Close
today i learned (til)

Fighting Physics

April 20, 2018

Though some may view it as violent, martial arts are an integral part of many cultures around the world. While some were created for warfare, such as Indonesia’s Pencak Silat, others, like China’s Kung Fu, were created to be an extension of one’s morals and principles. After centuries of evolution, some aspects of martial arts have been perfected to take full advantage of all the biological levers and pulleys that our bodies are made up of. With physics and data analysis, one can easily find the mechanics of what makes punches and kicks so deadly.

The Roundhouse Kick

The roundhouse kick is one of the simplest kicks in martial arts, but it takes full advantage of our built-in body mechanics. The fundamental principles of the roundhouse kick are dependent on the concepts of torque and angular velocity – or rotational force and speed, respectively. Your intuition might tell you that in order to have a stronger kick, you’d either have to put more force into it or make the kick go faster. However, the real question lies in how that’s done.

The force that is needed to overcome the mass of the leg and cause it to rotate is torque, which is defined as:

            τ = r x F

                        r = length of lever arm

F = magnitude of force applied

 

In the case of a Muay Thai kick (as shown in the video above), the entire leg is swung around in an arc. In a way, the entire leg acts like a baseball bat, rotating around the center of the body. However, torque means nothing in terms of impact strength unless you apply some kind of speed measurement to it. That’s where the equation for power comes in:

            P = τ x ω

                       τ = torque

ω = angular velocity

 

The mechanics involved in a roundhouse kick
The mechanics involved in a roundhouse kick

When lifting heavy things, people say that your power comes from your legs. Similarly, when kicking things, your power comes from your hips. Torque is produced by how quickly and how much you rotate your hips and foot, to some extent—you can see this in the feet and hips of the fighter above.

A study conducted by Colin Gavagan measured the various techniques of experts in Muay Thai, Karate and Taekwondo. He found that the average impact force of a roundhouse kick on a kicking shield from a professional Muay Thai practitioner was 1400 N! The force of this kick is roughly equivalent to a sledgehammer being swung as hard as possible at someone’s body! Through just a simple rotation of the hips and feet, one can deliver kicks strong enough to break bone.

 

The Straight Right

Similarly to the roundhouse kick, the straight right requires torque in order to generate enough power to do any considerable jab. Something most people may not know, however, is that most of the power from a straight right comes from the legs and hips. Rather than rely solely on the shoulder and arm muscles, a boxer uses his legs and hips to add to the power of his punch.

The mechanics of a punch are similar to that of a roundhouse kick
The mechanics of a punch are similar to that of a roundhouse kick

A punch is at a distinct disadvantage from a roundhouse kick in terms of power, however: a roundhouse kick uses the entire lower leg for contact, while a punch makes contact with only one’s hand. Whereas a leg might weigh 10% of someone’s weight, a fist would reasonably make up only 1%. How does a boxer make up for this difference in weight? The answer lies in the “snap”.

FΔt = mΔv

F = magnitude of force applied

Δt = change in time

m = mass

Δv = change in velocity

 

Δt stands for the total time it takes for the force to be applied, which in the case of a boxing glove on a punching bag or body, can’t be changed much. However, as the time of impact stays constant, if one increases the speed of their punch one can significantly increase the force of it. By focusing on the extension of the punch in your shoulder and forearm, you can significantly increase the speed your hand. The sheer speed of a punch can make up for it’s lack of mass behind it.

For such seemingly simple moves, there are so many fundamental processes going on at once that it’s impossible to analyze all the nuances that comprise them! There’s a lot of science that goes into something that, at first glance, seems like a simple punch.

References:

Kinoshita, M., Fujii, N. The Analysis of Kinematical Characteristics in Taekwondo Roundhouse Kick. Retrieved from https://ojs.ub.uni-konstanz.de/cpa/article/view/5737

Estevan, I. Falco, C. 2013 December. Mechanical Analysis of the Roundhouse Kick According to Height and Distance in Taekwondo. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3944546/

Gavagan, C., Sayers, M. 2017 August. A biomechanical analysis of the roundhouse kicking technique of expert practitioners: A comparison between the the martial arts disciplines of Muay Thai, Karate, and Taekwondo. Retrieved from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0182645

Tags:
0Likes
About Author

Noah is an undergraduate student at Georgia Tech looking to change the world one step at a time. He loves to hike, stargaze and build computers and spends his spare time dancing, working out and gaming.

You May Also Like

today i learned (til)
Should I Quit Chewing Gum?
Articles, today i learned (til)
Alex or Alexa: Gender Norms in Technology
today i learned (til)
Carbonation and Coca-Cola
today i learned (til)
Illegal Substances vs. Neurological Disorders: The Showdown

Your journey starts here! Let’s talk!

ThemeRex © 2026. All rights reserved.
  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.