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The karate Kiai has long been held to increase the power of strikes; while opening your mouth in a fight isn’t bright, a number of martial arts and combat sports disciplines do make use of loud grunts while delivering strikes or kicks. The yell is common in weight-lifting too, and even in more genteel pursuits, like Tennis. Maria Sharapova’s fierce yells have been called a form of cheating.

Now the practice has scientific backing. Via Grunting’s competitive advantage: Considerations of force and distraction, by Scott Sinnett , CJ Maglinti, and Alan Kingston, published by PLOS ONE.


Grunting is pervasive in many athletic contests, and empirical evidence suggests that it may result in one exerting more physical force. It may also distract one’s opponent. That grunts can distract was supported by a study showing that it led to an opponent being slower and more error-prone when viewing tennis shots. An alternative explanation was that grunting masks the sound of a ball being hit. The present study provides evidence against this alternative explanation by testing the effect of grunting in a sport—mixed martial arts—where distraction, rather than masking, is the most likely mechanism.

We first confirmed that kicking force is increased when a grunt is performed.

Twenty participants from a local MMA academy in Honolulu, Hawaii were recruited for this study. Each participant had at least one year of MMA/Muay Thai training, with several participants having professional competitive experience.

Each participant kicked a 100 lb. heavy bag. To measure the amount of g-force generated on the bag by each kick a Herman Digital Trainer accelerometer from Pro Sport Technology Limited was attached to the back of the bag at the participant’s hip level.


Each test session consisted of a grunting and a no grunting condition with their order counterbalanced across participants and sessions. Participants were able to generate significantly more g-force while kicking with a grunt than without. Specifically, participants generated 24.2 g-forces when grunting and 22 g-forces when not grunting (t(19) = 3.371, p = .003), a 9% increase in force.

We then adapted methodology used in the tennis study to mixed martial arts. Lifting the foot to kick is a silent act, and therefore there is nothing for a grunt to mask, i.e., its effect on an opponent’s response time and/or accuracy can likely be attributed to attentional distraction. Participants viewed videos of a trained mixed martial artist kicking that included, or did not include, a simulated grunt. The task was to determine as quickly as possible whether the kick was traveling upward or downward. Overall, and replicating the tennis finding, the present results indicate that a participant’s response to a kick was delayed and more error-prone when a simulated grunt was present.

Trainers have known yelling increases power since the birth of martial arts, but it’s still fun to have backing from science.


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