The side kick is a thrust kick which in the 1970s and 80s, at the height of traditional martial arts popularity, was considered to be the most dangerous and effective kick of all. Bruce Lee famously could generate a force seemingly disproportionate to his 130lbs bodyweight with this kick. In fact by leaping into this kick with a back foot behind front foot run up, it's possible to generate huge amounts of force. Unfortunately the power side kick with a skip up is both telegraphed and wildly inaccurate. For the readers unfamiliar with the kick which has featured in almost every martial arts film ever made, here's Bruce Lee demonstrating it.
The side kick to the abdomen remains a very hard kick to land - and a very easy kick to parry. By parrying the opponent's leg from the heel side as he kicks, it is possible to force him to land across himself and expose his back at close range. Even missing or glancing off of the opponent's torso with this kick means giving up your back. Consequently the side kick to the midsection has not been very popular in mixed martial arts, where giving the back generally means giving up a bad position or getting slammed on ones head.
What has become an absolute game changer in MMA in the last two years, however, is the lead leg side kick to the front of the knee. This technique has been used by Jon Jones, Anderson Silva and others to keep the opponent at range and out point them with ease. Anyone who picked up the old Bruce Lee Method books which were published in the late 1970s will remember that the side kick to the knee featured in almost every self defense scenario Lee presented. Such was his faith in the technique - and it's ability to stifle fighters as diverse as Demian Maia, Vitor Belfort and Quinton Jackson in recent years can certainly attest to that.
There is a somewhat popular view that kicks to the front of the knee are a sort of cheap shot - especially with how many athletic careers are effectively ended by knee injuries - but in truth few fighters are foolish enough to keep placing all their weight on their lead leg if the opponent is kicking it. That is the purpose of this kick - to get the opponent hesitant to plant his weight and throw hard shots or shoot a takedown - not to reverse his knee joint and cripple him for life. The occasional fighter will not know what to do and simply keep getting kicked until he is injured - such as Quinton Jackson against Jon Jones - but most will reassess their attack or at least have a corner intelligent enough to make them.
The side kick to the knee is a wonderful technique for both strikers and grapplers - above we have it being used in two contexts. On the left is Jon Jones, a wrestler with decent stand up defense against scary boxer / brawler Vitor Belfort. Jones used the side kick to the knee to keep Belfort out at range and any time Belfort backed onto the fence Jones would step in with a few punches before breaking off and side kicking again or clinching. On the right we have Anderson Silva, a striker against Jiu Jitsu ace Demian Maia - Silva used the side kick to keep Maia at bay, stay ahead on the cards and hurt Maia without the risk of Maia catching the kick.
As the combatants' lead legs are the closest point in any engagement at range, the side kick to the knee is the strike which maintains the most distance between the two fighters. Consequently if the kick glances off of the target - as side kicks often do - there is suitable distance to run and turn back to face the opponent without him grabbing a body lock from the back. Above Anderson Silva's kick glances off of Maia's leg but he is far enough away to simply retract his right foot, bring it to the mat, push off of it and turn to face Maia without giving his back to the Jiu Jitsu ace.
Side kicks to the body are a dangerous strike to attempt but two fighters who have found ways to alleviate the dangers are Jon Jones and Cung Le. That being said, Jon Jones doesn't so much act to alleviate the dangers of throwing a side kick to the body, it is more that as a tall, lengthy wrestler he is firstly not too concerned about conceding a bodylock on his back, and secondly the length of his legs means that even glancing kicks will keep the opponent at distance enough to prevent them immediately grabbing on to him.
Notice that Jones connects on the end of his kick - the most powerful portion - and is left with his back on display to Belfort - but Belfort has been clipped on the end of Jones' kick and simply can't get to Jones in time. Just as he did against Kazushi Sakuraba and Alistair Overeem when hit hard, Vitor chose to drop to guard rather than run after Jones with little hope of tying up.
Someone who has truly made a home for the side kick to the body in MMA is Cung Le. Firstly he rarely telegraphs his step up to kick, instead hopping in as he is kicking - making it harder to see coming. Secondly, and most importantly, Le uses missed kicks to land unorthodox attacks. Le's hands are pretty mediocre from a boxing stand point - everyone knows this and even Le himself comments often on how he is always attempting to improve his boxing - but his spinning backfists off of missed kicks are simply genius.
Notice above how Cung Le attempts a side kick on Wanderlei Silva - who parries it as he has obviously been training to do - then as Le's back is exposed, Le spins and connects a hard left hammer fist on Silva's jawline. This sent Wanderlei stumbling and made all of us who worry for the Axe Murderer's declining chin sweat. As it turned out, Cung Le was in horrible shape for his match with Silva and ended up gassing by the second round and getting stopped by Silva's legendary knees, but this technique is still genius. If you fancy seeing it applied in a slightly different way, check out the hook kick he used to spin into the backfist against Frank Shamrock in Strikeforce.
The side kick is the latest classical technique to find success in Mixed Martial Arts and it is moments like these which really make the sport so interesting for me. There will be an established way of thinking for a few years, or even months, and then someone will introduce a classical technique and start using it to win fights and everyone will rethink their view. It happened with Sakuraba's low single and spinning back kick, and it is happening now with the traditional martial arts' front snap kick and side thrust kick. I think we can certainly agree that even four or five years ago we certainly didn't think we'd see side kicks from Bruce Lee's self defense books published in the 1970s completely dictating the course of fights in 2011 and 2012.