. This process takes around 25 ms.
• In addition, there is a mechanical delay of approximately 10 ms, before the onset of force production.
• From there, the athlete start to produce force (to move). The force production itself will take time. 

Therefore, 5 + 50 + 25 + 10 + force production time will be approximately 100 ms. These durations are not exactly, may be more, may be less.

There is a level of force production that can be allowed, but if the force is excessively higher (before 100 ms), then the system register as false start. 

Currently, the threshold of force production that is allowed (before 100 ms) is relatively unclear. 

So there is also the possibility of committing a false start by just a "twitching" or "flinching" motion (eg, minor slip) on the block pedals.

It is important that if the gun is located 10m away from the athlete, there is a signal delay of 0.029s (29 ms) after firing the gun. This can be subsided by using a speaker placed on each blocks, which significantly reduces the delay, ie, only ~5 ms as noted above.

Scientific research

Scientists Mero and Komi studied human reaction in the early 1990s. They recruited 8 sprinters (PB ~10.76) to participate in their study. Interestingly, they found that different muscles (of the leg) produced different response time in response to a stimulus.

Human can actually react in less than 100 ms. The IAAF in 2009 confirmed this assertion. The IAAF published a report that scientists "found great variation in individual reaction times and confirmed previous reports of simple auditory reactions as fast as 80 ms."

The scientists then "recommend that the 100ms limit be lowered to 80 or 85ms"

The report also state that and "the IAAF urgently examines possibilities for detecting false starts kinematically, so that judges’ decisions are based on the first visible movement regardless of the body part."

The above can be done "with a system of high-speed cameras, which gives views of all the athletes on the start line."

More recently, scientists found that reaction time detected using a hand force plate occurred approximately 74 ms (Harrison et al., 2018).

Interestingly, a 0.086s reaction time made by Asafa Powell in the world championships in 2003, resulted in a disqualification.  

Thus, it is possible that human reaction time vary, possibly technology dependent, and cannot be confined by an arbitrary threshold (ie, 100 ms).

A good alternative of solution is therefore vital, so, the is a need for a false start (ie, 100 ms) rule changes. However, there is no easy way to handle this issue.

What can be done from here

• First, the 100 ms rule may be lowered to 80 or 85 ms. 
• Second, if the 100 ms threshold remains, athlete must be allowed to run under protest (if register a reaction time between 80/85ms to 99 ms), and only be investigated subsequently, thoroughly, including by scrutinizing the newly established high speed camera system to (see actual movement) determine if false start was committed or not.
• Third, any reaction between 991 to 999 ms will be rounded up to 100 ms, just like what we see in the official results, eg, 9.791s becomes 9.80s.
• Fourth, amend the rule slightly. So, if one registers 990 to 999 ms reaction time, only a yellow card will be given, and allowed to run once again, and advised to be careful. A repeated 'offence' gets a red card.

Additionally, here is a list of sprinters who can be considered to having the fastest reaction times.

• 0.100 s - Jon Drummond USA, Monaco, 1993
• 0.101 s - Bruny Surin CAN, Seville (world championships), 1999
• 0.104 s - Tim Montgomery USA, Paris, 2002

Finally, believe it or not, during the 1991 world championships in Tokyo, Dennis Mitchell recorded a reaction time of 0.090s en-route to finishing the 100m final in 3rd place, without being disqualified (ie, accepted by the IAAF).